Numerous economically and medically important insect species have been targets of genetic control efforts to reduce or eliminate their negative impacts. Selfish genes (coupled with deleterious alleles) are one possible genetic mechanism that could be deployed to suppress pest populations. We develop a population dynamics model describing how such control may be effective from a theoretical perspective, using a bark beetle – conifer system as a case study . We consider a cellular automaton model, allowing diffusion of beetles between cells, and assess whether beetle populations can be suppressed and maintained below outbreak thresholds. We found, that for biologically-relevant parameter values, this selfish genetic element is maintained in the population and that local beetle populations became suppressed or extirpated without substantially depleting their conifer resource. Under certain conditions, beetle movements between cells were sufficient to maintain suppressed beetle populations, which suggests that genetic control of bark beetle pests may be possible.
Insects and other arthropods face a number of (non-cognitive) decisions while foraging for resources, including whether to accept a current site for exploitation, which resources to exploit, when to abandon a resource patch, etc. These problems are often exacerbated by the state of the organism e.g. the state of its energy reserves, its knowledge state, eggload or ovipositor wear. We will use 2 examples that illustrate how we study in these problems theoretically and experimentally. In the first example, a foraging parasitic wasp depletes a patch of hosts that may harbour predators. We develop a dynamic state variable model to predict when a wasp will exit such a patch and then test our predictions in controlled experiments. In the second example, we explore the role of energy limitation in a bark beetle making host (habitat) selection decisions. Using a dynamic state variable model, we develop predictions about the quality of host trees accepted by energy-starved and non-starved beetles, and present experimental results testing predictions.
I tested the role of energy reserves on the host tree acceptance of mountain pine beetle (MPB) adults on host trees. Newly-emerged adult MPBs were kept at either high or low temperatures, depleting their lipid reserves at different rates, and thus to different levels. Beetles were introduced onto girdled lodgepole pine trees of either poor or high quality, and their willingness to accept the host was measured. Survival analysis showed low energy beetles accepted both poor quality trees sooner than did high energy beetles, and beetles on good quality trees accepted sooner than beetles on poor quality trees.
Individual variation in energy reserves and timing of emergence are expected to constrain host selection decisions of mountain pine beetle. We examined the timing and condition of emerging beetles, describing the probability of emerging on a particular day with a particular condition, which can be used in models of individual host selection and attack.
Reproduction in mountain pine beetle (Dendroctonus ponderosae Hopkins) is contingent upon successful search for and attack of a suitable host tree (Pinus spp., especially lodgepole pine, Pinus contorta var. latifolia). This search for a host involves both visual and olfactory cues, including beetle-produced pheromones involved in ‘mass attack’ which is required to overcome host defences. Beetles can be choosy while making host selection decisions, discriminating based on host size, quality, and defensive capability, as well as conspecific (attack) densities. However, the extent of their ‘choosiness’ appears to be modulated by individual energy (i.e. lipid) reserves. A stochastic dynamic programming model of individual beetle host-selection, based on an individual’s energy state, is presented. Field data are incorporated into model parameterization and the effects of host availability, quality, and vigour on individual’s decisions and their subsequent implications for attack dynamics of the population are discussed.