- I conducted field studies to determine the effect of insect herbivory and
intraspecific plant competition on ragwort Senecio jacobaea. The objectives were to
determine the patterns and causes in the distribution of the ragwort flea beetle
Longitarsus jacobaeae foraging among varying densities of ragwort, to measure the
behavioral and numerical responses of the beetle to changes in ragwort density, and to
estimate the impact of insect herbivory and intraspecific competition on ragwort
performance. Host density was manipulated by planting 1, 4, 8, or 16 plants per 0.5 x
0.5 m patch. Beetles were counted in each patch to assess the effect of host density on
the beetle population. I measured four components of reproductive success represented
by growth rate, development rate, reproduction, and annual survivorship to assess the
effect of herbivory and intraspecific plant competition on ragwort performance
In the first experiment, beetle populations were manipulated by establishing equal
numbers of beetles in patches with unequal number of hosts (1, 4, 8, 16 plants per
patch), which were then subsequently allowed to move freely about. Beetles rapidly re-distributed
themselves, such that the number of beetles was strongly and positively
correlated with the number of hosts. This indicates that ragwort flea beetles are highly
sensitive to local distribution of their food plants.
In the second experiment, host density was manipulated by planting ragwort in
densities of 1, 4, 8, 16 plants per patch, and beetles were then allowed to colonize the
experimental patches. Beetle behavioral response to a change in host density was dependent on host population size: the numbers of colonizing beetles increased asymptotically with increasing plant density. The number of beetle-days ranged from 261 for 1-plant patches to 1822 for 16-plant patches. In contrast, the numerical response (represented as observed multiplication rate per capita per generation per year) appears to be inexplicably low in the single plant population and levels off in the 4, 8, and 16 plant patches (grand mean for multiplication rate 1 was 5 and for multiplication rate 2 was 10.4 progeny per individual per generation). Combining these results, the beetles apparently respond to spatial variation in the density of hosts primarily by changes in their movement behavior rather than by changes in their per capita reproductive rates. These results highlight the importance of a natural enemy's colonizing behavior for controlling a sudden upsurge in pest abundance.
Both insect herbivory and intraspecific competition had an effect on ragwort performance. For example, over approximately one year, ragwort's rate of biomass accumulation was 48% lower, and seed-head production was 18% lower in exposed compared to protected plots, while intraspecific competition reduced ragwort's rate of biomass accumulation and seed-head production, such that a 16-fold increase in host density (in protected patches) led to a 12-fold decrease in biomass per plant and a 11-fold decrease in the number of seed-heads per plant. Herbivore effects were independent of host density: variation in plant density from 1 to 16 plants led to no detectable change in magnitude of the herbivore effect. This suggests there is no density-dependent refuge for host plants operating at these local scales of observation. Keywords: Host density effects, behavioral response, reproductive response, biological control agent, Longitarsus jacobaeae, Senecio jacobaea.