Foraging and life history theory

Adult female

Our laboratory focuses on relating foraging theory to life history in the context of their contributions to lifetime fitness. We concentrate this work on the crab spider Misumena vatia, a sit-and-wait predator on flowers. We have worked on foraging acts that convey especially large fitness payoffs, as measured by numbers of offspring recruited into the next generation. The most important is patch choice by adult females, who may increase in mass up to ten-fold over two-three weeks if they find an ideal hunting site. Variation in success at this time is great, however, both because of difficulties in locating an ideal hunting site and because some individuals eschew high-quality sites if presented with them. To solve this apparent paradox, we are currently studying the role of early experience, most likely encoded as learning, in affecting young individuals in their choice of hunting sites. We are particularly interested in whether this behavior can be passed over molting episodes to subsequent instars, especially adults. Where feasible, we work with groups of known sibs, so that we can simultaneously assess possible genetic aspects of variation in patch choice.

Sexual selection

A second focus explores the roles of sexual selection and natural selection to male and female fitness in Misumena. Males are small, sometimes no more than 1/100th the mass of gravid adult females, largely a consequence of going through fewer molt cycles than the females. These populations are female-biased, ranging from about 1.4 females per male at hatching to 2.5-5.0 females per male as adults. Densities of males are low and unmated adult females do not appear to advertise their presence to the males, as do other spiders thus far studied in this context.

Male mortality of most animals including spiders is generally thought to be high when they search for females, resulting in rapidly diminishing numbers. However, male numbers of Misumena do not decline significantly until late in the summer, after almost all females have mated. The small size and extreme agility of the males likely minimize their vulnerability at this time. Although male food needs are not critical at this time, males select the same flowers that females prefer. This preference could thus position males with females more often than predicted by chance.

A four-trophic-level system

We have also begun to study a four-trophic-level system involving a pyralid moth whose larvae feed on ferns. These larvae feed primarily on two abundant species, sensitive and marsh ferns, and on at least three other species of ferns where available, for a total of five ferns from three families. This spread suggests an unusual ability to process a wide range of defensive chemicals probably responsible for minimizing attacks from most herbivores. Densities of moth larvae reach extremely high levels in large colonies consisting of thousands of individuals. Most such sites are located in low-lying areas of old fields that support luxuriant fern growths. The moths are heavily parasitized by a braconid wasp, which in turn is hyperparasitized by ichneumonid and eulophip wasps. Investigations currently under way will establish the success of the moths on different fern species and the consequences of this success upon the corresponding success of their primary parasitoids and hyperparasitoids. The system is well suited for studies of the role of both bottom-up and top-down forces on interactions among these different trophic levels. Multitrophic-level studies usually do not incorporate hyperparasitoids in spite of their likely importance in mediating the success of primary parasitoids.

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Nest of pyralid moth Herpetogramma theseusalis on sensitive fern Onoclea sensibilis	Late-instar Herpetogramma larva in nest
Male Herpetogramma	Cocoon of braconid wasp parasitoid Alabagrus texanus
Female Alabagrus on Herpetogramma nest	Male ichneumonid wasp Mesostenus thoracicus. Parasitoid of Herpetogramma and hyperparasitoid of Alabagrus
Aprostocetus sp. (1.5 mm) Eulophid hyperparasitoids of Alabagrus