|Author: ||J.A. Stenberg|
|Keywords: ||Galerucella sagittariae, Asecodes mento, Fragaria × ananassa, biological control, density dependence, herbivory|
Organic strawberry (Fragaria × ananassa) plantations are often attacked by harmful leaf beetles of the genus Galerucella (Coleoptera: Chrysomelidae). An important natural enemy of Galerucella beetles is the hymenopteran parasitoid Asecodes mento. Here I describe an observational study, investigating the effects of plant density on beetle density and parasitism in Finnish organic strawberry plantations.
Eleven organic strawberry plantations, in the Pohjanmaa region of Finland, were selected.
All plantations were colonized by G. tenella, which is a common Galerucella species in northern Europe and Russia.
The density of strawberry plants varied between 3.2 and 7.1 plants/m2. At the beginning of July 2009 mean larval densities (larvae/plant) and parasitism (percentage of larvae parasitized by A. mento) were recorded in each plantation.
Mean larval densities varied between 1.9 and 33.2 larvae per plant and were strongly and negatively correlated to plant density.
Parasitism varied between 0 and 75%, and was not significantly related to plant or larval density.
The data suggest that the parasitoid can function as an important biocontrol agent against G. tenella, but parasitism varies widely between sites.
The reason for the low beetle densities in dense plantations is currently unknown; elucidating the mechanism behind this pattern will be important in order to understand the factors that regulate G. tenella in the field.
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