Mary L. Reid

SEARCH COSTS AND HABITAT SELECTION BY DISPERSERS

The effects of search costs on habitat selection by dispersers are largely unknown. We explore how habitat selection behavior is affected by the risk of mortality en route and by deferred search costs (i.e., costs incurred during search that reduce fitness after arrival in the new habitat), using a model designed for long-distance natal dispersers searching for scarce patches of suitable habitat embedded within a matrix of unsuitable habitat. In this situation, increases in the risk of mortality during search reduce disperser selectivity, where selectivity is reflected by the period during search when dispersers are only willing to accept a high-quality habitat. However, the effects of deferred costs on selectivity depend on other factors with pronounced effects on selectivity, including en- counter rates with high-quality habitats, relative habitat quality, and total search time. Surprisingly, under some sets of conditions, increases in deferred costs lead to increases in disperser selectivity. Overall, the effects of mortality and deferred costs on selectivity are small relative to the effects of other factors on selectivity. For instance, our model suggests that selectivity is much more strongly affected by total search time than by search costs, and it predicts a positive relationship between total search time and disperser selec- tivity across individuals in the same population, even in the face of considerable inter- individual variation in risk of mortality or deferred search costs.

FEMALE MATE CHOICE TACTICS IN A RESOURCE-BASED MATING SYSTEM: FIELD TESTS OF ALTERNATIVE MODELS

In this study we test theoretical models of female mate choice tactics in natural populations of pine engravers, Ips pini (Say) (Coleoptera: Scolytidae), a species with a resource‐based mating system and high search costs. We first develop distinguishing predictions for each of four models of mate choice: random, comparison tactics, and fixed and adjustable thresholds. These predictions relate to commonly collected field data that include the visiting behavior of females and the quality of accepted and rejected mates. Using these types of data, we conclude that pine engravers use an adjustable threshold mate choice tactic because females often accepted the first male encountered, rarely revisited males, visited similar numbers of males in patches of different quality, accepted higher‐quality males than those they rejected even on their first encounter with a male in a patch, and had higher acceptance thresholds in high‐quality patches than in low‐quality patches. This adjustable threshold tactic is consistent with a one‐step decision rule and is predicted to occur in species such as pine engravers in which search costs are high and females have information about patch quality before beginning a search in a patch.

Benefits of prolonged male residence with mates and brood in pine engravers (Coleoptera: Scolytidae)

In the pine engraver bark beetle, Ips pini (Coleoptera: Scolytidae), males stay in their breeding galleries for almost as long as their mates do - a period of several weeks. We examined several potential benefits of such a prolonged association of males with their mates in field studies. Attraction of additional females to male breeding sites and mate-guarding of current mates did not explain the observed male residence times, because the probability of arriving pine engravers of either sex was negligible within a week. Instead, a male-removal experiment showed that males significantly increased the reproductive rate of their mates, apparently by removing female-produced frass from the galleries, and males also defended the egg galleries against predators. Consequently, reproductive success tended to be greater when males were present. The breeding biology of bark beetles predisposes males to providing these types of assist

Effects of density on the reproductive success of pine engravers: is aggregation in dead trees beneficial?

1. The pine engraver bark beetle Ips pini (Say) (Coleoptera: Scolytidae), aggregates primarily on dead or dying pine trees. In this study pine engravers were laboratory‐reared on logs at a range of low densities to determine whether there was a fecundity advantage of breeding aggregations.

Choosing Hosts and Mates: The Value of Learning

Recent discussions of animal learning emphasize the importance of considering an animal’s ecology when interpreting its learning abilities (Johnston, 1982; Bolles and Beecher, 1988; Staddon and Ettinger, 1989). The implementation of this approach has so far been directed primarily at a few well-studied species. As a result of some success in revealing correspondences between what an animal can learn and its lifestyle, it has been recommended that a species’ ecology be thoroughly understood before learning experiments are conducted (Kamil and Mauldin, 1988). However, the risk here is that our understanding of learning will be limited to case studies that cannot be easily extended to other species. A useful companion approach would be to find general conditions favoring learning that can then be used as a framework for studying learning in individual species. As an example, the value of learning about a resource depends upon its variability or patchiness; if the resource is constant, then a fixed or innate response is favored, while increasing variability favors assessment and learning (Green 1980; see Stephens, this volume). Ideas such as this, generated from the functionalist’s perspective without concern for particular mechanisms, may generate important organizing principles in the study of learning in animals.

Condition-dependent tolerance of monoterpenes in an insect herbivore

The ability of a herbivore to tolerate plant defensive chemicals may vary with the herbivore’s energetic state. We investigated the effect of body condition on the survivorship of individual mountain pine beetles, Dendroctonus ponderosae, exposed to host monoterpenes at concentrations comparable to constitutive and induced levels of defence using fumigant exposure. Body condition index was calculated as the residual mass after fitting the relationship between fresh weight and body size. Differences in survivorship among the four monoterpenes tested (α-pinene, myrcene, terpinolene and limonene) were small. Beetles with a higher body condition index survived high monoterpene concentrations better than those in poorer condition. There was no direct effect of sex, but positive effects of body size and fat content on survivorship favoured females, the sex that pioneers attacks on live trees. Higher body condition index corresponded to both higher fat content and fat-free body mass; the same conclusions about monoterpene identity and size-dependent or energy-dependent tolerance of high monoterpene concentrations held if fat or fat-free body mass were used in place of body condition index. This study highlights the need to consider insect body condition in understanding insect–plant interactions.

Attack and Reproductive Success of Mountain Pine Beetles (Coleoptera: Scolytidae) in Fire-Damaged Lodgepole Pines

Abstract High-intensity fires are known to kill adult and larval bark beetles, but it is unclear how mountain pine beetles (Dendroctonus ponderosae Hopkins) respond to trees that have been damaged by lower-intensity ground fires at the periphery of burns. We conducted an experiment to determine whether mountain pine beetles preferentially attack trees that have been damaged by fire and to determine how fire damage affects beetles’ reproductive success. We simulated different intensities of ground fires by artificially burning a strip of bark that extended zero-thirds, one-third, two-thirds, or three-thirds around a tree’s circumference. Burn treatments were applied ∼7 wk before beetles emerged from surrounding trees. We found that beetles did not preferentially attack fire-damaged trees; fire damage had no effect on the number of beetles landing on a tree, which trees were attacked, attack rate, attack density, or the body size of beetles attacking a tree. Beetle reproductive success (number and condition of offspring) was also not affected by fire damage. Beetles were more likely to overcome tree defenses and produce successful egg galleries on fire-damaged trees than on undamaged trees, but this was only observed on trees with low beetle attack densities. If beetle attack density was high, trees were successfully attacked irrespective of burn treatment. Our results suggest that fire damage only affects mountain pine beetle reproduction and population growth in areas where attack densities are low. In other situations, fire damage will have negligible effects on beetle attack and reproductive success.

Who goes first? Condition and danger dependent pioneering in a group-living bark beetle (Dendroctonus ponderosae)

Among group-living organisms, some individuals initiate groups by being the first to attack a prey item or the first to colonize a new settlement site. In the group-living mountain pine beetle (Dendroctonus ponderosae), first attackers (known as pioneers) on live trees suffer higher mortality due to tree defenses than do beetles that join aggregations. This study examined factors that affect an individual’s propensity to initiate an aggregation. When placed on an unoccupied tree, the probability of successfully entering the tree was positively correlated with body condition (residual of mass versus length regression). However, beetles in better condition took longer to initiate tunnel construction than those in poorer condition, suggesting that pioneering is a “desperation” strategy used when low energy reserves preclude further dispersal or when potential trees are rare. These contrasting patterns suggest pioneering is a nonlinear behavioral response, such that beetles with the smallest energy reserves and beetles with the greatest energy reserves both avoid pioneering. We further found that pioneering was more likely when the environment favored success, such as in smaller diameter trees (which may have weaker defenses) and earlier in the season (when the probability of recruiting conspecifics is higher). Our results suggest that pioneers incorporate both internal and external variables in their decision to attack an uncolonized tree.

Forest Thinning Affects Reproduction in Pine Engravers (Coleoptera: Scolytidae) Breeding in Felled Lodgepole Pine Trees

Abstract Reproduction in bark beetles (Coleoptera: Scolytidae) is known to be affected by abiotic factors, especially temperature, and by the quality of individual beetles. Both of these factors are affected by forest structure, yet the effects of forest structure on reproduction in bark beetles have not been widely shown in field studies. Here we investigate how changes in forest structure due to thinning of mature lodgepole pine, Pinus contorta variety latifolia Engelmann, stands affect reproduction in pine engravers, Ips pini (Say), breeding in felled trees. To do this, we excavated pine engraver gallery systems in thinned and unthinned stands at the end of the breeding season. Males in thinned stands attracted more females than in unthinned stands. Also, females in thinned stands extended their egg galleries farther, laid more eggs, and had higher egg densities than in unthinned stands. These results are consistent with increased temperatures in thinned stands, but may also be attributable to differences in individual quality resulting from easier dispersal in thinned stands. Regardless, the observed increases in reproduction likely reflect higher reproductive success in thinned stands than in unthinned stands, and the effects of thinning on population dynamics of bark beetles should be further investigated.

Sub-lethal effects of monoterpenes on reproduction by mountain pine beetles

Plant defences may negatively affect the oviposition behaviour of insect herbivores. We exposed adult female mountain pine beetles Dendroctonus ponderosae (Hopkins) to monoterpene vapours and assessed their subsequent oviposition in lodgepole pine Pinus contorta. Exposure to 31.25 and 125 p.p.m. of α‐pinene and limonene for 24 h did not affect the survival or establishment of oviposition galleries compared with controls. Females had fewer, smaller eggs and therefore lower cumulative reproduction after exposure to 125 p.p.m. compared with the other treatments. Limonene negatively affected oviposition more than did α‐pinene. Female body condition, a measure of energetic state, influenced the responses (i.e. significant interactions between condition and monoterpene concentration). With exposure to 31.25 p.p.m., females in better condition tended to have fewer, larger eggs, whereas females in poorer condition tended to have more, smaller eggs, compared with controls. Overall, reproductive investment declined with body condition when exposed to monoterpenes. Egg size and number increased with female body size. The findings of the present study may indicate the toxic effects of monoterpenes or may be a result of females choosing not to invest in adverse environments depending on body condition. In either case, reduced reproduction when exposed to high plant defences may be increasingly important as climate change affects plant defences.