Travel costs, oviposition behaviour and the dynamics of insect–plant systems

Abstract

Insect attack can have major consequences for plant population dynamics. We used individually based simulation models to ask how insect oviposition behaviour influences persistence and potential stability of an herbivoreplant system. We emphasised effects on system dynamics of herbivore travel costs and of two kinds of behaviour that might evolve to mitigate travel costs: insect clutch size behaviour (whether eggs are laid singly or in groups) and female aggregation behaviour (whether females prefer or avoid plants already bearing eggs). Travel costs that increase as plant populations drop lead to inverse density dependence of plant reproduction under herbivore attack. Female clutch size and aggregation behaviours also strongly affect system dynamics. When females lay eggs in large clutches or aggregate their clutches, herbivore damage varies strongly among plants, providing probabilistic refuges that permit plant reproduction and persistence. However, the population dynamics depend strongly on whether insect behaviour is fixed or responds adaptively to plant population size: when (and only when) females increase clutch size or aggregation as plants become rare, refuges from herbivory weaken at high plant density, creating inverse density dependence in plant reproduction. Both herbivore travel costs themselves, and also insect behaviour that might evolve in response to travel costs, can thus create plant density dependence-a basic requirement for regulation of plant populations by their insect herbivores.

Resource concentration by insects and implications for plant populations

“…When no herbivory occurs, average plant reproduction is 7.5 flowering plants for each flowering plant. This is in line with Heard & Remer (2008); four plants plant −1 year −1 ), Myers & Risley (2000); 6.3–0.53 knapweed plants plant −1 year −1 ) after including density‐dependent plant reproduction and initial patch size density of 1–9 plants m −2 , and Rees & Paynter (1997); 25 scotch broom plants plant −1 year −1 ) if sites were available for plant colonization. The number of new plants produced by the patch i ( p i,t +1 ) is calculated by: …”

“…She showed that clumping could stabilize the insect–plant system, while overdispersion could lead to overexploitation and extinction. In Heard & Remer’s (2008) model, increased egg clumping in response to plant rarity led to inverse density dependence of plant reproduction. They concluded that the plant–insect system can be stabilized if insect oviposition responds to plant abundance so that eggs are aggregated on rare plants.…”

“…It is now commonly acknowledged that insects can influence the abundance of their host plants; however, our understanding of when insects are able to influence plant abundance remains poor (reviewed by Maron & Crone 2006). Spatial variation in herbivore load via insect aggregation on individual host plants has been suggested as a potential regulation mechanism for the populations of both plants and insects (Myers, Monro & Murray 1981; Heard & Remer 2008). Such aggregations may arise through a range of mechanisms that mediate the response of insects to plant density, for example sensory biases (visual vs. olfactory), dispersal biases, food requirements and diet breadth, competitive exclusion and predation (Kunin 1999).…”

Competitive release and outbreaks of non‐target pests associated with transgenic Bt cotton

“…, but see Rodriguez‐Saona and Thaler ). Heterogeneity in plant quality effectively increases travel costs for ovipositing females by increasing distance between high quality hosts (Heard and Remer ) and can reduce tenure time on both high and low quality hosts (Bernays ). Similarly, H. zea herbivory increased E. servus leaving rates from both damaged and nearby undamaged plants (Zeilinger et al.…”

“…Inter‐plant movement of herbivores, including leaving rates, can be a strong indicator of demographic effects from inter‐plant variation in resistance (either constitutive or induced; Bernays , Underwood ). At the same time, oviposition preference can be an important driver of herbivore population dynamics (Ohgushi , Heard and Remer ), in part because oviposition preference is often more sensitive than other life history traits to variation in plant quality (Gripenberg et al. ) and the presence of competitors (Kaplan and Denno ).…”