Model Comparison Tests to Determine Data Information Content.

In the context of inverse or parameter estimation problems we demonstrate the use of statistically based model comparison tests in several examples of practical interest. In these examples we are interested in questions related to information content of a particular given data set and whether the data will support a more complicated model to describe it. In the first example we compare fits for several different models to describe simple decay in a size histogram for aggregates in amyloid fibril formation. In a second example we investigate whether the information content in data sets for the pest Lygus hesperus in cotton fields as it is currently collected is sufficient to support a model in which one distinguishes between nymphs and adults. Finally in a third example with data for patients having undergone an organ transplant, we question whether the data content is sufficient to estimate more than 5 of the fundamental parameters in a particular dynamic model.

The effect of Wolbachia on the lifetime reproductive success of its insect host in the field.

Wolbachia is a widespread endosymbiont that induces dramatic manipulations of its host's reproduction. Although there has been substantial progress in the developing theory for Wolbachia-host interactions and in measuring the effects of Wolbachia on host fitness in the laboratory, there is a widely recognized need to quantify the effects of Wolbachia on the host fitness in the field. The wasp Anagrus sophiae, an egg parasitoid of planthoppers, carries a Wolbachia strain that induces parthenogenesis, but its effects on the fitness of its Anagrus host are unknown. We developed a method to estimate the realized lifetime reproductive success of female wasps by collecting them soon after they die naturally in the field, counting the number of eggs remaining in their ovaries and quantifying Wolbachia density in their body. We sampled from a highly infected A. sophiae population and found no evidence for Wolbachia virulence and possible evidence for positive effects of Wolbachia on realized reproductive success.

The evolution of polyembryony in parasitoid wasps.

Polyembryony has evolved independently in four families of parasitoid wasps. We review three main hypotheses for the selective forces favouring this developmental mode in parasitoids: polyembryony (i) reduces the costs of egg limitation; (ii) reduces the genetic conflict among offspring; and (iii) allows offspring to adjust their numbers to the quality of the host. Using comparative data and verbal and mathematical arguments, we evaluate the relative importance of the different selective forces through different evolutionary stages and in the different groups of polyembryonic wasps. We conclude that reducing the cost of egg limitation is especially important when large broods are favoured. Reducing genetic conflict may be most important when broods are small, thus might have been important during, or immediately following, the initial transition from monoembryony to polyembryony. Empirical data provide little support for the brood-size adjustment hypothesis, although it is likely to interact with other selective forces favouring polyembryony.

Predation on immature parasitoids and its impact on aphid suppression.

Many predatory arthropods eat both unparasitized herbivores and herbivores that are parasitized and contain the immature stages of endoparasitoids, a form of intraguild predation. Thus, the biological control of herbivorous arthropods can be either enhanced or disrupted by introducing a predator species to an existing host-parasitoid system. We evaluate the impact of introducing a predator, the convergent ladybird beetle, Hippodamia convergens, on the biological control of the cotton aphid, Aphis gossypii, by the parasitoid Lysiphlebus testaceipes, under field conditions. Predation on immature parasitoids by H. convergens was intense: 98-100% of aphid mummies were consumed by the end of the experiment, and H. convergens substantially reduced immature parasitoid populations. Despite the negative impact of H. convergens on aphid parasitoids, aphid population suppression was greatest in treatments containing both H. convergens and parasitoids. The parasitoid alone or in combination with H. convergens suppressed cotton aphids in a density-dependent manner and increased total plant leaf area and biomass, H. convergens did not substantially alter the percentage of aphids mummified by parasitoids and showed a partial feeding preference for unparasitized aphids over aphid mummies. We conclude that under conditions where a predator shows both a partial preference for unparasitized hosts and high levels of predation on unparasitized hosts, we may expect the predator to improve suppression of herbivores even if it produces high levels of intraguild predation. While intraguild predation is an important ecological interaction in the early-season cotton agroecosystem, it does not disrupt cotton aphid biological control.

Egg maturation, egg resorption and the costliness of transient egg limitation in insects.

Although there is widespread agreement that the cost of oviposition underlies selective oviposition in insects, there is no consensus regarding which factors mediate the cost of oviposition. Models have suggested that egg costs are often paramount in those insects that do not continue to mature eggs during the adult stage (pro-ovigenic insects). Here we address the hypothesis that egg costs are generally less significant in synovigenic insects, which can replenish oocyte supplies through continuous egg maturation. A dynamic optimization model based on the biology of a highly synovigenic parasitoid, Aphytis aonidiae, suggests that the maximum rate of egg maturation is insufficient to balance the depletion of eggs when opportunities to oviposit are abundant. Transient egg limitation therefore occurs, which imposes opportunity costs on reproducing females. Thus, whereas the most fundamental constraint acting on the lifetime reproductive success of pro-ovigenic species is the fixed total number of eggs that they carry at eclosion, the most fundamental constraint acting on a synovigenic species is the maximum rate of oocyte maturation. Furthermore, the ability of synovigenic species to reverse the flow of nutrients from the soma to oocytes (i.e. egg resorption) has a dramatic influence on the cost of oviposition. Whereas females in hostrich environments may experience oviposition-mediated egg limitation, females in host-poor environments may experience oosorption-mediated egg limitation. Both forms of egg limitation are costly. Contrary to initial expectations, the flexibility of resource allocation that typifies synovigenic reproduction actually appears to broaden the range of conditions under which costly egg limitation occurs. Egg costs appear to be fundamental in mediating the trade-off between current and future reproduction, and therefore are an important factor favouring selective insect oviposition.

Higher-order predators and the regulation of insect herbivore populations.

Empirical research has not supported the prediction that populations of terrestrial herbivorous arthropods are regulated solely by their natural enemies. Instead, both natural enemies (top-down effects) and resources (bottom-up effects) may play important regulatory roles. This review evaluates the hypothesis that higher-order predators may constrain the top-down control of herbivore populations. Natural enemies of herbivorous arthropods generally are not top predators within terrestrial food webs. Insect pathogens and entomopathogenic nematodes inhabiting the soil may be attacked by diverse micro- and mesofauna. Predatory and parasitic insects are attacked by their own suite of predators, parasitoids, and pathogens. The view of natural enemy ecology that has emerged from laboratory studies, where natural enemies are often isolated from all elements of the biotic community except for their hosts or prey, may be an unreliable guide to field dynamics. Experimental work suggests that interactions of biological control agents with their own natural enemies can disrupt the effective control of herbivore populations. Disruption has been observed experimentally in interactions of bacteria with bacteriophages, nematodes with nematophagous fungi, parasitoids with predators, parasitoids with hyperparasitoids, and predators with other predators. Higher-order predators have been little studied; manipulative field experiments will be especially valuable in furthering our understanding of their roles in arthropod communities.

Effects of time limitation and egg limitation on lifetime reproductive success of a parasitoid in the field.

We used field observations of freely foraging Aphytis aonidiae parasitoids in conjunction with results of laboratory studies of A. aonidiae and other Aphytis species to simulate lifetime patterns of behavior and reproduction. Field observations provided estimates of encounter rates with three classes of hosts, the mortality rate from predation on adult parasitoids, and host-handling times for oviposition and host feeding by adult wasps. A series of physiological parameters, including the egg maturation rate and the value of host-feeding meals, were estimated from previously published studies. Plasticity in parasitoid behavior was incorporated in two ways. For one set of simulations we used a behavioral rule derived empirically from observations of parasitoids made in the field, and for another we used a dynamic state-variable model to generate a set of behavioral rules that maximize lifetime reproductive success. As was expected, the empirically derived rule led to better matches with field observations than did simulations using the output of the dynamic model. Projections of lifetime reproductive success in the field ranged between three and 37 eggs within the 95% confidence intervals of the mortality rate and host encounter rate and depending on which behavioral rule was used. Lifetime reproductive success from the simulation with central estimates of the mortality and host encounter rates that incorporated the empirical rule was 6.25 eggs. Using the empirical versus the theoretical rule in the simulations led to a 10%-30% decline in projections of lifetime reproductive success, depending on mortality and host encounter rates. Regardless of the behavioral rule, the simulations underscored the observation that the host encounter rate was greater than the egg maturation rate. The overall oviposition rate was sufficiently high to lead to daily episodes of temporary egg limitation during which parasitoids must mature an egg before being able to oviposit.