Maya L. Evenden

Chemical similarity between historical and novel host plants promotes range and host expansion of the mountain pine beetle in a naïve host ecosystem

Host plant secondary chemistry can have cascading impacts on host and range expansion of herbivorous insect populations. We investigated the role of host secondary compounds on pheromone production by the mountain pine beetle (Dendroctonus ponderosae) (MPB) and beetle attraction in response to a historical (lodgepole pine, Pinus contorta var. latifolia) and a novel (jack pine, Pinus banksiana) hosts, as pheromones regulate the host colonization process. Beetles emit the same pheromones from both hosts, but more trans-verbenol, the primary aggregation pheromone, was emitted by female beetles on the novel host. The phloem of the novel host contains more α-pinene, a secondary compound that is the precursor for trans-verbenol production in beetle, than the historical host. Beetle-induced emission of 3-carene, another secondary compound found in both hosts, was also higher from the novel host. Field tests showed that the addition of 3-carene to the pheromone mixture mimicking the aggregation pheromones produced from the two host species increased beetle capture. We conclude that chemical similarity between historical and novel hosts has facilitated host expansion of MPB in jack pine forests through the exploitation of common host secondary compounds for pheromone production and aggregation on the hosts. Furthermore, broods emerging from the novel host were larger in terms of body size.

Factors Influencing Flight Capacity of the Mountain Pine Beetle (Coleoptera: Curculionidae: Scolytinae)

ABSTRACT The mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae: Scolytinae) is the most damaging pest of mature pine (Pinaceae) in western North America. Although mountain pine beetles have an obligate dispersal phase during which adults must locate a new host for brood production, dispersal is a poorly understood aspect of its ecology. This flight mill study was designed to test the effects of beetle size, sex, and age on flight capacity. Energy use during flight was assessed through measurements of weight before and after flight and fat content of flown versus control beetles. The mean flight distance achieved by mountain pine beetles varied between 2.12 and 5.95 km over the 23-h bioassay, but the longest total flight of an individual beetle was >24 km. Beetle preflight weight influenced flight initiation, flight distance, and duration. Bigger beetles are more likely to fly and once in flight fly longer and farther than smaller beetles. There was no direct effect of beetle sex on flight capacity. Flight capacity of beetles declined with age postemergence. Although individual flight capacity was variable, flight velocity was relatively constant between 1.55 and 1.93 km/h. Lipids are used to power flight in mountain pine beetles, as lipid content was lower in beetles flown on the flight mills compared with beetles that did not fly. Flight distance was negatively correlated with beetle postflight lipid content. The baseline flight capacity data revealed in this study have implications for understanding the population dynamics of this eruptive forest pest.

When mating disruption does not disrupt mating: fitness consequences of delayed mating in moths

The goals of pheromone‐mediated mating disruption are to interfere with mate finding, suppress insect population growth, and prevent crop damage. In addition to prevention of mating, pheromone treatment can also result in a delay of mating so that the fitness and subsequent population dynamics of the target insect pest are impacted. Females have a limited time to mate, mature eggs, and find suitable oviposition hosts, and a delay in mating can have large consequences on female fitness. As a result, delayed mating could be considered an indirect method by which mating disruption works to control pest populations. We perform a meta‐analysis on 24 experimental studies to assess the consequences of delayed mating on female moth fecundity, egg fertility, adult longevity, and pre‐oviposition period. Our goal is to determine whether the effect of delayed mating on female moth fitness is influenced by the following explanatory variables: moth family, voltinism, larval diet breadth, adult dispersal capacity, female mating strategy, and egg development strategy. Across species, the effect of delayed mating on female moths significantly decreases fecundity, fertility, and pre‐oviposition period and increases female longevity. The effect of delayed mating on female fitness is only marginally affected by the explanatory variables tested. We discuss the observed patterns and argue that delayed mating can be an important method by which population regulation is achieved through pheromone‐mediated mating disruption. Finally, we highlight areas where future research could add to the growing body of knowledge on mating disruption‐imposed delayed mating of female moths.

Peripheral and behavioral plasticity of pheromone response and its hormonal control in a long-lived moth

SUMMARY Reproductive success in many animals depends on the efficient production of and response to sexual signals. In insects, plasticity in sexual communication is predicted in species that experience periods of reproductive inactivity when environmental conditions are unsuitable for reproduction. Here, we study a long-lived moth Caloptilia fraxinella (Ely) (Lepidoptera: Gracillariidae) that is reproductively inactive from eclosion in summer until the following spring. Male sex pheromone responsiveness is plastic and corresponds with female receptivity. Pheromone response plasticity has not been studied in a moth with an extended period of reproductive inactivity. In this study, we ask whether male antennal response and flight behavior are plastic during different stages of reproductive inactivity and whether these responses are regulated by juvenile hormone. Antennal response to the pheromone blend is significantly reduced in reproductively inactive males tested in the summer and autumn as compared with reproductively active males tested in the spring. Reproductively inactive autumn but not summer males show lower antennal responses to individual pheromone components compared with spring males. Treatment with methoprene enhances antennal response of autumn but not summer males to high doses of the pheromone blend. Behavioral response is induced by methoprene treatment in males treated in the autumn but not in the summer. Plasticity of pheromone response in C. fraxinella is regulated, at least in part, by the peripheral nervous system. Antennal and behavioral response to pheromone differed in reproductively active and inactive males and increased with methoprene treatment of inactive males.

Effects of nutrition and methoprene treatment upon reproductive diapause in Caloptilia fraxinella (Lepidoptera: Gracillariidae)

Abstract Female Caloptilia fraxinella exhibit a prolonged reproductive diapause immediately post adult emergence in mid‐summer until the next spring when mating, egg development and oviposition on fresh Fraxinus spp. leaflets occur. Factors that effect the termination of reproductive diapause are investigated in this species. Caloptilia fraxinella diapausing adults held in overwintering conditions (2 °C, LD 0 : 24 h) for 24 weeks terminate diapause after placement for 2 weeks in simulated summer conditions (24 °C, LD 16 : 8 h) only if they are provided with 10% sugar water. Exogenous application of the Juvenile Hormone (JH) analogue methoprene to moths in both early‐ (summer) and mid‐ (autumn) reproductive diapause demonstrates that JH affects diapause termination but a carbohydrate nutrition source also mediates mating and vitellogenesis. Mating between moth pairs early in diapause occurs only after treatment with methoprene and provision with sugar water. However, there is no impact of mating on the propensity of females to produce vitellogenic oöcytes. Moths collected in the autumn in mid‐diapause respond in a dose‐dependent fashion to methoprene treatment and the response is greater than that of moths early in diapause collected in the summer. Treatment with methoprene and access to sugar water results in vitellogenic oöcytes in 18.75% of females from mid‐diapause moth pairs treated with 0.01 μg methoprene per insect and in all females from pairs treated at the two highest doses of methoprene (0.1 and 1 μg per insect). Mating occurs only between moths in mid‐diapause treated with the two highest doses of methoprene and these doses induce 91% and 100% mating, respectively. Both control and methoprene‐treated males in mid‐diapause held under summer conditions mate successfully and pass a spermatophore to their methoprene‐treated female partner. These data demonstrate that female C. fraxinella undergo a prolonged reproductive diapause in which termination is dependent on JH and further mediated by a carbohydrate nutrition source. The production of vitellogenic oöcytes is independent of mating. These data also provide evidence that response of moths in diapause to exogenous applications of methoprene differs throughout the diapause period and between male and female C. fraxinella.

Body Size, Age, and Disease Influence Female Reproductive Performance in Choristoneura conflictana (Lepidoptera: Tortricidae)

Abstract We determined the effect of body size and adult female longevity on the realized fecundity and fertility of the large aspen tortrix, Choristoneura conflictana (Walker) (Lepidoptera: Tortricidae), an outbreaking forest lepidopteran. We examined these relationships from an ecological perspective and included the impact of age at pairing and infection with a microsporidian pathogen in our analyses. The relationship between body size, as measured by pupal weight and forewing length and area, and realized fecundity and fertility was best for healthy females that were paired immediately after eclosion. A delay in age of pairing or sublethal pathogen infection eroded these relationships. A delay in age of pairing resulted in a lower percentage of females that produced fertile eggs and a significant reduction in the number of eggs laid. Although sublethal pathogen infection indirectly disrupted the body size-realized fecundity and fertility relationship, infection did not directly decrease egg production. Wing area was the best predictor of total and fertile egg production. The relationship between pupal weight and wing area was correlated, but females eclosing from small pupae had proportionately larger wings than females from large pupae, which may indicate a cost of large body mass in this actively dispersing species. Longevity of females did not predict the total or fertile number of eggs laid by females. There was no consistent relationship between pairing treatment and longevity of female moths, suggesting that there is no trade-off between longevity and reproductive fitness in this species. Sublethal pathogen infection significantly decreased female moth longevity, but this decrease was not reflected in reduced fecundity of infected females. Our data illustrate that although body size is an important determinant of realized fecundity and fertility in C. conflictana, these relationships are mediated by several factors that may vary greatly under field conditions and at varying population densities.

The lodgepole × jack pine hybrid zone in Alberta, Canada: a stepping stone for the mountain pine beetle on its journey East across the boreal forest?

Historical data show that outbreaks of the tree killing mountain pine beetle are often preceded by periods of drought. Global climate change impacts drought frequency and severity and is implicated in the range expansion of the mountain pine beetle into formerly unsuitable habitats. Its expanded range has recently reached the lodgepole × jack pine hybrid zone in central Alberta, Canada, which could act as a transition from its historical lodgepole pine host to a jack pine host present in the boreal forest. This field study tested the effects of water limitation on chemical defenses of mature trees against mountain pine beetle-associated microorganisms and on beetle brood success in lodgepole × jack pine hybrid trees. Tree chemical defenses as measured by monoterpene emission from tree boles and monoterpene concentration in needles were greater in trees that experienced water deficit compared to well-watered trees. Myrcene was identified as specific defensive compound, since it significantly increased upon inoculation with dead mountain pine beetles. Beetles reared in bolts from trees that experienced water deficit emerged with a higher fat content, demonstrating for the first time experimentally that drought conditions benefit mountain pine beetles. Further, our study demonstrated that volatile chemical emission from tree boles and phloem chemistry place the hybrid tree chemotype in-between lodgepole pine and jack pine, which might facilitate the host shift from lodgepole pine to jack pine.

Factors influencing flight potential of Choristoneura conflictana

Abstract Factors affecting the flight potential of Choristoneura conflictana, an insect that undergoes cyclical changes in population density, are investigated using computer‐linked flight mills. Female and male moths are flown for a 12‐h period and the longest single flight and total flight distance of each moth is recorded. After flight bioassays, moths are subjected to lipid extraction with a soxhlet apparatus to determine the effect of body lipid content on flight. Larger C. conflictana moths fly farther than smaller moths. Female C. conflictana fly farther than male moths but the effects of both mating status and moth age on the distance flown are dependent on sex. Mated females fly farther than virgins and older females fly farther than young females but these relationships do not occur in males. Body lipid content affects the distance flown by both females and males through a significant interaction with sex and age. The factors examined in this study will vary with density in natural populations and are predicted to be important indicators of flight potential and dispersal in this species.

Plasticity of male response to sex pheromone depends on physiological state in a long-lived moth

Plasticity of response to sexual signals is predicted to occur in long-lived animals that have periods of reproductive inactivity. Response should be adaptive to avoid wasted energy mate searching when conditions are unfavourable. Here we test that male response to female-produced sex pheromone in a long-lived moth species, Caloptilia fraxinella, is plastic and depends on male physiological state. Electrophysiological and behavioural studies indicated that the sex pheromone is composed of two compounds: (Z)-11-hexadecenal and (Z)-11-hexadecen-1-ol. Experiments were conducted to examine how males that differed in physiological state responded to signals of varying quality and quantity produced by manipulating synthetic pheromone blends. Reproductively active males had a lower pheromone response threshold and showed an upper threshold compared to males in reproductive diapause, which responded only to high pheromone doses with no upper threshold. The nature of pheromone response also varied depending on the males physiological state. Reproductively active males showed a more narrowly tuned response than males in diapause. However, reproductively active males did not discriminate among pheromones with (Z)-11-hexadecenal and 1–50% of (Z)-11-hexadecen-1-ol. Males in diapause were not attracted to pheromone composed of (Z)-11-hexadecenal and 1% (Z)-11-hexadecen-1-ol, showing reduced signal perception in moths in diapause. Taken together, these results indicate that male C. fraxinella pheromone response is plastic with respect to threshold and specificity and depends on physiological state of the responder.

The Effect of Water Limitation on Volatile Emission, Tree Defense Response, and Brood Success of Dendroctonus ponderosae in Two Pine Hosts, Lodgepole, and Jack Pine

The mountain pine beetle (MPB; Dendroctonus ponderosae) has recently expanded its range from lodgepole pine forest into the lodgepole × jack pine hybrid zone in central Alberta, within which it has attacked pure jack pine. This study tested the effects of water limitation on tree defense response of mature lodgepole and jack pine (Pinus contorta and Pinus banksiana) trees in the field. Tree defense response was initiated by inoculation of trees with the MPB-associated fungus Grosmannia clavigera and measured through monoterpene emission from tree boles and concentration of defensive compounds in phloem, needles, and necrotic tissues. Lodgepole pine generally emitted higher amounts of monoterpenes than jack pine; particularly from fungal-inoculated trees. Compared to non-inoculated trees, fungal inoculation increased monoterpene emission in both species, whereas water treatment had no effect on monoterpene emission. The phloem of both pine species contains (-)-α-pinene, the precursor of the beetle’s aggregation pheromone, however lodgepole pine contains two times as much as jack pine. The concentration of defensive compounds was 70-fold greater in the lesion tissue in jack pine, but only 10-fold in lodgepole pine compared to healthy phloem tissue in each species, respectively. Water-deficit treatment inhibited an increase of L-limonene as response to fungal inoculation in lodgepole pine phloem. The amount of myrcene in jack pine phloem was higher in water-deficit trees compared to ambient trees. Beetles reared in jack pine were not affected by either water or biological treatment, whereas beetles reared in lodgepole pine benefited from fungal inoculation by producing larger and heavier female offspring. Female beetles that emerged from jack pine bolts contained more fat than those that emerged from lodgepole pine, even though lodgepole pine phloem had a higher nitrogen content than jack pine phloem. These results suggest that jack pine chemistry is suitable for MPB pheromone production and aggregation on the host tree.