Bernard D. Roitberg

Instar-specific defense of the pea aphid,Acyrthosiphon pisum: Influence on oviposition success of the parasiteAphelinus asychis (Hymenoptera: Aphelmidae)

The parasite Aphelinus asychisWalker (Hymenoptera: Aphelinidae) oviposits in all four instars of the pea aphid, Acyrthosiphon pisum(Harris) (Homoptera: Aphididae). Searching females display a highly stereotyped sequence of behaviors when encountering a host. Once recognized, an aphid is examined and probed by the wasp with the everted ovipositor prior to oviposition. Oviposition success is influenced by aphid behavior that is related to aphid size and expressed through instarspecific escape and defense reactions. Being smaller and less able to defend themselves, first and early-second instars of pea aphid are more susceptible to successful parasitism than third and fourth instars, in that order. Observed patterns of preference by Aphelinus females for particular aphid species and instars reflect the outcome of behavioral interactions between the hosts and the parasites, rather than preference in the strict sense.

On the evolutionary ecology of marking pheromones

SummaryMany parasitic insects mark hosts with a pheromone after oviposition. The evolutionary ecology of such marking pheromones was studied to determine (i) under what ecological and behavioral conditions such pheromones could evolve and (ii) why so many of these marking pheromones are water-soluble and thus short-lived. We used a number of different techniques. First, the fitness values of individual normal (nonmarking) and mutant (marking) insects foraging for hosts were computed using dynamic state-variable models. Second, population level models were used to study when a population of non-marking individuals can be invaded by marking individuals. Third, behavior-rich simulations (developed originally for apple maggot,Rhagoletis pomonella) were used to test ‘experimentally’ some of the hypotheses generated using the individual and population-level models. Finally, we developed a model for the ‘benefit’ over time to an individual by marking. This model shows that when benefit is measured in terms of larval survival, nearly all of the benefit to a mother is obtained from short-lived marks. Genetical theories of pheromone evolution and the connection between our results and existing theories of altruistic behavior are discussed.

THE INFLUENCE OF PREDATORS ON THE MOVEMENT OF APTEROUS PEA APHIDS BETWEEN PLANTS

SUMMARY (1) This paper examines the influence of predators on the dispersal of apterous pea aphids in the laboratory and field. (2) Individual pea aphids can be placed in one of two categories, searchers and runners, depending upon the behaviour they exhibit after dropping from their host plant. (3) In field cages, pea aphids readily disperse to new host plants in the presence of predators. Dispersal of apterous aphids is rare when active predators are absent. (4) Evidence is presented which suggests fecundity is reduced the day after an aphid disperses. (5) The distance dispersed by aphid nymphs is positively correlated with the density of aphids on the plant the aphid leaves. (6) The significance of dispersal by apterous aphids is discussed in the light of evidence presented.

Foraging Behaviour of Rhagoletis pomonella, a Parasite of Hawthorn (Crataegus viridis), in Nature

(4) Flies never or rarely oviposited in non-host and marked fruit respectively, and in both cases emigrated from trees harbouring those fruit soon after examining the fruit. (5) Flies exhibited success-motivated search following discovery of and oviposition in uninfested, unmarked fruit. (6) Flies visited more fruit, oviposited more often and remained longer in trees harbouring high v. low densities of fruit clusters. (7) Flies emigrated sooner after the last egg they laid on trees harbouring high, v. low, densities of fruit clusters (Giving Up Time shorter). (8) Rhagoletis pomonella foraging behaviour is discussed in relation to current foraging theory.

Does mother really know best? Oviposition preference reduces reproductive performance in the generalist parasitoid Aphidius ervi

The reproductive success of female parasitoids is dependent on their ability to accurately assess the suitability of a host for larval development. For generalist parasitoids, which utilize a broad range of species and instars as hosts, a set of assessment criteria determines whether a host is accepted or rejected. The suitability of a host, however, can only be imperfectly assessed by the female parasitoid, which can result in the selection of lesser quality hosts for oviposition. In this study we explored the disparity between host quality and host preference using the generalist koinobiotic parasitoid Aphidius ervi Haliday (Hymenoptera: Aphidiidae) and the host Aulacorthum solani (Harris) (Homoptera: Aphididae), the foxglove aphid. The second instar hosts produced the highest level of reproductive success, while third and fourth instars resulted in a substantially reduced reproductive performance. When given a choice of host instars, parasitoids preferred the older hosts for oviposition disregarding their reduced suitability for larval development. Results are discussed in context of mechanisms involved in A. ervi host selection and biases in the criteria used to assess hosts that may arise when parasitoids transfer host species between generations.

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

On the Evolution of Masting Behavior in Trees: Predation or Weather?

We address the problem of parasite-mediated selection for the evolution of masting (episodic, synchronized seed production) in trees. As a strategy, masting works very well in swamping seed predators when it is common in the population. It is not clear, however, how such a behavior can invade a nonmasting ancestral population when parasites and predators are kept at high densities by an annual abundance of fruits. We demonstrate that rare masting behavior enjoys higher fitness than nonmasting, as long as seed parasites are aggregated and all masters are perfectly synchronized. We develop a dynamic optimization model that demonstrates that synchronized fluctuations in seed production can develop as long as there is some year-to-year variation in weather-mediated resource availability. We suggest that synchronized fluctuations in seed production must exist before any predator or parasite can select for masting.

Ontogeny of alarm pheromone secretion in pea aphid, Acyrthosiphon pisum.

When attacked by a predator, an aphid may secrete a droplet of fluid from its cornicles containing a volatile alarm pheromone component, (E)-β-farnesene. This study investigated both qualitative and quantitative aspects of alarm pheromone production in the pea aphid, Acyrthosiphon pisum. The best predictor of cornicle droplet emission was reproductive phase, rather than instar, as prereproductive aphids were more likely to secrete cornicle droplets than either reproductive or postreproductive individuals. Analogously, alarm pheromone amounts were highest in prereproductive aphids. (E)-β-Farnesene quantities (mean ± SE) increased significantly from first instar (1.5 ± 0.6 ng) to second instar (11.2 ± 3.7 ng) and did not significantly change during third (12.8 ± 3.0 ng) or fourth instars (11.0 ± 3.7 ng). Alarm pheromone amounts then decreased significantly in adults (4.8 ± 2.3 ng). We suggest that prereproductive aphids have been selected to produce higher levels of pheromone because of their more clustered colony structure and higher levels of predation, as compared with adult aphids.

To mark the host or the patch: Decisions of a parasitoid searching for concealed host larvae

We found evidence for patch marking in the parasitic wasp Halticoptera laevigata (Hymenoptera: Pteromalidae) foraging for concealed hosts. Wasps attack larvae of the fruit fly Myoleja lucida (Diptera: Tephritidae) in fruits of honeysuckle. A special feature of this host-parasitoid system is the limited food supply of a patch (i.e. a fruit of honeysuckle), which allows the successful development of only a single host fly larva. Females of the parasitoid H. laevigata were found to mark the host patch with a pheromone and to abandon the patch following oviposition into a single host larva. Field data revealed that eggs of the parasitoid were spread out evenly among infested patches, with several larvae of the host fly left unparasitized in those patches that contained more than one host. Since many parasitic insects mark the parasitized host after oviposition, we assumed host marking to be the ancestral character state and studied the patch-marking behaviour of H. laevigata as a derived character state as an alternative foraging strategy. We used stochastic dynamic modelling to investigate under what conditions mutant (patch) markers would be able to invade a population of normal (larval) markers. The models suggested that, under a variety of conditions, wasps marking the patch obtained higher fitness than wasps only marking the larva. Consequently, the results from our model predict the evolution of the patch-marking behaviour found in the empirical investigation. Finally, we discuss alternative pathways to the evolution of patch marking and point out under what circumstances the evolution of a patch-marking behaviour can generally be expected.

Individuals on the Landscape: Behavior Can Mitigate Landscape Differences among Habitats

We present a case study of the rose hip fly, Rhagoletis basiola, to demonstrate how one can connect landscape to population and evolutionary dynamics through the responses of individuals. Survey data from six different isolated rose habitats (Rosa sp.) near Vancouver, Canada were analyzed to determine the spatial distribution of rose hips within and among bushes. Rose hips were clumped at all sites; there was statistically significant variation in degree of clumping among sites. However, analyses using dynamic life history theory suggest that much of this variation may be mitigated by oviposition and movement response of individual flies to fruit distribution : sites that differ structurally may not differ evolutionarily. With this in mind, we provide five different indices that could be used to evaluate fly performance on different landscapes: I) the probability that a cell r units away contains resource, given that the current cell contains resource, 2) the probability that a cell r units away contains resource, given that the current cell is devoid of resource, 3) the optimal distance to move from a cell that harbors fruit, 4) the optimal distance to move from a cell that is devoid of fruit, and 5) the Expected Reproductive Success of a fly in a particular habitat. Those indices provide the link between landscape and individual behavior and suggest another way of addressing habitat conservation issues.