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An antagonistic insect/host-plant system : the problem of persistence Green, Wren Quinton

Abstract

Tansy ragwort (Senecio jacobaea L.), a biennial weed, is eaten, and occasionally defoliated over large areas, by larvae of the cinnabar moth (Tyria jacobaeae (L.)). .Nonetheless such outbreaks do not eradicate the plant. How does so vulnerable a plant manage to persist? and likewise, how can a herbivore that devastates its food supply persist after population crashes? My conclusions are as follows after a four-year study of this herbivore-plant system near Nanaimo, British Columbia. 1. Ragwort has no effective resistance against Tyria; its persistence depends on its ability to recover by: (a) producing a second seed crop following defoliation; (b) changing to a perennial growth form. The second seed crop was only 10% the size of a normal crop; but 50% of some experimentally defoliated plants became perennial compared with only 30% of the controls. Also, perennials were more likely to survive defoliation than were biennials. Vegetative regrowth, in fact, was the main process responsible for local persistence. Seeds enable tansy ragwort, a pioneer species, to colonize new habitats, and the secondary seed crop ensures the annual production of at least some seed for colonization. 2. Female moths usually lay each of several clusters of 30-60 eggs on a different plant. The surviving larvae from an average-sized cluster eat most of the biomass of the average tansy ragwort; hence the spacing of clusters is adaptive, and at low moth densities promotes efficient use of the food resource. At high densities, however, the distribution of clusters is contagious, since moths prefer ovipositing on large plants, and fail to discriminate against plants that already have clusters on them. Consequently many plants are overloaded (i.e. the larval food requirements exceed the plant biomass), whereas some plants have no clusters on them, and thus become food refuges. 3. These food refuges are important, I suggest, for maintaining the moth when populations crash through starvation, since they provide adequate food for the dispersing larvae that find them. Dispersal in the fifth instar is density-dependent, and is associated with an antagonistic 'head-flicking' behaviour. Larvae also disperse when food is plentiful, however, for in one population over 75% of the larvae dispersed once during the fifth instar. In that particular study larvae that dispersed had, on average, more food after dispersal than before, although the risk of not reaching a plant was sometimes high: at 0.5 plants/m² only 20% of fifth-instar larvae found plants compared with 80% at 5.0 plants/m². From these observations I conclude that under some conditions populations of the cinnabar moth can regulate their own numbers before all the host plants are stripped, but that under other conditions they cannot do so, and crash through starvation, A necessary condition for an outbreak seems to be that larvae suffer little mortality during dispersal, a condition that is satisfied when plant density is high. Data on plant density from 11 outbreak areas are consistent with this idea. The essential features of the regulatory mechanism proposed for the cinnabar moth could be identified in other insect species. Outbreaks in these species were also associated with high host-plant densities.

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