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Plant shape affects the ability of insect predators to locate prey by altering search paths or by providing partial refugia for prey. Changes in predator foraging efficiency can have significant consequences for population dynamic of both predators and prey. Yet, the relationship between plants and insect predators is not well understood despite its relevance to agriculture and biological control. The effect of plant gross morphology on predator foraging success was tested using multicoloured Asian ladybeetles, Harmonia axyridis Pallas (Coleoptera: Coccinellidae), and green lacewing larvae, Chrysoperla carnea Stephens (Neuroptera: Chrysopidae), preying on pea aphids, Acyrthosiphon pisum Harris (Hemiptera: Aphididae). These predators differed in body size and therefore might be expected to have different responses to a given plant morphology. Experiments were conducted using four different pea plant morphologies (Pisum sativum L.) that differed in fractal dimension, but which were controlled for surface area. The consumption rate of each predator on each pea morph was determined by measuring the number of aphids consumed in a 48 hour foraging period at 3 prey densities. I also tracked predator search paths using 2D time-lapse photography to determine if the two predators search plants differently. I found that both predators were more successful at capturing prey on plants with a higher leaf edge to leaf area ratio (lower fractal dimension). Plants with more edges were easier for predators to grip, thus increasing their mobility and manoeuvrability. Also, plants with more edges and fewer leaf surfaces had fewer locations where aphids could hide. As a result, predators are more effective at locating and capturing prey on these morphologies.

Plant shape affects the ability of insect predators to locate prey by altering search paths or by providing partial refugia for prey. Changes in predator foraging efficiency can have significant consequences for population dynamic of both predators and prey. Yet, the relationship between plants and insect predators is not well understood despite its relevance to agriculture and biological control. The effect of plant gross morphology on predator foraging success was tested using multicoloured Asian ladybeetles, Harmonia axyridis Pallas (Coleoptera: Coccinellidae), and green lacewing larvae, Chrysoperla carnea Stephens (Neuroptera: Chrysopidae), preying on pea aphids, Acyrthosiphon pisum Harris (Hemiptera: Aphididae). These predators differed in body size and therefore might be expected to have different responses to a given plant morphology. Experiments were conducted using four different pea plant morphologies (Pisum sativum L.) that differed in fractal dimension, but which were controlled for surface area. The consumption rate of each predator on each pea morph was determined by measuring the number of aphids consumed in a 48 hour foraging period at 3 prey densities. I also tracked predator search paths using 2D time-lapse photography to determine if the two predators search plants differently. I found that both predators were more successful at capturing prey on plants with a higher leaf edge to leaf area ratio (lower fractal dimension). Plants with more edges were easier for predators to grip, thus increasing their mobility and manoeuvrability. Also, plants with more edges and fewer leaf surfaces had fewer locations where aphids could hide. As a result, predators are more effective at locating and capturing prey on these morphologies.

Lygus hesperus is an important pest on different crops including cotton and alfalfa in the western U.S. Anaphes iole is a common parasitoid of Lygus spp. eggs in the U.S. and has potential as a biological control agent against L. hesperus in different crops. Its foraging behavior has been studied to a limited extent, but it is unknown whether A. iole females rely on plant volatiles to locate host habitats. L. hesperus feeding and oviposition are known to induce emission of plant volatiles in cotton and maize, but no studies have addressed the role of plant volatiles in the host searching behavior of A. iole. The objectives of this study were to evaluate the attraction of A. iole females toward volatiles derived from L. hesperus habitats and flight response of A. iole females toward cotton plants harboring L. hesperus eggs or treated with methyl jasmonate. Results from olfactometry bioassays showed that female wasps were attracted to odors emanating from different plant-L. hesperus complexes and from adult L. hesperus, while plants damaged by non-hosts or mechanically-damaged were not attractive. These findings suggested that A. iole females use specific plant volatiles released following L. hesperus feeding and oviposition to locate host habitats. In addition, in flight chamber tests A. iole females discriminated between cotton plants with moderate (41 eggs) and high (98 eggs) levels of L. hesperus infestations relative to uninfested plants, but not between plants with low (7 eggs) infestations compared to uninfested plants. In larger scale experiments conducted in the greenhouse, female wasps responded to L. hesperus-infested plants but not to methyl jasmonate-treated plants under similar conditions. Overall, results from this study revealed that A. iole females employ volatile signals to locate its host’s habitat and that they are attracted to plants damaged by L. hesperus feeding and oviposition. However, further research should seek to identify the chemical elicitors involved in the release of plant volatiles attractive to A. iole females.

In recent years it has become apparent that when studying plant-herbivore interactions, it is important to also consider the multitrophic context, i.e. to what extent generalist and specialist enemies act as bodyguards for the plant. I have performed a field experiment with 25 genets, varying in plant defense levels, of meadowsweet (Filipendula ulmaria) and its herbivore, the chrysomelid beetle Galerucella tenella. Each genet was treated with four different density manipulations of G. tenella (5, 15, 30, and 60 larvae per genet). The experiment was set up on an archipelago island with natural levels of a specialist hymenopteran parasitoid Asecodes parviclava and generalist natural enemies. I set up the following hypotheses: 1) larval survival will be higher on less defended plants than on more defended plants, 2) the proportion of parasitoids will be higher on more defended plants, and 3) larval survival will increase with increasing larval densities. The most important findings of the experiment were an overall very low total survival of larvae (G. tenella pupae and A. parviclava mummies), a generally high parasitism by A. parviclava on more defended plants, and that two groups of generalist predators (hunting and web-forming spiders) were more frequent on less defended plants. My study suggests that herbivore survival may not be higher on less defended plants under situations with abundant occurrence of generalist predators, as they are able to trace high-quality foraging sites.

Lygus hesperus is an important pest on different crops including cotton and alfalfa in the western U.S. Anaphes iole is a common parasitoid of Lygus spp. eggs in the U.S. and has potential as a biological control agent against L. hesperus in different crops. Its foraging behavior has been studied to a limited extent, but it is unknown whether A. iole females rely on plant volatiles to locate host habitats. L. hesperus feeding and oviposition are known to induce emission of plant volatiles in cotton and maize, but no studies have addressed the role of plant volatiles in the host searching behavior of A. iole. The objectives of this study were to evaluate the attraction of A. iole females toward volatiles derived from L. hesperus habitats and flight response of A. iole females toward cotton plants harboring L. hesperus eggs or treated with methyl jasmonate. Results from olfactometry bioassays showed that female wasps were attracted to odors emanating from different plant-L. hesperus complexes and from adult L. hesperus, while plants damaged by non-hosts or mechanically-damaged were not attractive. These findings suggested that A. iole females use specific plant volatiles released following L. hesperus feeding and oviposition to locate host habitats. In addition, in flight chamber tests A. iole females discriminated between cotton plants with moderate (41 eggs) and high (98 eggs) levels of L. hesperus infestations relative to uninfested plants, but not between plants with low (7 eggs) infestations compared to uninfested plants. In larger scale experiments conducted in the greenhouse, female wasps responded to L. hesperus-infested plants but not to methyl jasmonate-treated plants under similar conditions. Overall, results from this study revealed that A. iole females employ volatile signals to locate its host’s habitat and that they are attracted to plants damaged by L. hesperus feeding and oviposition. However, further research should seek to identify the chemical elicitors involved in the release of plant volatiles attractive to A. iole females.

Prior exposure to plants cues can enhance assortative mating in insects. We hypothesized that, as previously reported for females, males of Cotesia congregata would display inherent responses to plant cues that could be modified by postemergence experience and further, that males originating from two different host-plant complexes (HPCs) would display different behavioral responses to these HPCs. In no-choice contact assays with a non-host plant, searching responses of males and females increased sharply at Day 2 and remained stable through Day 4. In no-choice assays with potential host plants, males searched longer on catalpa than tobacco; responses were not modified by postemergence experience. In choice assays with both HPCs, naïve males did not display orientation preferences; however, males experienced with their natal plant preferred their natal HPC. Results indicate that postemergence experience on the natal host plant induces an orientation preference for the natal HPC and thus, can facilitate assortative mating.

Among the predicted impacts associated with global climate change, warming is of special interest because the rates of physiological processes are temperature-dependent. Insects and other ectotherms are likely to be affected due to their limited ability to control body temperature. In this study, I measured the tolerance to extreme high temperatures, i.e., critical thermal maximum (CTmax), of component species in a tri-trophic system, including an herbivore (Manduca sexta), a primary larval parasitoid (Cotesia congregata) and a hyperparasitoid (genus Silochalcis). For wild insects, the parasitoid had the lowest CTmax, the hyperparasitoid had the highest, and the herbivore was intermediate. For laboratory insects, the parasitoid had a lower CTmax than the herbivore. Results suggest that laboratory colonies can be used to predict relative thermal performance of interacting species in the field. Variations in tolerance to high temperature among component species could disrupt the outcome of interactions in multi-trophic systems.

Solenopsis invicta, the red imported fire ant, has recently become associated with Antonina graminis, an invasive pest, and Neodusmetia sangwani, biological control agent, and maybe negatively affecting established biological control. A preliminary survey outlined the range of A. graminis and its parasitoids, and found N. sangwani was present at a reduced rate in South Texas and in the southeastern United States. A greenhouse experiment demonstrated that S. invicta decreased the rate of parasitism of A. graminis by N. sangwani, with S. invicta directly interfering with oviposition. Interactions between S. invicta and A. gaminis may be facilitating the spread and establishment of two invasive pests which has a negative impact on established classical biological control of A. graminis by N. sangwani.

To understand how plant defensive traits will evolve, we need to consider the biotic context for plant-herbivore interactions. I investigated how predators affect selection on defensive traits in plants. First, I established the timing of resistance in three soybean genotypes. Next, I examined the combined effects of resistance and predators on plant fitness. I reared Mexican bean beetles (MBBs) with or without spined soldier bugs (SSBs) on soybeans with constitutive resistance (CR) or no resistance (NR). SSBs fed more on MBBs that fed on NR than on CR plants, and this translated into an increased fitness benefit from predators for NR plants over CR plants. Selection for some types of resistance in plants should thus be stronger with lower predation rates. Similarly, I reared MBBs with or without SSBs on soybeans with early induced resistance (EI), late induced resistance (LI), or CR. SSBs fed more on MBBs reared on LI plants than on beetles raised on CR plants, but no more on beetles reared on EI plants than on beetles reared on CR plants. LI plants were the only of the three soybean varieties to receive a fitness benefit from predators, which could help explain the evolution of this type of plant defense. The results of both experiments also suggest that predator introductions may be more beneficial to LI or NR crop plants than EI or CR crops. Finally, I present a model that determines the optimum amount of induced resistance (IR) and CR for a plant growing with and without neighbors. Unlike earlier models, our plants have a probability of being attacked that is modified by short- and long-term feedback of plant defenses to herbivores. Higher costs of defense favor IR over CR, while increasing herbivore attack rates or increasing the overall effectiveness of defense results in more CR. Plants with neighbors might be selected to evolve higher or lower levels of CR than if they were growing alone. Adding neighbors also selects for more mixed induced/constitutive strategies for all parameters. Having defended neighbors could thus be part of the reason why plants have evolved such mixed defense strategies. / Dissertation