Ian F. Harvey

FLEXIBLE LARVAL GROWTH ALLOWS USE OF A RANGE OF HOST SIZES BY A PARASITOID WASP

Recent optimality models of host-parasitoid associations have assumed that host quality varies with host size or age at parasitism. This is based on the fact that larger hosts provide more resources, making size a reliable indicator of the amount of resources available for parasitoid development. Few studies have examined this in parasitoids that allow their hosts to continue development after parasitism (koinobiont parasitoids). In this study we compared growth trajectories of the koinobiont ichneumonid endoparasitoid Venturia canescens developing in four larval instars of one of its hosts, the moth Plodia interpunctella. Hosts were reared with excess food and parasitized as late second, third, fourth, and fifth instars. Hosts were dissected at intervals after parasitism, and host and parasitoid dry mass determined. The survivorship, development time, and size of eclosing adult wasps were also monitored. Hosts parasitized by Venturia continued to grow and become prepupae, although their maximum masses were progressively smaller the earlier that the hosts were parasitized: second-instar (L2) hosts grew to 70% of control (unpar- asitized) size, while L4 and L5 hosts reached control size. The maximum larval dry mass, as well as eclosing adult size, varied with host instar at parasitism. Venturia larvae spent prolonged periods as first instars when developing from L2 hosts and grew most rapidly during early stages in L5 larvae. The data show that for Venturia, host quality is not a direct function of host size at parasitism. Furthermore, the pattern of development shown by Venturia differs markedly from that shown by aphidiid koinobiont parasitoids.

Life Expectancy and Egg Load Affect Oviposition Decisions of a Solitary Parasitoid

Life-history theory predicts that animals should be sensitive to both the amount of resources available and life expectancy in making reproductive decisions. Because it is easier to control the mortality of insect parasitoids (insects whose larva develop in or on another insect) than many other groups of animals, the best tests of these predictions have used them. However, because of the inter-correlation of several of the variables of interest, much of this evidence is equivocal, and experimental manipulations have failed to isolate the most important factors. Here we report an experiment which circumvents such problems by comparing the superparasitism rates of fed and starved parasitoids. By using the asexual solitary hymenopteran parasitoid Venturia canescens, we demonstrate that starved wasps with a reduced life expectancy lay eggs in low-quality hosts more frequently than those with a greater expected lifespan, as do parasitoids with higher egg loads and hence more resources available for reproduction.

Effects of simulated climate warming on macrophytes in freshwater microcosm communities

We monitored macrophyte communities (Lagarosiphon major, Elodea nuttallii, Potamogeton natans) growing in freshwater microcosms that mimicked north temperate shallow lake and pond environments. The microcosms were subjected to a 2 years simulation of two climate warming regimes (3 °C above ambient year-round and 3 °C above ambient during the summer-only), in combination with a nutrient addition (eutrophication) treatment and the presence or absence of fish. Throughout the experiment, total macrophyte abundance remained relatively high and was unaffected by warming. However, the proportion of each community made up by L. major and the growth rate of L. major increased under continuous warming. Warming did not significantly influence the abundance or growth rate of E. nuttallii. For P. natans, flowering occurred earlier in the season under continuous warming and floating leaf surface area increased under both warming treatments. The effects of nutrient addition and the presence of fish were almost exclusively warming-independent and, where of statistical significance, tended to increase and decrease macrophyte abundance, respectively. Only the growth rate of E. nuttallii was significantly increased by nutrient addition. Our observations suggest that, as a functional component of north temperate shallow lake and pond ecosystems, elodeid macrophyte communities may be broadly resilient to the small increases in temperature associated with climate warming, even when these temperature increases occur in combination with increased nutrient loading and the presence of fish. Nevertheless, it may be of some concern that the species apparently most favoured by warming in our microcosms, L. major, is an exotic which has the known potential to become problematic as a weed in temperate waters.

Lifetime reproductive success in the solitary endoparasitoid, Venturia canescens

Parasitoid wasps have long been considered excellent organisms in studies examining the evolution of reproductive and life-history strategies. In examining the lifetime reproductive success of parasitoids in the laboratory, most investigations have provided the insects with excess hosts and food, where they exist in a relatively constraint-free environment. Importantly, these conditions may not accurately reflect the true heterogeneity of natural systems, where suitable hosts and food sources are likely to be limiting. This study examines the influence of differences in host and food availability on reproductive and life-history parameters in an asexual strain of the solitary endoparasitoid, Venturia canescens (Hymenoptera: Ichneumonidae). Lifetime reproductive success in V. canescens was measured in response to temporal variations in host and food (honey solution) access. Cohorts of parasitoids were provided with 200 fifth-instar larvae of the Indian meal moth, Plodia interpunctella (Lepidoptera: Pyralidae), and food for variable periods daily after eclosion. V. canescens is synovigenic, and host-deprived wasps continued to mature eggs over the first few days after eclosion until the egg storage capacity was reached in the oviducts. When these wasps were subsequently provided with hosts, oogenesis resumed and continued until later in adult life. Constantly fed wasps lived longer and produced more progeny than wasps from cohorts which were alternately fed and starved or were starved from eclosion. Moreover, wasps with constant host and food access produced most progeny early in life and usually experienced prolonged periods of postreproductive survival. In contrast, the reproductive period of wasps with limited host access was more evenly distributed throughout the adult life. Consequently, the cumulative progeny production by V. canescens with constant food access was fairly uniform irrespective of host availability. Longevity and fecundity in V. canescens were positively correlated with adult size. However, variable host access had little effect on the longevity of wasps which were constantly supplied with honey. Over the first 2 days of adult life, variation in food access also had no effect on progeny production by V. canescens. We argue that manipulating temporal host and food access to parasitoids in the laboratory more closely approximates natural conditions, where these resources are likely to be spatially separated. Moreover, our findings suggest that many highly synovigenic parasitoids like V. canescens, which produce microtype (=hydropic) eggs, have a considerably higher reproductive potential than ovary dissections have revealed. Our findings are discussed in relation to life-history evolution in the parasitic Hymenoptera.

Can't tell the caterpillars from the trees: countershading enhances survival in a woodland

Perception of the bodys outline and three-dimensional shape arises from visual cues such as shading, contour, perspective and texture. When a uniformly coloured prey animal is illuminated from above by sunlight, a shadow may be cast on the body, generating a brightness contrast between the dorsal and ventral surfaces. For animals such as caterpillars, which live among flat leaves, a difference in reflectance over the body surface may degrade the degree of background matching and provide cues to shape from shading. This may make otherwise cryptic prey more conspicuous to visually hunting predators. Cryptically coloured prey are expected to match their substrate in colour, pattern and texture (though disruptive patterning is an exception), but they may also abolish self-shadowing and therefore either reduce shape cues or maintain their degree of background matching through countershading: a gradation of pigment on the body of an animal so that the surface closest to illumination is darker. In this study, we report the results from a series of field experiments where artificial prey resembling lepidopteran larvae were presented on the upper surfaces of beech tree branches so that they could be viewed by free-living birds. We demonstrate that countershading is superior to uniform coloration in terms of reducing attack by free-living predators. This result persisted even when we fixed prey to the underside of branches, simulating the resting position of many tree-living caterpillars. Our experiments provide the first demonstration, in an ecologically valid visual context, that shadowing on bodies (such as lepidopteran larvae) provides cues that visually hunting predators use to detect potential prey species, and that countershading counterbalances shadowing to enhance cryptic protection.

Fluctuating asymmetry and lifetime mating success are correlated in males of the damselfly Coenagrion puella (Odonata: Coenagrionidae)

Abstract. 1 The influence of fluctuating asymmetry on lifetime mating success of males of the damselfly Coenagrion puella (Odonata: Coenagrionidae) was investigated. Fluctuating asymmetry was measured as the difference in length of left and right fore and hind wings. 2 Males with more symmetrical wings enjoyed higher lifetime mating success. 3 Larger males, in contrast to a previous study of this species, also had higher mating success. This may be attributed to differences in the weather conditions prevailing at the time of the studies.

The effect of superparasitism on development of the solitary parasitoid wasp, Venturia canescens (Hymenoptera: Ichneumonidae)

Abstract. 1 The effects of superparasitism on the rate of development, adult size and mortality of Venturia canescens (Hymenoptera: Ichneumonidae) were investigated. Parasitoids were reared from third (L3) and fifth (L5) instars of one of its hosts, Plodia interpunctella (Lepidoptera: Pyralidae) containing one, two or four parasitoid eggs. 2 Superparasitism increased the development time of Venturia reared from both instars, but the developmental delay was more evident in wasps from L5 hosts. 3 The size of parasitoids from L3 hosts was unaffected by egg number, but wasps from both superparasitized L5 treatments were significantly smaller than those from singly parasitized hosts. 4 Parasitoid mortality was significantly higher in L5 than L3, but within instars did not differ significantly with egg number. 5 The results confirm that superparasitism may affect the fitness of both the adult female wasp and her progeny, and should therefore be incorporated into models of superparasitism as an adaptive foraging strategy.

The effect of host nutrition on growth and development of the parasitoid wasp Venturia canescens

We investigated the effect of host (Plodia interpunctella; Lepidoptera: Pyralidae) nutritional status on development of the solitary endoparasitoid, Venturia canescens (Hymenoptera: Ichneumonidae). Parasitoids from 3rd (L3) instars reared on a deficient diet during early parasitism took longer to develop and suffered higher mortality than those reared from hosts fed ad libitum although there was not a significant difference in the size of eclosing wasps from the two groups. L5 hosts reared at high density produced smaller parasitoids, which developed more rapidly than those reared from hosts from low density containers, although mortality was higher in the latter.

Countershading enhances cryptic protection: an experiment with wild birds and artificial prey

Of the many traits seen in cryptic prey animals, countershading (darker pigmentation on those surfaces exposed to the most lighting) is one of the commonest, and paradoxically one of the least understood. Countershading has been hypothesized to enhance crypsis by shadow-obliteration, in which lighter coloration on the undersides compensates for increased shadow in these regions, thus reducing detection by visually hunting predators. We tested the hypothesis that countershading enhances crypsis in two experiments with artificial prey presented to free-living birds. In the first experiment, artificial prey were presented on lawns to a range of bird species. In the second experiment, the prey were presented on green boards to individual blackbirds, Turdus merula. In both experiments countershaded prey had significantly lower levels of predation than controls. Our results show that countershading can enhance cryptic protection and has important implications for the evolutionary ecology of prey defences.

Evolutionarily stable foraging speeds in feeding scrambles: a model and an experimental test

An evolutionarily stable strategy (ESS) model of scramble effort (for example, foraging speed) in competition for food or mates, where payoff may depend on competitor density, is described. An individual can increase speed (at some energetic cost) to gain a greater share of resources. The predictions are that if food input per competitor is constant (density independence), the ess foraging speed should increase with density, but if food input per patch is constant (density dependence), then the ESS foraging speed should decrease with density. The predictions of this model are tested in an experiment using different densities of cichlid fish, Aequidens portalegrensis, scrambling for food at a discrete resource patch. The results provide support for the model in a situation where payoffs are density dependent, but fit less well where they are density independent.