Journal of Economic Entomology Advance Access published June 6, 2015 ECOLOGY
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BEHAVIOR
Egg Cannibalism and its Life History Consequences Vary with Life Stage, Sex, and Reproductive Status in Hippodamia convergens (Coleoptera: Coccinellidae) MOHAMED H. BAYOUMY1 AND J. P. MICHAUD2,3
J. Econ. Entomol. 1–10 (2015); DOI: 10.1093/jee/tov148
ABSTRACT Egg cannibalism is common in Coccinellidae, but its biological consequences have not been fully explored. We examined egg cannibalism by neonates, fourth instars, and adults of Hippodamia convergens Guerin-Meneville for effects on development, reproduction, and progeny fitness. We also tested female adults for ability to avoid cannibalizing their own eggs and first-instar larvae, and both sexes for changes in cannibalism propensity following mating, all in the presence of ad libitum food [larvae: eggs of Ephestia kuehniella Zeller (Lepidoptera: Pyralidae), adults: Schizaphis graminum (Rondani)]. Cannibalism by neonates reduced developmental time and increased male body size. Cannibalism in the fourth instar accelerated pupation and led to the production of eggs that hatched faster, regardless of which parent cannibalized. However, egg fertility was improved only by maternal cannibalism in the fourth instar. Females recognized their own egg clusters, sometimes added eggs to them, and preferentially cannibalized nonfilial clusters. Most gravid females cannibalized a first-instar larva within 30 min, whether filial or not. Adult egg cannibalism was similar for virgin males and females, but declined after mating in males, and increased in females, although it had no effect on fecundity or fertility. Daughters of cannibal pairs were heavier than those of other mating combinations, but offspring of noncannibal parents had the fastest development. Reproductive females appeared to use egg cannibalism to reduce risk for their own eggs, increasing the number cannibalized with the number laid. Thus, egg cannibalism in coccinellids varies with life stage, sex, and reproductive condition, independent of food availability, and benefits are life stage specific. KEY WORDS development, interference competition, kin recognition, reproduction
Cannibalism behavior is observed in a wide range of organisms, generating many theories about its costs and benefits (Fox 1975, Polis 1981, Elgar and Crespi 1992, Pfennig 1997). Two of the most obvious benefits of cannibalism are the acquisition of nutrients and the elimination of a conspecific competitor, direct and indirect benefits, respectively. Clearly, the magnitude of these benefits is context-dependent and will vary with the nutritional state of the individual and the intensity of intraspecific competition, respectively. Benefits can also vary according to the life stage in which cannibalism occurs. As the primary function of the larval stage is growth and development, we expect direct benefits for larvae in terms of development rate, immature survival, and adult body size, perhaps sometimes extending to effects on adult reproductive performance (e.g., Richardson et al. 2010). Apart from avoidance of starvation, the only direct benefits possible for adults are reproductive, i.e., improved fecundity, egg fertility, or offspring quality. The indirect benefits of
1 Mansoura University, Faculty of Agriculture, Economic Entomology Department, Mansoura 35516, Egypt. 2 Department of Entomology, Kansas State University, Agricultural Research Center-Hays, 1232 240th Ave, Hays, KS 67601. 3 Corresponding author, e-mail:
[email protected].
cannibalism may also diverge between larval and adult stages; larval cannibals may reduce their immediate competition for a resource (e.g., food, space, etc.), whereas adult cannibals can benefit by reducing future competition for their progeny. Nevertheless, very few studies have yet tried to disentangle the relative contributions of direct and indirect benefits of cannibalism to life history plasticity and expression of the behavior in particular life stages (Block and Stoks 2004). Potential costs of cannibalism include disease transmission (e.g., Saito and Bjornson 2006) and the loss of inclusive fitness because of consuming a close relative (Hamilton 1964), which often leads to the evolution of linkages between cannibalism and kin recognition mechanisms (Pfennig 1997, Schausberger 2003). The magnitude of this cost will vary with the probability of encountering relatives and their degree of relatedness (Blaustein and O’Hara 1986); the greater the probability of encounter and the higher the degree of relatedness, the stronger selection will be for kin recognition. Thus, adult females are often capable of recognizing their own eggs, but in cases of sex role reversal or male parental care, males may also exhibit kin recognition and cannibalism avoidance (e.g., Mehlis et al. 2010). Agarwala and Dixon (1993) showed that female Adalia bipunctata L. are able to recognize and avoid eating their own eggs. Larvae demonstrate kin recognition
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and reluctance to cannibalize related larvae in some coccinellid species (Joseph et al., 1999), but not in others (Michaud 2003). Thus, any examination of adult cannibalism behavior should also include a test for kin recognition ability. When nutritional benefits drive cannibalism, eggs are often the target (see Richardson et al. 2010 for relative rates of cannibalism on different life stages in noncarnivorous insects); they typically lack defenses, are rich in proteins and lipids (Sloggett and Lorenz 2008), and have high digestibility by virtue of a lack of differentiation. Even species that rarely cannibalize in nature have been reared successfully on an exclusive diet of conspecific eggs (e.g., Rock 1968). Among the eusocial Hymenoptera, the use of trophic eggs as a means of storing and transporting food is ubiquitous and well-recognized (Crespi 1992). In many insects, it is common for neonates to consume their own chorions following eclosion to scavenge the protein, and it is only a small step further to the consumption of unhatched eggs (e.g., Barros-Bellanda and Zucoloto 2001). For example, when predatory bugs (Anthocoridae) are deprived of food, they typically begin cannibalizing eggs in preference to all other life stages (Pires et al. 2011). Phytoseiid mites present an interesting exception in that adults prefer to cannibalize the larval stage because eggs can be quite difficult for them to pierce (Schausberger 2003). Although some eggs may possess chemical defenses against predation, these are typically ineffective against conspecifics (Michaud 2002, Cottrell 2004). In Coccinellidae, most aphidophagous species lay their eggs in clusters, sometimes including a fraction of infertile eggs, ostensibly to facilitate some egg cannibalism by neonates (Osawa 1992, Michaud and Grant 2004, Perry and Roitberg 2005). Sibling egg cannibalism will increase the time a first-instar larva can survive until it captures its first aphid (Brown 1972) and, depending on the quality of the subsequent food supply, can result in faster larval development to a larger adult size (Osawa 1992, 2002, Michaud and Grant 2004, Pervez et al. 2006, Omkar et al. 2007, Roy et al. 2007). Furthermore, because female coccinellids face a high risk of egg cannibalism by unrelated conspecifics (Mills 1982, Michaud and Belliure 2000, Schellhorn and Andow 1999), cannibalism-avoidance has shaped their oviposition behavior. Females avoid laying eggs on plants contaminated with larval tracks (Doumbia et al. 1998, Ruzicka 2003) and normally oviposit some distance from an aphid colony, thus balancing the risk of egg cannibalism or predation against the risk that neonates will hatch too far from their food source (Osawa 1989, Michaud and Jyoti 2007). Although egg cannibalism by neonate coccinellid larvae and its fitness consequences have been well-studied, its implications for late-instar larvae and adults have received less attention. The fourth-instar coccinellid is the most voracious larval stage, and the possible life history benefits of egg cannibalism by this stage have not yet been fully explored. Snyder et al. (2000) showed that larvae of Harmonia axyridis Pallas could improve their survival and development on poor-quality
prey by cannibalizing conspecific larvae, but did not test conspecific eggs. A large proportion of coccinellid eggs in the field is probably cannibalized by adults (Michaud and Belliure 2000, Cottrell 2005, Takizawa and Snyder 2011). The potential benefits of adult egg cannibalism for coccinellid reproductive success have also not yet been examined, despite the fact that adults can be more voracious egg cannibals than larvae (Bayoumy and Michaud, 2015). For example, long-term reproductive studies on Coleomegilla maculata DeGeer require the separation of pairs once oviposition begins to prevent egg cannibalism by males (Vargas et al. 2012). Furthermore, among adult beetles, there may be sexual differences in cannibalism propensity. In flour beetles, Tribolium spp., females cannibalize more eggs than males (Rich 1956, Sonleitner 1961), ostensibly because of the greater nutritional demands of egg production. Similarly, adult females of the herbivorous coccinellid Henosepilachna pustulosa (Kono) are more prone to cannibalize egg clusters than are males and use the resources for their own reproduction (Nakamura and Ohgushi 1981), as do viviparous females of Elaphrothrips tuberculatus (Hood) (Thysanoptera: Phlaeothripidae) (Crespi 1990). Alternatively, egg cannibalism by reproductive females could represent interference competition with other females that reduces their own risk of egg cannibalism and the competition for resources their progeny will face. However, reproductive adults will risk filial cannibalism, and its attendant loss of fitness, unless they can either recognize their own offspring, or rarely encounter them. Thus, female reproduction may be negatively associated with cannibalism in the absence of kin recognition (Lewis et al. 2010). For example, Wagner (1995) demonstrated that inhibition of spiderling cannibalism in adult female wolf spiders, Schizocosa ocreata (Hentz), was associated with maternal care of an egg sac, whether the female had produced it herself or not. The inhibition was also plastic and disappeared 2 wk after spiderlings dispersed, which would coincide with the period of risk for filial cannibalism. Similarly, adult aphidophagous coccinellids face a substantial risk of filial cannibalism; they oviposit in large bursts over a short period near aphid colonies that tend to be highly aggregated in habitats. Hippodamia convergens Guerin-Meneville is a wellstudied, nearctic coccinellid that is a valuable biological control agent of aphids in a wide variety of crops, particularly in cereals on the High Plains (Michaud 2012). Here, female H. convergens often survive months of prey scarcity in reproductive diapause and then produce a large burst of oviposition when aphids become available (Michaud and Qureshi 2006, Vargas et al. 2012). We tested five hypotheses in laboratory experiments. 1) Female H. convergens can recognize their own egg clusters and selectively cannibalize nonfilial eggs in a choice situation. To test whether kin recognition would extend to early-instar larvae, we also presented adult females with first-instar larvae, either their own offspring, or that of another female. Contingent on the first hypothesis, we predicted that 2) reproductive females would cannibalize more eggs than
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BAYOUMY AND MICHAUD: H. convergens EGG CANNIBALISM
prereproductive females. If this was true in the presence of ad libitum prey, it would be indicative of females using egg cannibalism to interfere with the reproduction of other females, rather than to obtain nutritional benefits. Because we did not expect males to be capable of recognizing eggs they had sired, we hypothesized that 3) virgin males would be more cannibalistic than mated males, as the former are not yet at risk of filial cannibalism. Finally, we hypothesized that 4) late-stage larval egg cannibalism and 5) adult egg cannibalism would yield measurable benefits for reproductive success, either in terms of female fecundity and fertility, or in parental effects on progeny development. Materials and Methods Insect Colony. A stock colony of H. convergens was established from 90 adults collected from a sunflower field in Hays, KS (38 510 N, 99 200 W), in April 2013. Following transfer to the laboratory, the insects were held in a growth chamber set to a photoperiod of 16:8 (L:D) h and a constant temperature of 23.0 6 1.0 C. For reproduction, adult females were isolated in polystyrene vials (145 cc) with ventilated caps and fed an ad libitum diet of greenbug, Schizaphis graminum (Rondani), a natural prey. The latter were raised on sorghum seedlings grown in a greenhouse and fed to beetles on excised leaves of their host plants. Eggs laid on the inner surfaces of the vials were collected daily by transferring the beetles to new vials. Larvae of the first laboratory generation were labeled according to their maternal lineage and reared in plastic Petri dishes (5.5 cm in diameter) in sibling groups of five on a diet of frozen eggs of the flour moth, Ephestia kuehniella Zeller (Lepidoptera: Pyralidae) ad libitum, with water provided on a small cube of sponge, both refreshed daily. Previous work has shown that the moth eggs are suitable prey for larval development of this species, but aphids are required for reproduction (Michaud and Qureshi 2006). All larvae were held in a growth chamber with a photoperiod of 16:8 (L:D) h day length at a temperature of 25.0 6 1.0 C. Once adults emerged, they were sexed, isolated in plastic Petri dishes (5.5 cm in diameter), and assigned a number indicating their individual identity and maternal lineage. These adults were then used in experiments. All experimental trials were conducted in a climate-controlled growth chamber set to a temperature of 25.0 6 1.0 C and a photoperiod of 16:8 (L:D) h and in the presence of ad libitum prey (S. graminum) to render them as conservative as possible and eliminate hunger as the primary driver of cannibalism behavior. Filial Cannibalism of Eggs. A series of virgin adults, 10–11-d-old postemergence, were paired in Petri dishes (n ¼ 25) with eggs of E. kuehniella for about 6 h and directly observed to ensure all pairs completed to at least one copulation. The females were then isolated in vials (as in Insect Colony) with greenbugs provided ad libitum and refreshed daily. Eggs laid in clusters on either the side of the vial or under the lid were outlined with different colored fine-tipped felt
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pens and were used in experiments within 24 h. Clusters laid on lids were then used to cap vials with clusters laid on the sides, ensuring that equal numbers of filial and nonfilial clusters were presented on lids and vials, respectively. The experiment consisted of a series of 20 females, each presented with a choice of a filial egg cluster and a nonfilial cluster of approximately similar size (range ¼ 20–40 eggs) in the presence of abundant prey (greenbugs on sorghum leaves). After 24 h, all egg clusters, including any newly laid by the female, were examined for cannibalism, which leaves behind stained fragments of chorion adhering to the container. The proportions of filial and nonfilial eggs cannibalized were subjected to a paired t-test for analysis. Filial Cannibalism of Larvae. To test whether females could recognize their own larval offspring, we designed a no-choice assay of first-instar cannibalism by reproductive adult females. A series of 24 mated females, each 13-d-old postemergence, were isolated in plastic Petri dishes (5.5 cm in diameter) containing greenbugs on excised sorghum leaves and presented with a single first-instar larva. In the treatment, females (n ¼ 12) received a filial larva, and in the controls (n ¼ 12), the larva of another female. Each female was directly observed for 30 min and all larval encounters and cannibalism events were tallied. Each female was tested only once. Given the a priori hypothesis that cannibalism of filial larvae would be avoided, cannibalism events were analyzed by a two-sided test of proportions weighted by sample size (StatSoft 2000, Statsoft Inc., Tulsa, OK). Cannibalism and Mating Status. To test whether the propensity for adult egg cannibalism was sensitive to mating status, a series of 120 larvae from four maternal lineages were reared on eggs of E. kuehniella until they emerged as adults. Adults were isolated in plexiglass cylinders (as in Insect Colony) and fed ad libitum greenbugs, refreshed daily. A series of 40 females and 40 males were each divided into two groups; adults of the first group were mated to a nonsibling on day 10 of adult life, whereas those of the second group remained unmated. All beetles were tested on their 11th day of adult life. Trials consisted of transferring individual beetles to a vial containing greenbugs on sorghum leaves and a cluster of conspecific (nonfilial) eggs (20) that were
