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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.

Overbrowsing by ungulates decimates plant populations and reduces diversity in a variety of ecosystems, but the mechanisms by which changes to plant community composition influence other trophic levels are poorly understood. In addition to removal of avian nesting habitat, browsing is hypothesized to reduce bird density and diversity through reduction of insect prey on browse-tolerant hosts left behind by deer. In this study, we excluded birds from branches of six tree species to quantify differences in songbird prey removal across trees that vary in deer browse preference. Early in the breeding season, birds preyed on caterpillars at levels proportional to their abundance on each host. Combining these data with tree species composition data from stands exposed to experimentally controlled deer densities over 30 years ago, we tested whether overbrowsing by white-tailed deer reduces prey biomass long after deer densities are reduced. Our analysis predicts total prey availability in the canopy of regenerating forests is fairly robust to historic exposure to high deer densities, though distribution of prey available from host species changes dramatically. This predicted compensatory effect was unexpected and is driven by high prey abundance on a single host tree species avoided by browsing deer, Prunus serotina. Thus, while we confirm that prey abundance on host trees can act as a reliable predictor for relative prey availability, this study shows that quantifying prey abundance across host trees is essential to understanding how changes in tree species composition interact with ungulate browse preference to determine prey availability for songbirds.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19534号 / 理博第4194号 / 新制||理||1602(附属図書館) / 32570 / 京都大学大学院理学研究科生物科学専攻 / (主査)教授 髙林 純示, 教授 永益 英敏, 教授 石田 厚 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

The red imported fire ant, Solenopsis invicta, is an abundant and aggressive component of early-successional communities in the southeastern United States. After disturbance, it rapidly invades new habitats, and once there, it has strong competitive and predatory effects on the existing arthropod community. In upland coastal plain pine forests at the Savannah River Site in South Carolina, I conducted two studies of fire ant ecology. In my first study (chapter 1), I investigated the way in which fire ants colonize early-successional road and powerline cuts through forests, and I tested whether some of these linear habitats provided better fire ant habitat than others. I found that fire ant mound density (#/ha) was similar in narrow dirt roads and in wider roads with the same intermediate level of mowing disturbance, and that density was lower in wide powerline cuts where the vegetation is only removed every five years. Furthermore, mound density was greatest near the edges of cleared roads and powerline cuts and was also greater on the northern sides of roads and powerline cuts where there was less shading from the adjacent forest. Results from this study suggest that allowing increased shading from adjacent forest vegetation, especially along northern roadside edges, would limit the suitability of road and powerline cuts as fire ant habitat, thereby slowing invasion. In my second study (chapter 2), I examined the impact of fire ants on arthropod and plant species in early-successional forest gaps. In a tritrophic system, I tested whether the top-down effect of insect herbivore consumption by fire ants was strong enough to cascade through two trophic levels and improve plant growth and fitness. I compared this potential effect to that of other arthropod predators in the community. I found that fire ants controlled the level of tissue damage to plant leaves by herbivores, but that the damage was not severe enough to influence plant growth or fitness. Fire ants had stronger negative interactions with insect herbivores than other predators in the community, and for this reason, fire ants can be considered keystone predators. This project provides further evidence that fire ants successfully invade even small patches of early-successional habitat that exist within larger matrices of uninhabitable, late-successional forest, and that once there, they have a dramatic effect on the arthropod community. Restricting the amount of early-successional habitat within southeastern forests, either as permanent road and powerline cuts or as temporary gaps, would limit the potential for strong and detrimental effects by the invasive fire ant. / Master of Science