Robert G. Lalonde

Host Confusion and the Evolution of Insect Diet Breadths

It is observed that many herbivorous insects develop well, or even better, on hosts which they will not use than on hosts they regularly use. We present a verbal model which explains this often imperfect correlation between host use and host suitability. Our model focuses on limitations in an insects ability to perceive and integrate host-associated stimuli. We argue that confusion of «good» hosts with «poor» hosts results from an imperfect identification process and will result in selection for either a change in the behavioral response of an insect to host associated stimuli, or a change in the insects ability to perceive and integrate these stimuli. Specifically, we argue that there are four evolutionary solutions for dealing with the confusion of «good» and «poor» hosts

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.

OPTIMAL OFFSPRING PROVISIONING WHEN RESOURCES ARE NOT PREDICTABLE

I begin with two basic premises: first, that organisms frequently must base offspring allocation decisions on expected levels of resource availability, and second, that allocated resources are at least partially nonrecoverable. Given this, how many offspring should an organism begin to provision? Here, I develop an analytical model to address this question. In this model I make offspring fitness a function of the integral of all possible resource states, multiplied by the probability density function for parental resources. Parental fitness is the product of offspring fitness and the number of offspring. I solve this basic model numerically and find that the optimum offspring number for iteroparous organisms is not strongly affected by variance. In contrast, semelparous organisms maximize their expected reproductive success by initiating fewer offspring as variance increases. I use my results to provide a theoretical explanation for seasonal seed size decline in plants.

Seasonal effects on superparasitism by Rhagoletis completa

1. We investigate a host-parasite system in which superparasitism is a more viable strategy than is initial parasitism. Walnut husk flies (Rhagoletis completa Cresson) readily use sting holes made by conspecifics as oviposition sites. 2. Superparasitism is probably a viable strategy because husks have sufficient food for more than one flys offspring and are difficult to parasitize initially due to the toughness of the husks. Flies preferentially attacked nuts with significantly softer husks than those which were unattacked. 3. Furthermore, flies oviposited in nuts with experimental oviposition punctures in preference to unmanipulated controls at the beginning of the season when husks were tough

Host marking enhances parasitism risk for a fruit-infesting fly Rhagoletis basiola

We demonstrated that chemical marking of hosts following oviposition by the rosehip fly, Rhagoletis basiola, increases risk of egg parasitism by the wasp Halticoptera rosae. This is because wasps allocate more time and locate eggs more readily when searching marked versus unmarked hosts. Thus, the benefit of host marking (reduced rate of larval competition) may be offset by increased risk of parasitism. We considered this tradeoff through development of analytical parasitoid-host models that evaluate the success of marking and non-marking flies in the presence of parasitoids

Host selection behavior of a thistle-feeding fly: choices and consequences

SummaryFemale Canada thistle seed flies (Orellia ruficauda) preferentially oviposit into seed heads which are a single day from opening. When flies are forced to oviposit into flower heads at other stages of development, offspring typically do slightly poorer: they attain a mature mass of about 15% less than do larvae derived from preferred hosts. Larval mass correlates strongly with reproductive success: heavy larvae develop into adults that produce eggs at a faster rate than do those developing from small larvae. After laying a clutch of eggs, flies circumscribe the rim of the flowerhead with their extended ovipositor and deposit a clear fluid. Flies reject previously-attacked hosts, bearing this apparent marking pheromone, significantly more often than they reject unattacked hosts. Costs of superparasitism in this system are relatively small, inasmuch as there is only a weak relationship between clutch size and larval success at the densities measured in this study. We speculate that flies are highly selective, when the apparent costs of making a mistake are rather low, because the information provided by phenological cues and by the putative marking pheromone is highly reliable, and low fecundity and time costs allow sufficient time to express a high level of discrimination.

Field studies of seed predation in an introduced weedy thistle

Predation on flower heads of Cirsium arvense by larvae of Orellia ruficauda has a remarkably small impact upon survivorship of achenes; either at the population level, or the level of the individual seed head. Flies avoid laying eggs into male flower heads and preferentially exploit female flower heads, but larvae in heads at typical field densities do not consume a large proportion of the available achenes. There were no apparent effects of crowding upon larval size at maturity at the densities measured in the field. Furthermore, there is no apparent relationship between per head larval density and the probability of dying due to either cannibalism, parasitism or disease

Chaotic Dynamics Can Select for Long‐Term Dormancy*

Extended dormancy in a population is evolutionarily costly unless some variance in season‐to‐season fitness (usually driven by variance in environmental quality) makes bet hedging useful. Consequently, dormancy in a population is usually accepted as evidence of environmental variance. Using a Ricker‐type model with heritable variation in dormancy, we show that this need not be so. Intrinsic population dynamics can generate chaotic fluctuations in the absence of environmental variance. Chaotic dynamics increase the frequency of a range of dormant strategists under natural selection, even when mortality during dormancy is relatively high. The buffering effect of dormant individuals then eliminates chaotic dynamics or generates periodic orbits of relatively low amplitude. These stabilized populations harbor a high frequency of dormant individuals that express a range of propensities to enter dormancy.

Catching ariadne by her thread: how a parasitoid exploits the herbivore's marking trails to locate its host

Chemical signals that can be associated with the presence of a host insect often work as arrestants in close range host location by parasitoids, leading to longer searching times on patches where such signals are present. Our current view of parasitoid host location is that by prolonging the search times in patches, randomly searching parasitoids enhance their chance of detecting host insects. However, prolonged search times are not necessarily the only modification in parasitoid behaviour. In this study, we examine the exploitation of host‐fruit marking pheromone of rose‐hip flies, Rhagoletis basiola Osten‐Sacken (Diptera: Tephritidae) by the specialized egg‐larval parasitoid Halticoptera rosae Burks (Hymenoptera: Pteromalidae). We provide evidence that the instantaneous probability that a host egg will be located by a searching parasitoid wasp differs markedly between pheromone‐marked and unmarked fruits. The arresting response to the marking pheromone, i.e., the prolonged time a wasp is willing to search on marked fruits, can only account for a small fraction of the difference in successful host location on marked and unmarked fruits. We further demonstrate that the time wasps require to locate the host egg is independent of the size of the rose‐hip harbouring the fly egg, and thus is independent of the area the wasp potentially has to search. A comparison of our findings with results of different search algorithms for parasitoid wasps suggests that wasps use the flys pheromone marking trail as a guide way to the flys oviposition site and thus the host egg.

Nontarget herbivory by a weed biocontrol insect is limited to spillover, reducing the chance of population-level impacts

Insects approved for classical biocontrol of weeds are often capable of using close relatives of their target weed for feeding, oviposition, or larval development, with reduced preference and performance. When nontarget herbivory occurs and is suspected to reduce survival, growth, or fecundity of individual plants, and insects are capable of reproducing on their nontarget host, characterization of spatial and temporal patterns of the occurrence and intensity of herbivory is valuable for predicting potential population-level effects. Here, we perform a novel post-release manipulative field experiment with a root-feeding biocontrol weevil, Mogulones crucifer, released in Canada to control the rangeland weed Cynoglossum officinale, to test for its ability to establish on the nontarget plant Hackelia micrantha. After Cynoglossum, M. crucifer exhibits its highest preference for and performance on Hackelia spp. We released M. crucifer on Canadian rangeland sites with naturally occurring populations of H. micrantha growing interspersed with the target weed or in the near absence of the target weed. Adult weevil feeding on surrounding plants was monitored for three summers after release (years 0, 1, and 2), and, subsequently, subsets of plants were destructively sampled to determine M. crucifer oviposition levels. Additional oviposition and larval development data were obtained from seven non-experimental sites where weevils were released zero, three, or four years earlier. M. crucifer was not detected on experimental sites without C. officinale after two years, and nontarget herbivory was restricted to rare, low-level spillover. Visible evidence of adult herbivory (i.e., scars on shoots) was associated with oviposition in 90% of targets but only 30% of nontarget plants. We infer, through ecological refuge theory, that nontarget population-level impacts from M. crucifer spillover are unlikely because of temporal, spatial, and probabilistic refuges from herbivory, and make recommendations for monitoring and management of biocontrol systems with similar attributes, such as removing target plants around nontarget populations of interest. Because M. crucifer is among the least host-specific of the modern weed biocontrol agents, and H. micrantha is likely one of its most highly preferred nontargets, these conclusions are, arguably, generally applicable to other nontarget plants and biocontrol systems.