Functional Response and Superparasitism by Diachasmimorpha longicaudata (Hymenoptera: Braconidae), a Parasitoid of Fruit Flies (Diptera: Tephritidae)

Functional Response and Superparasitism by Diachasmimorpha longicaudata (Hymenoptera: Braconidae), a Parasitoid of Fruit Flies (Diptera: Tephritidae) Author(s): Pablo Montoya, Pablo Liedo, Betty Benrey, Juan F. Barrera, Jorge Cancino, and Martin Aluja Source: Annals of the Entomological Society of America, 93(1):47-54. 2000. Published By: Entomological Society of America DOI: http://dx.doi.org/10.1603/0013-8746(2000)093[0047:FRASBD]2.0.CO;2 URL: http://www.bioone.org/doi/full/10.1603/0013-8746%282000%29093%5B0047%3AFRASBD %5D2.0.CO%3B2

BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder.

BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.

ECOLOGY AND POPULATION BIOLOGY

Functional Response and Superparasitism by Diachasmimorpha longicaudata (Hymenoptera: Braconidae), a Parasitoid of Fruit Flies (Diptera: Tephritidae) PABLO MONTOYA, PABLO LIEDO,1 BETTY BENREY,2 JUAN F. BARRERA,1 JORGE CANCINO, AND MARTIN ALUJA3 Programa Moscamed, Direccion General de Sanidad Vegetal, SAGAR, Apartado Postal 368, 30700 Tapachula, Chiapas, Mexico

Ann. Entomol. Soc. Am. 93(1): 47Ð54 (2000)

ABSTRACT The functional response and the effect of superparasitism of Diachasmimorpha longicaudata (Ashmead) in larvae of Anastrepha ludens (Loew) was determined under laboratory conditions. Adult parasitoids were tested individually and in groups of 5. Third-instar A. ludens larvae were exposed for 3 h to experienced, 5-d-old females at the ratios of 1, 5, 20, 30, 40, 50, and 60 host larvae per wasp. For individual females, functional response was type III, whereas for females in groups, a type II curve was observed. In the presence of conspeciÞcs, females increased their parasitization activity. Females showed a strong tendency for self-superparasitism (the same female laying ⬎1 egg in the same host) with a range of 38.9 Ð57.9% of larvae superparasitized, even when there was no competition and a high availability of host larvae. In the superparasitism experiment, when the parasitoid/host ratio was 1 or greater, larval mortality was consistently high but never exceeded 90%. Successful parasitoid emergence decreased as the parasitoid/host ratio increased. We discuss and question the detrimental effects that have been attributed to superparasitism in solitary endoparasitoids. KEY WORDS Diachasmimorpha longicaudata, Anastrepha ludens, self-superparasitism, host discrimination, intraspeciÞc competition

THE TEPHRITID FRUIT ßy parasitoid Diachasmimorpha longicaudata (Ashmead), a solitary braconid endoparasitoid, is currently considered as one of the most important biological control agents for augmentative releases (Clausen et al. 1965, Sivinski 1996). Knipling (1992) suggested that this species might be used successfully for the control of the oriental fruit ßy, Bactrocera dorsalis Hendel, and the Caribbean fruit ßy, Anastrepha suspensa (Loew), and, if the augmentative releases were combined with the release of sterile ßies, eradication of these pests might be possible. In Mexico, this parasitoid is being mass-reared (50 million per week) as part of the National Fruit Fly Campaign (Cancino et al. 1996). The effectiveness of a natural enemy to regulate pest populations has been traditionally related to its functional response (Hassell 1978, Fujii et al. 1986), which is deÞned as the relationship between the number of prey taken by the predator as a function of prey density (Holling 1959). It is believed that a natural enemy is more likely to be effective if the functional response is density-dependent (Solomon 1949, Ni1 El Colegio de la Frontera Sur, Apartado postal 36, 30700 Tapachula, Chiapas, Mexico. 2 Instituto de Ecologia, Universidad Nacional Autonoma de Mexico, Apartado postal 70 Ð275, 04510 Coyoacan, Mexico DF. 3 Instituto de Ecologia, Apartado postal 63, 91000 Xalapa, Veracruz, Mexico.

cholson 1958), although there is controversy concerning the role of density-dependent factors in regulating populations (Stiling 1987, 1989; Brown 1989). Another important attribute for successful biological control agents includes the ability of a parasitoid to discriminate among parasitized hosts that differ in the number of parasitoid eggs (van Lenteren et al. 1978). This ability is necessary to avoid superparasitism and to minimize the waste of time and energy associated with this behavior (Mackauer 1990, Godfray 1994). However, van Alphen and Visser (1990) argue that under certain circumstances, superparasitism might be adaptive. As a part of a comprehensive assesment of D. longicaudata as a natural enemy of Anastrepha fruit ßies, our goals in this study were as follows: (1) to determine the type of functional response of this parasitoid exposed to varying densities of Anastrepha ludens (Loew) larvae, (2) to determine the prevalence of superparasitism and how it is related to functional response, and (3) to evaluate the effect of superparasitism on the development and emergence of parasitoid progeny. Materials and Methods Study Site. Work was carried out in the Biological Control Laboratory of the Mediterranean fruit ßy pro-

0013-8746/00/0047Ð0054$02.00/0 䉷 2000 Entomological Society of America

48

ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA

gram in Metapa, Chiapas, Mexico. Laboratory conditions were 26 ⫾ 2⬚C, 65 ⫾ 5% RH, and a photoperiod of 12:12 (L:D) h. Cubic cages of 30 cm per side comprising a wooden frame covered with mesh (as described by Wong and Ramadan [1992]), were used as experimental units. Fly larvae were mixed with artiÞcial diet in petri dishes and were exposed to parasitism in these cages. Biological Material. A. ludens larvae and D. longicaudata were obtained from the Moscafrut mass-rearing facility in Metapa, Chiapas, Mexico. This facility produces 300 million sterile A. ludens ßies and 50 million parasitoids per week, as part of the actions of the National Fruit Fly Campaign. In the mass-rearing facility, fruit ßy adults are maintained in cages where the females lay their eggs through a panel of plastic mesh. Eggs are collected daily, incubated in containers with water, and aerated by a gently bubbling air ßow. At egg hatch, the newborn larvae are placed in an artiÞcial diet made of yeast, sugar, corn cob powder, citric acid, sodium benzoate, and nipagin. Larval development takes 9 d. Third-instars (8 d old) are separated from the diet, irradiated at 4.5 Krads and used as hosts for rearing D. longicaudata. The irradiated larvae are placed in cassette-type containers covered with mesh and exposed to adult parasitoids in aluminum frame mesh-covered cages (30 by 30 by 41 cm high). After 2 h of exposure, the host larvae are collected and placed in containers with vermiculite to allow pupation. After 14 d, the parasitized pupae are packed in paper bags for adult eclosion and Þeld release. The A. ludens rearing process has been fully described by Dominguez (1996), and the parasitoid rearing process was described by Cancino (1996). Functional Response Experiment. Eight densities of host larvae were tested independentlyÑ1, 5, 10, 20, 30, 40, 50, and 60 larvae per petri dish Þlled with larval diet and covered with mesh. Each density was exposed to a single 5-d-old randomly selected female. The 8 cages, each with 1 individual female parasitoid and 1 host density, were run simultaneously for 3 h. This exposure time was established after preliminary observations indicated that some females remained over the parasitization unit for ⬎2:30 h. Before the experiment, parasitoid wasps were held in 30-cm wooden frame cages at a density of ⬇250 parasitoids per cage. One day before the assays, they were provided with honey and host larvae in petri dishes for a period of 5 h. Each treatment was replicated 42 times. The following parameters were recorded: (1) number of ovipuncture scars (small, dark, melanized spots on the host puparia) per attacked larva; (2) number of 1st instars of D. longicaudata (as per Pemberton and Willard 1918) per host larva, found by dissection 48 h after exposure; (3) percentage parasitism, calculated as the number of adult parasitoids that emerged divided by the sum of fruit ßy adults and adult parasitoids in the sample; and (4) percentage self-superparasitism (percentage of females that oviposited more than once in the same host), determined as the number of host pupae with ⱖ2 scars or parasitoid larvae.

Vol. 93, no. 1

After exposure, all the host larvae were placed in plastic containers with vermiculite for pupation. Forty-eight hours later, a 10% subsample was taken from each treatment to determine the number of scars and parasitoid larvae per host pupa. To determine percentage succesful parasitism, pupae were maintained in vermiculite until parasitoid or ßy eclosion. To determine the inßuence of conspeciÞc parasitoids in the same cage on the functional response, a similar experiment was performed, using the same ratios of host larvae per female parasitoid, but multiplied by a factor of 5. Therefore, the host densities were 5, 25, 50, 100, 150, 200, 250, and 300 larvae simultaneously exposed to a group of 5 female parasitoids. Each density treatment was run in a separate cage. This experiment was replicated 24 times. Superparasitism Experiment. The parasitoid/host ratios tested in this experiment were 4:1, 2:1, 1:2, 1:4, 1:8 and as a control to estimate natural host mortality, 0:1. The actual number of parasitoids per host larvae per cage was 500:125, 500:250, 500:1,000, 250:1,000, 125:1,000, and 0:500. These ratios were established on the basis of the ratio used for mass rearing at the Moscafrut facility, which is 2 host larvae per female parasitoid (Cancino 1996). The total densities per cage were based on the work by Wong and Ramadan (1992). Female parasitoids were 5 d old and had been exposed for 3 h to 3rd-instar host larvae 24 h before the experiment. After exposure to parasitoids, 100 host larvae per treatment were placed in plastic containers with vermiculite to record adult eclosion. Percentage mortality was estimated as the fraction of host ßy pupae that did not emerge, from the sum of emerged fruit ßies plus parasitoids, plus pupae that did not emerge. Mortality in the control was used to correct mortality in each treatmentusing the Abbott formula (Abbott 1925). To determine the effect of superparasitism on preadult D. longicaudata development, a sample of 10 pupae per treatment was dissected 48 h after exposure, and the number of parasitoid 1st instars per host was recorded. This process was repeated 96 h after exposure when the parasitoid 2nd instars appeared. Before dissection, the number of scars per pupa was recorded. Data Analysis. A completely randomized block design was used in both experiments. In the case of functional response for females in groups, the Holling (1959) disk equation was used. Na ⫽ a⬘TNo/共1 ⫹ a⬘ThNo兲, where Na is the number of larvae attacked, No is the number of larvae exposed, a’ is the instantaneous host discovery rate, T is the total time of parasitoidÐ host exposition, and Th is handling time. In the case of single females, because density-dependence was observed on the proportion of larvae attacked at low densities, the data were Þtted to the equation proposed by Hassell et al. (1977): Na ⫽ bNo 2T/共1 ⫹ cNo ⫹ bThNo 2兲, where b and c are constants. The parameters in this equation were Þtted by an iterative nonlinear least

January 2000

MONTOYA ET AL.: FUNCTIONAL RESPONSES BY SUPERPARASITISM IN D. longicaudata

49

Table 1. Number (mean ⴞ SE) of attacked larvae, scars per host pupa, and D. longicaudata 1st-instar per host pupa caused by a single D. longicaudata and grouped females at different adult parasitoid/host larvae ratios Treatments Parasite:host

No. larvae attacked by 乆

% larvae attacked

1 1:1 2 1:5 3 1:10 4 1:20 5 1:30 6 1:40 7 1:50 8 1:60

0.24 ⫾ 0.07 1.33 ⫾ 0.27 3.36 ⫾ 0.48 5.14 ⫾ 0.89 5.64 ⫾ 1.08 6.92 ⫾ 1.29 9.90 ⫾ 1.34 9.22 ⫾ 1.18

23.8 26.7 33.6 25.7 19.6 16.8 20.6 15.5

1 5:5 2 5:25 3 5:50 4 5:100 5 5:150 6 5:200 7 5:250 8 5:300

0.71 ⫾ 0.05 3.01 ⫾ 0.22 5.47 ⫾ 0.39 7.21 ⫾ 0.80 7.93 ⫾ 1.01 9.90 ⫾ 1.19 9.85 ⫾ 0.73 10.34 ⫾ 0.92

70.7 61.1 54.7 36.0 26.2 25.1 19.7 17.2

a

No. scars per pupae

% pupae with ⬎1 scar

No. parasitoid larvae per attacked pupa

Single females 7.13 ⫾ 1.66 4.73 ⫾ 0.86 2.82 ⫾ 0.34 2.14 ⫾ 0.19 1.82 ⫾ 0.12 1.80 ⫾ 0.11 1.75 ⫾ 0.09 1.76 ⫾ 0.11

100.0 65.2 59.3 57.9 41.3 46.7 42.7 38.8

5.89 ⫾ 2.60 4.01 ⫾ 0.02 2.47 ⫾ 0.30 1.61 ⫾ 0.40 1.43 ⫾ 0.09 1.45 ⫾ 0.19 1.13 ⫾ 0.07 1.36 ⫾ 0.15

Females in groupa 10.78 ⫾ 0.41 3.69 ⫾ 0.59 2.48 ⫾ 0.22 2.42 ⫾ 0.19 1.61 ⫾ 0.11 1.52 ⫾ 0.09 1.44 ⫾ 0.14 1.51 ⫾ 0.07

85.0 65.1 59.7 56.9 38.0 39.1 30.6 35.1

7.73 ⫾ 1.24 2.55 ⫾ 0.42 1.86 ⫾ 0.20 1.91 ⫾ 0.16 1.55 ⫾ 0.75 1.46 ⫾ 0.07 1.31 ⫾ 0.12 1.24 ⫾ 0.09

Data per one female.

squares regression using the Gauss-Newton method (NLIN; SAS Institute 1985) according to suggestions by Hassell et al. (1977) and Juliano (1983). To determine if data from single females Þtted a type II or III curve, we used a logit analysis for maximum likehood (CATMOD; SAS Institute 1985, Trexler et al. 1988, Juliano 1993). Differences between the functional response curves of single females and females in groups were analyzed by slope comparisons (Sen and Srivastava 1990). In the superparasitism experiment, separate analysis of variance (ANOVA) and Tukey multiple mean comparisons (␣ ⫽ 0.05) were used to analyze the differences in the number of scars, number of parasitoid larvae, and percentage adult eclosion among treatments. An arcsine square root transformation was applied to the proportion data to normalize their distribution. The relationships between the number of scars

and the number of parasitoid larvae per host pupa, and between the number of scars per pupa and percentage of adult eclosion, were analyzed by simple linear regression. Differences between total mortality and percentage parasitism were analyzed by a Student t-test. All analyses were done using the Statgraphics (1993) program, except where otherwise indicated. Results Functional Response. In the single-female experiment, the greatest number of host larvae attacked (9.90 ⫾ 1.34 [mean ⫾ SE]) was observed at the density of 50 larvae (Table 1). After this density, no further increments were observed (Fig. 1). The greatest percentage of host larvae attacked (33.6%) was observed at the ratio of 10 larvae per female, whereas the average number of scars per pupae was always ⬎1. The

Fig. 1. Functional response of D. longicaudata for single females and females in groups. (a) Single females. (b) Females in groups. F, number of larvae attacked; f, proportion of larvae attacked; ⫹, predicted values from model.

50

ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA

Vol. 93, no. 1

Table 2. SAS output data analysis of D. longicaudata attacking different proportions of A. ludens larvae

␹2

Source Intercept N0 N02 N03 N04 Likelihood ratio Effect

DF

Prob.

Max Likelihood ANOVAa 31.57 1 13.65 1 21.38 1 24.23 1 25.39 1 3008.95 347

0.0000 0.0002 0.0000 0.0000 0.0000 0.0000

Parameter

Estimate

SE

␹2

Prob.

31.57 13.65 21.38 24.23 25.39

0.0000 0.0002 0.0000 0.0000 0.0000

a

Intercept N0 N02 N03 N04 a

Analysis of max likelihood estimates 1 ⫺1.5585 0.2774 2 0.1894 0.0513 3 ⫺0.0135 0.00291 4 0.0003 0.000063 5 ⫺2.35E-6 4.661E-7

Procedure CATMOD.

greatest mean number of scars per pupa was 7.13 (⫾1.66) at the lowest host density. The percentage of pupa with ⬎1 scar and the mean number of parasitoid 1st instars per host pupa dissected showed a similar trend, where the highest Þgures were found in the treatments with low host densities (Table 1). The functional response curve for single females Þts a type III response (Fig. 1a), where a density-dependence zone on the proportion of host larvae attacked can be observed. Table 2 shows that the quadratic parameter (NO2) was negative, which together with the high Þt of observedÐ expected proportions of host larvae attacked (Fig. 1a), indicated a type III functional response (Juliano 1993). For females in groups, data showed a signiÞcant Þt (␹2 ⫽ 0.094, df ⫽ 7, P ⬍ 0.05) to the Holling type II functional response (Fig. 1b). The searching rate (a’) for single females ranged between 0.081 and 0.129, whereas for females in groups, a’ was 0.259. Handling time (Th) was 0.189 for single females and 0.221 for grouped females. The analysis of slopes showed that the curves for both bioassays were signiÞcantly different (F ⫽ 46.75; df ⫽ 2, 12; P ⬍ 0.05). Superparasitism. In treatments with low host densities, the number of scars per pupae was greater and the percentage of parasitoid eclosion was signiÞcantly

Fig. 2. Relationship between number of scars per pupa and percentage of D. longicaudata emergence from A. ludens larvae.

lower than in high-density treatments (F ⫽ 186.36; df ⫽ 5, 66; P ⬍ 0.001) (Table 3). There was a signiÞcant negative correlation between the number of scars per pupae and percentage adult parasitoid eclosion (r ⫽ ⫺0.7092, r2 ⫽ 0.503, P ⬍ 0.001) (Fig. 2). A highly signiÞcant positive correlation (r ⫽ 0.9492, r2 ⫽ 0.901, P ⬍ 0.001) was found between the number of scars per pupa and the number of parasitoid 1st instars per dissected pupa (Fig. 3). At 96 h after exposure, 85.7% of the parasitoid 1st instars were complete and without signs of physical encounters with their conspeciÞcs. There was a signiÞcant difference (t ⫽ 2.71, df ⫽ 5, P ⬍ 0.05) between the percentage of parasitoid eclosion and total corrected mortality. Figure 4 shows that when the parasitoid/host ratio increases ⬎1:4, percentage parasitism (or adult parasitoid eclosion) decreases, but host mortality increases up to 90% (at 1:2 parasitoid/host ratio) and was maintained at this level. Discussion Functional Response. It is recognized that functional response derived from laboratory studies may bear little resemblance to those that could be measured in the Þeld (Munyaneza and Obrycki 1997).

Table 3. Number (mean ⴞ SE) of scars per host pupa, number of D. longicauda 1st-instar per host pupa, and percentage of emergence of D. longicaudata adults at different adult parasitoids/host larvae ratios Treatments Parasite:host

No. scars

No. larvae

% emergence of adults

1 4:1 2 2:1 3 1:2 4 1:4 5 1:8 6 0:1

28.95 ⫾ 3.22a 19.59 ⫾ 2.28b 8.08 ⫾ 1.25bc 6.51 ⫾ 0.74c 4.03 ⫾ 0.67cd 0.00d

11.05 ⫾ 2.14a 7.99 ⫾ 2.39ab 5.83 ⫾ 0.83abc 3.25 ⫾ 0.20bc 1.88 ⫾ 0.33c 0.00c

33.83 ⫾ 3.41b 44.28 ⫾ 4.15b 62.37 ⫾ 4.62ab 67.27 ⫾ 1.64a 66.58 ⫾ 3.01a 00.00c

Values with different letters in each column are signiÞcantly different (Tukey ␣ ⫽ 0.05).

Fig. 3. Relationship between number of scars per pupa and number of 1st-instar D. longicaudata in A. ludens pupae.

January 2000

MONTOYA ET AL.: FUNCTIONAL RESPONSES BY SUPERPARASITISM IN D. longicaudata

51

Fig. 4. Percentage D. longicaudata emergence and total A. ludens larval mortality at different parasitoid/host ratios.

Houck and Strauss (1985) pointed out, however, that laboratory functional response studies can be used to infer basic mechanisms underlying natural enemyÐ preyÐ host interactions. Such studies provide valuable information for biological control programs. For example, comparisons of the attributes of different parasitoid species can be made, and baseline information can be established for quality control standards in mass-rearing projects. According to Fujii et al.(1986), the type III functional response showed by D. longicaudata is more characteristic of vertebrate predators that can learn to concentrate on a prey as it becomes abundant. However, Hassell et al. (1977) argued that sigmoid type III responses may be much more common than previously supposed, even for invertebrate predators. Learning inßuences the searching behavior of many predators, and it has been shown that parasitoids have the capacity to learn cues used in the process of host location (Turlings et al. 1993). Several authors have tried to explain why a type III response is less common than a type II (van Lenteren and Bakker 1978, Hofsvang and Hagvar 1983). They argue that in laboratory tests, parasitoids are forced to remain in the patch, whereas under natural Þeld conditions they would probably leave the patch because of the very low host density or because most hosts are already parasitized. Thus, under natural Þeld conditions, type III responses may be common. Collins et al. (1981) showed a sigmoid-type response in Aphelinus thomsoni Graham when parasitoids were allowed to emigrate from the experimental arena; in contrast, when they were enclosed with their host for a Þxed period of time, a type II response was observed. The functional response of D. longicaudata can be described by a ratio-dependent model, because over

a lower range of host densities (1Ð10 larvae per female) the proportion of hosts attacked increased. Several different mechanisms have been proposed to generate ratio dependence, mainly nonrandom foraging activity and the presence of refuges for the host. Both situations are typical for parasitoids. Kairomones play an important role in the foraging activity of parasitoids (Vet and Dicke 1992, Knipling 1992); according to Murdoch (1994) and Hochberg and Hawkins (1994), at low host density and in a heterogeneous habitat, hunting efÞciency is reduced and the importance of refuges increases. In this study, although the habitat was not heterogeneous, it is possible that at lower densities, the probability of discovery was reduced and, as such, this may represent a form of refuge from the searching parasitoid. Differences in handling times (Th) and searching rates (a’) found between the 2 assays suggest that, at low host densities in the presence of conspeciÞcs, females increase their search activity. Female parasitoids spent more time searching in the presence of other females, although direct interference was not apparent among them. The signiÞcant differences between slopes of the functional response curves supports the idea that at low host densities and in the presence of conspeciÞcs, D. longicaudata increases its parasitization activity. There also is some controversy regarding the determination of key parameters (i.e., searching rate a’ and handling time Th) in functional response models (Fan and Petitt 1994, 1997, Williams and Juliano 1996). In the current study, we felt justiÞed in using the equations proposed by Holling (1959) and Hassell et al. (1977), even though these equations do not include hostÐprey depletion (Rogers 1972). This is because we observed that D. longicaudata self-superparasitizes a

52

ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA

large fraction of the available hosts, which reduces the effect of host depletion. The high prevalence of pupae attacked with ⬎1 scar (Table 1), seems to be contrary to the criteria of host discrimination proposed by Salt (1961) and discussed by Godfray (1994), in which it is expected that an adequate parasitoid discriminatory capability will result in most hosts receiving 1 egg and very few receiving ⬎1 egg. However, several authors (see van Alphen and Jervis 1996) have shown that such criteria are not always supported. The number of scars per attacked pupa was highly correlated with the mean number of parasitoid 1st instars per host. This is in agreement with observations by Lawrence et al. (1978) on D. longicaudata parasitizing A. suspensa larvae. These authors pointed out that the melanized scars on the larval host provide an estimate of the number of ovipositions made by the female parasitoid. Our results indicate that the same female probed and oviposited more than once in a great number of the host larvae found. This suggests a consistent trend toward self-superparasitism. van Alphen and Visser (1990) consider that in solitary endoparasitoids, self-superparasitism might be adaptive if the presence of ⱖ2 eggs per host increases the overall survival probability of their offspring. This might be the case if the same host is attacked later by another parasitoid or as a response to the immunological defenses of the host. In our case, this selfsuperparasitism was found under situations where the females have many unparasitized host larvae available in the absence of immediate local competition. This suggests that this behavior might be adaptive, increasing the chances of offspring survival, if encapsulation of parasitoid 1st instars occurs as reported in A. suspensa (Lawrence 1988). Superparasitism. The negative correlation found between the number of scars per pupa and the percentage of emergence of D. longicaudata could be attributed to negative effects of superparasitism, because the treatment with the highest mean number of scars showed a 33.4% decrease in parasitoid emergence. However, despite the fact that in other treatments there were signiÞcant differences in the number of scars per pupa and in the number of D. longicaudata larvae per host pupa, the percentages of parasitoid emergence were very similar. These results suggest that D. longicaudata can efÞciently eliminate supernumerary conspeciÞc larvae without obvious costs in terms of the probability of parasitoid development. This contrasts with certain detrimental effects attributed to superparasitism in earlier studies (see van Alphen and Visser 1990). When dissecting A. ludens pupae with large numbers of parasitoid 1st instars, we commonly observed that these larvae were aggregated side by side or one in front of the other, although we never noticed any physical combat among them. When 96-h dissections were made to detect 2nd instars, 85.7% of the 1st instar were dead but did not show obvious signs of having been attacked by conspeciÞcs. This suggests that the elimination of supernumerary larvae may have been

Vol. 93, no. 1

mainly by physiological suppression as proposed by Mackauer (1990). During the course of this study, incomplete parasitoid 1st instars with melanized wounds were observed, but at an extremely low frequency (2.38%). Therefore, physical combat may be a secondary mechanism to eliminate supernumerary individuals. Lawrence (1988) reported that physical combat among D. longicaudata larvae might result, in some cases, in the death of both individuals from the wounds inßicted. However, she also concluded that the main cause of mortality of D. longicaudata larvae exposed to superparasitism must be through physiological suppression mechanisms. The mortality produced by the parasitoids was greater than that which might be estimated from parasitoid emergence. The difference between mortality and parasitoid emergence may be interpreted as the effect of superparsitism. This is in agreement with Van Driesche (1983) and Van Driesche et al. (1991), who noted that in such a situation, percentage parasitism would represent an underestimation of parasitoid impact over their host populations. This has some important practical implications because in most Þeld studies, percentage parasitism is commonly used to measure the effectiveness of parasitoid releases, and additional mortality produced by action of the parasitoids is often not considered. We believe that our results have 3 important implications. First, D. longicaudata is a parasitoid with a density-dependent functional response with a strong tendency to superparasitism, raising questions about the adaptiveness of this phenomenon and how widespread it is among other species of parasitoids. Second, mean host larvae mortality was never ⬎90% even at very high parasitoid/host ratio (4:1). According to Knipling (1992), parasitization rates will increase as the ratio of parasitoid/host encounters to host larvae increases. Our data do not support this assumption, and suggest the host mortality from parasitism will hardly be ⬎99%. This provides clues on how much we can expect from parasitoid releases in the Þeld. Third, our results also suggest ways of optimizing the massrearing process, because an increase in the parasitoid/ host ratio of ⬎1:4 resulted in higher host mortality but did not increase overall parasitoid production. Mass rearing will be more efÞcient if lower parasitoid/host ratios are maintained.

Acknowledgments We are grateful to Trevor Williams (El Colegio de la Frontera Sur) for critical review of an early version of the manuscript. We thank Mauricio Zenil, Ramon Hernandez, and Carlos Estrada (Programa Moscamed) for technical assistance. We also thank Salvador Flores (Programa Moscamed) and Javier Valle-Mora (El Colegio de la Frontera Sur) for statistical assistance. We gratefully acknowledge support from Jesus Reyes. This work was Þnanced by the Campan˜ a Nacional contra Moscas de la Fruta, the Programa Moscamed (DGSV, SAGAR), and Project No. SIBEJCONACYT A-004.

January 2000

MONTOYA ET AL.: FUNCTIONAL RESPONSES BY SUPERPARASITISM IN D. longicaudata References Cited

Abbott, W. S. 1925. A method of computing the effectiveness of an insecticide. J. Econ. Entomol. 18: 265Ð267. Brown, M. W. 1989. Density dependence in insect hostparasitoid systems: a comment. Ecology 70: 776Ð779. Cancino, J. 1996. Procedimientos y fundamentos de la crõ´a masiva de Diachasmimorpha longicaudata, parasitoide de moscas de la fruta, pp. 409 Ð 418. In Memorias del “X Curso Internacional sobre Moscas de la Fruta.” Programa Moscamed. DGSV-SAGAR, Metapa de Domõ´nguez, Chiapas, Me´ xico. Cancino, J., S. de la Torre, S. Ruı´z, F. de M. Moreno, E. Herna´ ndez, and M. Zenil. 1996. Establecimiento de la produccio´ n masiva de Diachasmimorpha longicaudata en Metapa de Domõ´nguez, Chiapas, Me´ xico, pp. 82Ð 83. In Proceedings, 2nd Meeting of the Working Group on Fruit Flies of the Western Hemisphere, 3Ð 8 November 1996. Vin˜ a del Mar, Chile. Collins, M. D., S. D. Ward, and F. G. Dixon. 1981. Handling time and the functional response of Aphelinus thomsoni, a predator and parasite of the aphid Drepanosiphum platanoidis. J. Anim. Ecol. 50: 479 Ð 487. Clausen, C. P., D. W. Clancy, and Q. C. Chock. 1965. Biological control of the oriental fruit ßy Dacus dorsalis Hendel. U.S. Dep. Agric. Tech. Bull. 1332. Domı´nguez, G. J. 1996. Me´ todos de crõ´a masiva de Anastrepha spp., pp. 329 Ð336. In Memorias del “X Curso Internacional sobre Moscas de la Fruta.” Programa Moscamed. DGSV-SAGAR, Metapa de Domõ´nguez, Chiapas, Mexico. Fan, Y., and F. L. Petitt. 1994. Parameter estimation of the functional response. Environ. Entomol. 23: 785Ð794. Fan, Y., and F. L. Petitt. 1997. Functional response, variance and regression analysis: a reply to Williams and Juliano. Environ. Entomol. 26: 1Ð3. Fujii, K., C. S. Holling, and P. M. Mace. 1986. A simple generalized model attack by predators and parasites. Ecol. Res. 1: 141Ð156. Godfray, H.C.J. 1994. Parasitoids. Behavioral and evolutionary ecology. Princeton University Press, Princeton, NJ. Hassell, P. M. 1978. The dynamics of arthropod predatorprey systems. Princeton University Press, Princeton, NJ. Hassell, P. M., J. H. Lawton, and J. R. Beddington. 1977. Sigmoid functional responses by invertebrate predators and parasitoids. J. Anim. Ecol. 46: 249 Ð262. Hochberg, M. E., and B. A. Hawkins. 1994. The implications of population dynamics theory to parasitoid diversity and biological control, pp. 451Ð 471. In B. A. Hawkins and W. Sheehan [eds.], Parasitoid community ecology. Oxford University Press, Oxford. Hofsvang, T., and E. B. Hagvar. 1983. Functional responses to prey density of Ephedrus cerasicola (Hym.: Aphidiidae), an aphidiid parasitoid of Myzus persicae (Hom.: Aphididae). Entomophaga 28: 317Ð324. Holling, C. S. 1959. Some characteristics of simple types of predation and parasitism. Can. Entomol. 91: 385Ð398. Houck, M. A., and R. E. Strauss. 1985. The comparative study of functional responses: experimental design and statistical interpretation. Can. Entomol. 115: 617Ð 629. Juliano, S. A. 1993. Non-linear curve Þtting: predation and functional responses curves, pp. 159 Ð182. In S. M. Schiner and J. Gurevitech [eds.], Design and analysis of ecological experiments. Chapman & Hall, New York. Knipling, F. E. 1992. Principles of insect parasitism analyzed from new perspectives. U.S. Dep. Agric. Handb. 693.

53

Lawrence, P. O. 1988. IntraspeciÞc competition among Þrst instars of the parasitic wasp Biosteres longicaudatus. Oecologia (Berl.) 74: 607Ð 611. Lawrence, P. O., P. D. Greany, and R. M. Baranowsky. 1978. Oviposition behavior of Biosteres longicaudatus, a parasite of the Caribbean fruit ßy Anastrepha suspensa. Ann. Entomol. Soc. Am. 71: 253Ð256. Mackauer, M. 1990. Host discrimination and larval competition in solitary endoparasitoids, pp. 41Ð 62. In M. Mackauer, L. E. Ehler, and J. Roland [eds.], Critical issues in biological control. Intercept, Andover. Murdoch, W. W. 1994. Population regulation in theory and practice. Ecology 75: 271Ð287. Munyaneza, J., and J. J. Obrycki. 1997. Functional response of Coleomeguilla maculata (Coleoptera: Coccinellidae) to Colorado potato beetle eggs (Coleoptera: Chrysomelidae). Biol. Control 8: 215Ð224. Nicholson, A. J. 1958. Dynamics of insect populations. Annu. Rev. Entomol. 3: 107Ð136. Pemberton, C. E., and H. F. Willard. 1918. A contribution to the biology of fruit ßy parasites in Hawaii. J. Agric. Res. 25: 419 Ð 437. Rogers, D. 1972. Random search and insect population models. J. Anim. Ecol. 41: 369 Ð383. Salt, G. 1961. Competition among insect parasitoids. Symp. Soc. Exp. Biol. 15: 96 Ð119. SAS Institute. 1985. SAS userÕs guide: statistics, version 5. SAS Institute, Cary, NC. Sen, A., and M. Srivastava. 1990. Regression analysis. Theory, methods and applications. Springer, New York. Sivinski, J. M. 1996. The past and potential of biological control of fruit ßies, pp. 365Ð375. In B. A. McPheron and G. J. Steck [eds.], Fruit ßy pests. A world assessment of their biology and management. St. Lucie, DelRay Beach, FL. Solomon, M. E. 1949. The natural control of animal population. J. Anim. Ecol. 18: 1Ð35. Statgraphics. 1993. Statgraphics/statistical reference manual, version 7:1. Statistical Graphics, Cambridge, MA. Stiling, P. D. 1987. The frequency of density dependence in insect host parasitoid systems. Ecology 68: 844 Ð 856. Stiling, P. D. 1989. Density dependenceÑa reply to Brown. Ecology 70: 779 Ð783. Trexler, J. C., C. E. McCulloch, and J. Travis. 1988. How can the functional response best be determined? Oecologia (Berl.) 76: 206 Ð214. Turlings, T.C.J., F. L. Wa¨ kers, L.E.M. Vet, W. J. Lewis, and J. H. Tumlinson. 1993. Learning of host-Þnding cues by hymenopterous parasitoids, pp. 51Ð78. In D. R. Papaj and A. C. Lewis [eds.], Insect learning. Ecology and evolutionary perspectives. Chapman & Hall, New York. van Alphen, J.J.M., and M. A. Jervis. 1996. Foraging behavior, pp. 32Ð36. In M. A. Jervis and N. Kidd [eds.], Insect natural enemies. Practical approach to their study and evaluation. Chapman & Hall, London. van Alphen, J.J.M., and M. E. Visser. 1990. Superparasitism as an adaptive strategy for insect parasitoids. Annu. Rev. Entomol. 35: 59 Ð79. Van Driesche, R. G. 1983. Meaning of “percent parasitism” in studies of insect parasitoids. Environ. Entomol. 12: 1611Ð1622. Van Driesche, R. G., T. S. Bellows, Jr., J. S. Elkinton, and N. D. Ferro. 1991. The meaning of percentage parasitism revisited: solutions to the problem of accurately estimating total losses from parasitism. Environ. Entomol. 20: 1Ð7. van Lenteren, J. C., and K. Bakker. 1978. Behavioural aspects of the functional response of a parasite (Pseudocolia

54

ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA

bochei Weld) to its host (Drosophila melanogaster). Neth. J. Zool. 28: 213Ð233. van Lenteren, J. C., K. Bakker, and J.J.M. van Alphen. 1978. How to analyse host discrimination. Ecol. Entomol. 3: 71Ð75. Vet, L.E.M., and M. Dicke. 1992. Ecology of infochemical use by natural enemies in a tritrophic context. Annu. Rev. Entomol. 37: 141Ð172. Williams, F. M., and S. A. Juliano. 1996. Functional response revisited. Environ. Entomol. 25: 549 Ð550.

Vol. 93, no. 1

Wong, T.T.Y., and M. M. Ramadan. 1992. Mass rearing biology of larval parasitoids (Hymenoptera: Braconidae: Opiinae) of tephritid ßies (Diptera: Tephritidae) in Hawaii, pp. 405Ð 426. In T. E. Anderson and N. C. Leppla [eds.], Advances in insect rearing for research and pest management. Westview, Boulder, CO. Received for publication 22 December 1997; accepted 3 November 1998.