The Effects of Egg Production on Longevity in the Parasitoid Mastrus ridibundus (Hymenoptera: Ichneumonidae) Jay A. Francisco Abstract Parasitoid behaviors are predicted by dynamic optimization models based on variables

Parasitoid behaviors are predicted by dynamic optimization models based on variables that describe the insects physiology and surrounding environment. Many of these models assume an interaction between the cost of reproduction and longevity. However, the details of this of the trade off function are still undescribed. This paper investigates the effects of egg production on longevity in a synovigenic parasitoid Mastrus ridibundus. Reproduction efforts were manipulated and their corresponding longevities are analyzed with a series of linear regression tests. The results show a negative correlation between both egg production and the number of hosts attacked. However a conclusive trade-off function could not be described. Introduction Parasitoids are a group of insects whose larvae develop by feeding on arthropod hosts. Adult females often paralyze hosts with an injection of venom that is delivered via the ovipositor. Eggs are then deposited in or on the host, and the larvae feed on the host’s hemolymph. They are a unique group, with a developmental strategy that distinguishes them from both predators and parasites. Unlike predators, the consumption of a single host facilitates development into an adult; they differ from parasites in that larval feeding usually kills the host (Gauld and Bolton 1988). Their diversity and ability to adapt is evident in the range of host stages exploited (Mills 1994). As a group, parasitoids comprise an invaluable component of terrestrial ecosystems and are represented in the orders Hymenoptera, Coleoptera, Diptera, Lepidoptera, Neuroptera, and Trichoptera (Eggleton and Belshaw 1992; Wells 1992). Studies on parasitoids, have led to tremendous advancements in a number of academic fields. Their success in agricultural systems, as biological control agents in integrated pest management programs, has not only reduced the application of environmentally degrading insecticides but has proved that sustainable pest management can be achieved (Huffaker and Messenger 1976). Within the field of population biology, the host-parasitoid relationship contributed to the development of models that describe species interactions and ecosystem stability (Walde and Murdoch 1988). These models are designed to predict optimal behavioral responses in different environments. However, details of the “currency” that is optimized, and thus the behavior carried out, remains an issue under contention. Jaenike (1978), Charnov and Skinner (1984), and Ives (1989) proposed models within the framework of the optimal foraging theory; behavior and reproduction are a function of the rate of energy gained by ovipositing on the host of marginal quality or by searching for an alternative host. In these models, time and environmental conditions are the primary factors governing behavioral responses. For example, the time spent locating and handling a host of poor quality is time that could be spent finding a more suitable host. Therefore, the poor host should be abandoned and a new one should be sought out because the rate of fitness gained by finding the better host outweighs the fitness incurred from the poor host. Other studies contend that optimal behavior is based on a lifetime reproductive success (Iwasa 1984; Houston et al. 1988; Mangel 1988; Heimpel et al. 1996; Rosenheim 1996). These dynamic state variable models emphasize the physiological state of the searching parasitoid. Thus, optimal behavior is a function of not only time and environmental conditions, but also a function of the parasitoids age, egg load, and nutritional status. For example, an old female with a large egg load should deposit eggs on hosts of marginal quality because that would return a fitness gain, whereas dying having laid no eggs returns no fitness. Although many of these models include functions illustrating a trade-off between reproduction and longevity, there are only a few empirical studies directly supporting this assumption (Ellers et al. 2000). The basis for a trade-off between the cost of reproduction and longevity lies within the nutrient limitation theory. In general, this theory assumes that all activities draw on a common resource pool. Any activities performed during the life of the insect compete for the same resources. Therefore, allocation of resources to any activity would reduce the amount of energy available for maintaining life (van Noordwijk and de Jong 1986; Boggs 1997). Several experiments conducted on other organisms support the nutrient limitation theory and phenotypic plasticity. For example, Tatar et al. (1993) and Tatar and Carey (1995) found that early reproductive efforts caused an increase in age-specific mortality in the beetle Callosobruchus maculatus. In waterstriders, Kaitala (1991) reported decreased longevity and increased egg production given high quality environments. In Drosophila species, multiple mating decreased female longevity (Partridge et al. 1987; Fowler and Partridge 1989; Chapman et al. 1995; Chapman et al. 1998). Scott and Barlow (1984) reported that greater flight activity decreased longevity in the Syrphid Metasyrphus corollae. Applying the theory to parasitoids requires the use of species that exhibit the synovigenic method of egg production. These types of species are able to mature eggs throughout their adult stage (this contrasts the proovigenic strategy, where adult females have a fixed number of oocytes within their ovarioles). Therefore, increased egg production draws on the energy reserves that could be allocated to extending longevity. Two theories predicting the trade-off function exist within the literature. Rosenheim (1996) proposed a reciprocal relationship, defined by the function L = 1/(aR), where (L) is longevity, (a) is a constant, and (R) is the reproductive investment. This theory states that the first eggs laid cause a greater reduction in longevity relative to eggs laid latter in life. The logic behind it involves a reduction in egg size as the parasitoid ages. Alternatively, Sevenster et al (1998) proposed a linear trade-off described by the equation L = T – aE, where (L) is longevity, (T) is the total amount of available resources, (a) is a constant, and (E) is egg production. Here, each egg laid reduces the total amount of available resources by the same increment. Ellers et al. (2000) is the only known study that directly assesses this trade-off function in parasitoids. Their results suggest a linear trade-off function. However, their analysis was based on a small data set, and the assumption of a trade-off between egg production and intrinsic mortality rate could not be confirmed. This experiment will assess the trade-off function between egg production and longevity in the parasitoid M. ridibundus (Hymenoptera: Ichneumonidae). Experimental Subjects M. ridibundus is a synovigenic parasitoid that attacks the codling moth, Cydia pomonella (Lepidoptera: Tortricidae), during the prepupal stage of development. M. ridibundus is a gregarious ectoparasitoid (several eggs are deposited and the larvae feed from the surface of the host). The females are synovigenic. At 26 C, M. ridibundus develops, from egg to adult emergence, in approximately 18 days. We regulated their population with a weekly provision of cardboard strips containing codling moths suspended in the prepupal stage. The culture was supplied with an allotment of honey and water three times per week. Cydia pomonella (Lepidoptera: Tortricidae) is a major pest of apples, walnuts, peaches, and other stone fruits. The Okanagan-Kootenay sterile insect release program in Canada supplied us with egg sheets. The larvae fed on thinned, fuji apples at a temp of 20 C. Specially designed cardboard strips served as pupation sites for the developing larvae. We induced diapause with a photoperiod of eight hours, and the strips were removed and placed into a refrigerator. This maximized the availability of the host stage needed for laboratory experiments. Methods To impose the treatments of differential reproductive efforts, I created five groups delimited by a spread of total eggs laid (control = 0 eggs laid; group 1 = 1-10 eggs; group 2 = 11-20 eggs; group 3 = 21-30 eggs; group 4 = eggs laid for entire life). I conducted three trials (see Table 1). Each treatment group in the three trials contained a proportion of the total number of females for that group. All of the females used in the control group came from trial 1. Small vials (diameter = 4cm, height = 7cm) equipped with either honey cards alone (control), or honey cards and four hosts (all treatments), housed the parasitoids. I ensured the highest potential longevity within my sample population by selecting newly emerged females. To limit the frequency of mating, I placed each female and two males into a vial for 24 hours. All specimens were kept in an incubation chamber set at 22 C. I randomly assigned parasitoids to groups after their first successful oviposition. Thus, all individuals that oviposited on the first day were distributed to a group; these individuals were excluded from further assignment. I followed this protocol daily, until every parasitoid was included in a group. This method of distribution across all groups controlled for Tatar et al.’s (1993) finding, which linked a high early reproductive effort to increased age-specific mortality. Females of each treatment were exposed to four hosts per day until the target number of eggs laid was reached. Daily examination of the hosts yielded the number of eggs laid by each parasitoid. Lares that contained eggs larve that were paralysed yet lacked the deposition of eggs were scored as a host attacked. Once a female reached the target egg range, I discontinued the supply of available hosts and recorded total longevity in days since eclosion. Mortality was recorded and the honey cards were replenished daily. Upon death, I measured the female’s hind tibia and dissected their abdomens to determine the total number of mature oocytes remaining in the ovarioles. Mature oocytes were scored based on their opaque, banana shaped appearance. Immature oocytes were transparent and flat (Alexander and Rozen 1987). For each female, I determined the total number of eggs laid, the total number of eggs produced (eggs laid plus mature oocytes contained in the ovarioles), the number of hosts attacked, the hind tibia length (HTL), and the longevity since eclosion. I analyzed the data using a series of simple linear regression tests. Each variable was tested within each of the three trials. I also performed a linear regression with the combined data set. Results The results on the series of simple linear regression tests for each trial are summarized in Table 1. All of the variables were tested against the dependent variable, longevity. Neither the total number of eggs produced nor HTL had a significant effect on longevity in any of the three trials. In trial 1, only the number of hosts attacked had a significant effect (p = 0.0311, R = 0.0932). None of the variables yielded a significant effect in trial 2. In trial 3, the number of hosts attacked and the number of eggs laid revealed significant effects on longevity (p = 0.0014, R = 0.587 and p = 0.0065, R = 0.473 respectively). Table 1 displays the results from the regression tests for the combined data set. The number of hosts attacked and eggs laid each had a significant effect on longevity (p < 0.001, R = 0.10 and p = 0.005, R = 0.087 respectively; Fig. 1 and Fig. 2). There is a negative correlation between eggs laid and longevity. However, there is a stronger negative relationship between longevity and the number of hosts attacked. Total eggs Eggs Number hosts treatment HTL produced Laid attacked trial 1 Rsqr = 0.0118 0.0224 0.067 0.0932 n = 50 P = 0.452 0.2994 0.0696 0.0311 trial 2 Rsqr = 0.0015 0.00918 0.0355 0.0396 n = 26 P = 0.8535 0.6487 0.3673 0.34 trial 3 Rsqr = 0.0747 0.251 0.473 0.587 n = 13 P = 0.3444 0.0681 0.0065 0.0014 complete set Rsqr = 0.00166 0.038 0.087 0.1 n = 89 P = 0.7045 0.067 0.005 0.0025 Table1 displays the results of the linear regression tests for each trial and the entire data set. Each variable was tested against the dependent variable longevity. Eggs laid and number of hosts attacked both had significant effects on longevity for trial 1, 2, and the entire data set. The increasing R values and the decreasing p-values following the variables HTL, total eggs produced, eggs laid, and number of hosts attacked. An interesting pattern exists across all three trials and the combined data set. There was a gradual increase in the R values and a decrease in the p-values following the variables from the total number of eggs produced, to eggs laid, and finally to the number of hosts attacked (Table 1). Thus, the number of hosts attacked had the largest impact in reducing longevity. Using this variable in a least squares regression model yielded a linear trade-off (Fig 2; p = 0.0025; equation Longevity = 25.1 – 0.283 * the number of hosts attacked). I used the same variable to test the reciprocal model (Fig 3; p > 1E20 ; the statistical software failed to report an equation). The R for the reciprocal fit had to be discarded because the sum of squares of the residuals was too large and resulted in a value greater than 1. Comparison of these tests suggests that the linear trade model better describes the trade off function.