Edward W. Evans

Timing of reproduction in a prairie legume: seasonal impacts of insects consuming flowers and seeds

SummarySeasonal patterns of insect damage to reproductive tissue of the legume Baptisia australis were studied for three years in native tallgrass priairie. Contrasting seasonal patterns of damage were associated with the major species of insect consumers. The moth Grapholitha tristegana (Olethreutidae) and the weevil Tychius sordidus (Curculionidae), which together infested 80–100% of developing fruits (pods), consistently damaged more seeds on average in early than in late maturing pods. But while late opening flowers were less subject to attack from moths and weevils, they were more subject to attack from chewing insects, particularly blister beetles (Epicauta fabricii, Meloidae), which destroyed >80% of all flowers and developing young pods (including moth and weevil larval inhabitants). The blister beetles arrived late in the flowering season and fed particularly on young reproductive tissue, allowing larger, older pods that had developed from early opening flowers to escape destruction. The relative abundances and impacts of blister beetles, moths, and weevils varied from year to year. Adding to the uncertainty of reproductive success of the host plant were the large and variable amounts of damage to immature buds inflicted by insects (including the blister beetles and weevil adults) and late killing frosts. Thus, timing of flowering is critical to success in seed production for B. australis. The heavy impacts of insects and weather can result in a very narrow window in time (which shifts from year to year) during which B. australis can flower with any success. The opposing pressures exerted by insects and weather on floral reproductive success may act in concert with other features of the plants biology to foster the maintenance of considerable diversity in flowering times among individuals in local populations of B. australis.

HABITAT DISPLACEMENT OF NORTH AMERICAN LADYBIRDS BY AN INTRODUCED SPECIES

How introduced species displace native species is often poorly understood. Various studies suggest that several species of native ladybird beetles (Coleoptera: Coccinellidae) have declined in abundance in particular habitats (especially crops) and/or geographic regions following the colonization of North America by the seven-spotted ladybird beetle (Coccinella septempunctata L.). However, the factors responsible for such declines are unclear. Long-term sampling of alfalfa fields in northern Utah revealed that as C. septempunctata established itself and increased in numbers, densities of native species decreased significantly over the period 1992–2001. The decline in numbers of native ladybirds mirrored a decline in numbers of their principal prey in alfalfa, pea aphids. Field experiments with induced aphid outbreaks were conducted to test the hypothesis that dispersing native adults have shifted their foraging efforts away from alfalfa as the introduced species has increased in numbers and depressed prey ...

Asymmetric larval interactions between introduced and indigenous ladybirds in North America.

Understanding the mechanisms that result in the success of introduced species will contribute to predicting future invasions and managing invaded systems. We examined interactions between larvae of two predatory ladybird species recently introduced to North America, Coccinella septempunctata (CS) and Harmonia axyridis (HA), and two indigenous ladybirds, Coccinella transversoguttata (CT) and Hippodamia convergens (HC). By pairing young and old larvae in the laboratory at low and high levels of aphid availability, we assessed the degree of asymmetry in intraguild predation (IGP), the strength of competitive effects on growth and development of larvae escaping predation, and the nature of attack and escape behavior among the species. Interactions were generally asymmetric, with larvae of introduced species acting most frequently as intraguild predators and larvae of indigenous species serving most frequently as intraguild prey (the two Coccinella spp., however, preyed on each other at similar rates). Because they were especially aggressive and because other larvae were least successful in escaping their attacks, larvae of HA had stronger negative effects on larvae of the two indigenous species than did larvae of CS. Such negative effects, expressed most strongly when aphid availability was low, were especially adverse for the smaller of the two indigenous species, HC. In general, older larvae interacted with each other more strongly than young larvae did, and older larvae had especially strong negative effects on young larvae when interactions occurred between age classes. Our results suggest that HA more than CS may represent a threat to indigenous ladybirds as an intraguild predator, and that IGP in turn may play a stronger role for HA than for C. septempunctata in promoting the successful invasion of North America.

Essential versus alternative foods of insect predators: benefits of a mixed diet

Abstract Although many predatory insects appear to be opportunistic generalists in their selection of prey, only a subset of prey species may in fact serve as “essential foods” capable of supporting immature growth and adult reproduction. It has been suggested that other, “alternative foods” serve only to maintain the predator when essential foods are not available, but little research has evaluated the significance of a mixed diet of essential and alternative foods for predator growth or reproduction. Here we test the general hypothesis that although alternative prey may be inadequate to support reproduction when consumed alone by adult predators, consumption of such prey may enhance the predators reproductive output when the predator also has access to essential prey. We compared egg production by two aphidophagous lady beetles, Coccinella septempunctata and C. transversoguttata, provided with diets of aphids (essential prey) and weevils (alternative prey). As predicted, female predators produced greater numbers of eggs when a diet of pea aphids in limited number was supplemented by alfalfa weevil larvae. The predators laid no eggs when provided only with weevils or only with sugar. But once aphids were added to the diet, females of C. transversoguttata (but not C. septempunctata) laid eggs in greater numbers when they had fed previously on weevils than on sugar. Females of both species also produced eggs in modest numbers when provided with both weevils in excess and sugar, but this diet supported a lower rate of egg production than did a diet of weevils in excess plus a limited number of aphids. Although C. septempunctata has a longer history of association with the alfalfa weevil than does C. transversoguttata, the former species was not more effective in exploiting this alternative prey in support of reproduction. The tendency of generalist predators such as adult lady beetles to consume alternative as well as essential prey probably enhances considerably their ability to capitalize on short-lived and scattered opportunities as they seek out suitable sites in which to reproduce.

Intra versus interspecific interactions of ladybeetles (Coleoptera: Coccinellidae) attacking aphids

SummaryThe nature and relative strengths of intra versus interspecific interactions among foraging ladybeetle larvae were studied experimentally by measuring short-term growth rates of predators and reductions in population sizes of prey in laboratory microcosms. In these microcosms, ladybeetle larvae foraged singly or as conspecific or heterospecific pairs, for pea aphids on bean plants over a two-day period. Similarly sized third instar larvae ofHippodamia convergens andH. tredecimpunctata, H. convergens andH. sinuata, andH. convergens andCoccinella septempunctata, were tested in experiments designed to ensure that paired larvae experienced moderate competition. Interspecific competition in these experiments did not differ significantly from intraspecific competition, in that an individuals weight gain did not depend on whether its competitor was heterospecific or conspecific. Furthermore, aphid populations were reduced equally by heterospecific and conspecific pairs. These results suggest that there is little or no difference between intra and interspecific interactions among larvae of these ladybeetles when two similarly sized individuals co-occur on a host plant. Thus, the species diversityper se of assemblages of ladybeetle larvae may have little influence over the short term on the reduction of aphid populations by ladybeetle predation.

Indirect Interactions in Biological Control of Insects: Pests and Natural Enemies in Alfalfa

The population dynamics of insect pests in agroecosystems may often be linked to those of other phytophagous species through the foraging activities of natural enemies; these indirect interactions provide both opportunities and challenges for biological control. We conducted field experiments to explore indirect interactions in the biological control of the alfalfa weevil (Hypera postica) in northern Utah. Honeydew produced by pea aphids (Acyrthosiphon pisum) in alfalfa fields may be important as a source of nutrition for adults of the wasp Bathyplectes curculionis and may thereby enhance parasitism of the wasps host, the alfalfa weevil. The presence of aphids may also promote aggregation of lady beetles, which consume weevil larvae in alfalfa. But by feeding heavily on pea aphids as well, these predators could indirectly enhance weevil survivorship by undercutting the wasps capacity to parasitize its host. In one field experiment, we added pea aphids and/or adults of the lady beetle Coccinella septempunctata to caged field plots of alfalfa during the first (spring) hay crop. Adding aphids increased levels of parasitism among late-maturing alfalfa weevil larvae. Adding lady beetles decreased numbers of late-maturing weevil larvae, but also decreased levels of parasitism among these larvae. In another experiment, repeated application of artificial honeydew to caged alfalfa plots during the first crop increased levels of parasitism among weevil larvae. In a third experiment, a single application of artificial honeydew to open (uncaged) alfalfa plots in mid-May also had a direct positive effect on rates of parasitism among larvae. However, this application had an indirect negative effect on parasitism as well: the presence of artificial honeydew caused aggregation of lady beetles and thus led to decreased aphid abundance. Consequently, provision of artificial honeydew as a food source for parasitoid adults was neutralized by a subsequent reduction in naturally occurring honeydew. These field experiments illustrate the complexity with which indirect interactions can affect exploiter-victim relationships in biological control. They also raise questions con- cerning the long-term consequences that may result from the purposeful introduction of the Old World lady beetle C. septempunctata to alfalfa and other crops in North America. Artificial honeydew may prove useful in enhancing parasitism of the alfalfa weevil, but frequent applications may be required to compensate for associated reductions in availability of naturally occurring aphid honeydew. The effects of honeydew producers on other her- bivores through natural enemies fall under the general concepts of short-term apparent competition and related ecological relationships among species, and highlight the impor- tance of temporal and spatial factors in indirect interactions.

The harlequin ladybird, Harmonia axyridis: global perspectives on invasion history and ecology

The harlequin ladybird, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), is native to Asia but has been intentionally introduced to many countries as a biological control agent of pest insects. In numerous countries, however, it has been introduced unintentionally. The dramatic spread of H. axyridis within many countries has been met with considerable trepidation. It is a generalist top predator, able to thrive in many habitats and across wide climatic conditions. It poses a threat to biodiversity, particularly aphidophagous insects, through competition and predation, and in many countries adverse effects have been reported on other species, particularly coccinellids. However, the patterns are not consistent around the world and seem to be affected by many factors including landscape and climate. Research on H. axyridis has provided detailed insights into invasion biology from broad patterns and processes to approaches in surveillance and monitoring. An impressive number of studies on this alien species have provided mechanistic evidence alongside models explaining large-scale patterns and processes. The involvement of citizens in monitoring this species in a number of countries around the world is inspiring and has provided data on scales that would be otherwise unachievable. Harmonia axyridis has successfully been used as a model invasive alien species and has been the inspiration for global collaborations at various scales. There is considerable scope to expand the research and associated collaborations, particularly to increase the breadth of parallel studies conducted in the native and invaded regions. Indeed a qualitative comparison of biological traits across the native and invaded range suggests that there are differences which ultimately could influence the population dynamics of this invader. Here we provide an overview of the invasion history and ecology of H. axyridis globally with consideration of future research perspectives. We reflect broadly on the contributions of such research to our understanding of invasion biology while also informing policy and people.

Invasions by ladybugs, ladybirds, and other predatory beetles.

Species of predatory Coleoptera have become abundant in new geographic regions recently, raising concerns for invaded ecosystems. We address this topic by focusing on invasive alien ladybird beetles (Coccinellidae; known also as ladybugs). Humans appear directly or indirectly responsible for all or most ladybird invasions. Factors hypothesized to have promoted ladybird invasions include genetic diversity (e.g., for polymorphism), phenotypic plasticity, adaptation and genetic shift, generalized diet and habitat preferences, flexible life history and reproduction, large body size, and release from enemies. Factors such as climate, habitat and prey availability, and biotic resistance may sometimes prevent or slow ladybird invasions. Indigenous species (e.g., herbivores) may suffer from invasions, and biological control programs may be affected. Species of indigenous ladybirds throughout the world are reported to have declined in abundance following ladybird invasions, with increased competition and/or intraguild predation most often hypothesized or inferred. Similar recent studies especially of ground beetles (Carabidae) also make clear the potential of invasive alien predatory Coleoptera to disrupt invaded natural and agricultural ecosystems.

Egg production in response to combined alternative foods by the predator Coccinella transversalis

Insect predators may commit to reproduction to varying degrees depending on the nature of the prey they consume. I compared egg production by females of the aphidophagous ladybird beetle Coccinella transversalis (F.) (Coccinellidae) maintained on differing diets. As expected, females laid most eggs on a diet of aphids (pea aphids, Acyrthosiphon pisum[Harris]). Upon being switched from a diet of aphids, females laid no eggs after the first 3 days when maintained on a water solution of sucrose, and laid almost no eggs when maintained on a diet of second‐instar larvae of the moth Helicoverpa armigera (Hübner). However, females laid eggs in small numbers (on average, 2.7 eggs per day) when provided both sucrose and Helicoverpa larvae. Females laid similar numbers of eggs when aphid honeydew was substituted for sucrose in combination with Helicoverpa larvae; in both cases, egg production was only 10–15% of that on a diet of aphids. The production of small numbers of eggs from consumption of non‐aphid foods may enhance the ability of aphidophagous ladybirds to optimize their timing of reproduction at short‐lived colonies of their preferred aphid prey.

Influence of Carbohydrate Foods and Mating on Longevity of the Parasitoid Bathyplectes curculionis (Hymenoptera: Ichneumonidae)

Abstract By promoting adult longevity, carbohydrate food sources can influence the effectiveness of parasitoids in biological control of insect pests. Adults of Bathyplectes curculionis (Thomson), a parasitoid of the alfalfa weevil (Gyllenhal)(Coleoptera: Curculionidae), benefit greatly from continual nourishment: field-collected adults lived significantly longer when provided sugar-water than those provided water only. Access to one potential source of adult nutrition in the field, dandelion flowers, enabled laboratory-reared wasps to live only slightly but nevertheless significantly longer than those that had access to water only, foliage of alfalfa, or flowers of Phacelia tanacetifolia Bentham. The wasps were observed foraging readily on dandelion but could not insert their heads into and did not pierce florets to obtain nectar. For each of the foods provided (dandelion, phacelia, alfalfa, or water alone), virgin females lived significantly longer than mated females. This was not the case, however, when wasps were provided a honey-water solution; both virgin and mated females lived more than 20 d on average. These results suggest that even in the absence of oviposition, female longevity is reduced by the simple act of mating when inferior but not superior foods are available. The striking contrast in longevity of wasps provided honey versus other carbohydrate foods raises the question of how much female wasps in nature realize their potential lifespan and fecundity. Dandelions and pea aphid honeydew appear to be the most likely sources of carbohydrates for the wasp in alfalfa fields. Supplemental foods such as floral plantings may promote realization of such potential longevity, but the quality and accessibility of floral foods for the wasp will need to be evaluated carefully.