Pine weevil update July 2016
The large pine weevil Hylobius abietis is a major pest of trees in replanted coniferous forests in northern Europe. The use of entomopathogenic nematodes applied against developing stages for population suppression is increasingly recognised as an effective alternative to plant protection using chemical pesticides.
A series of small-scale trials was conducted over two years, in Poland and Ireland. The focus in the Polish trials was on comparing the entomopathogenic nematodes species, while the Irish trials investigated the influence of factors such as soil type on their efficacy.
Comparison of results for trials in the two countries indicate that the moist temperate climate of Ireland is more suitable for use of entomopathogenic nematodes against pine weevils than the warmer drier conditions prevailing in Poland at time of application. Nevertheless, despite drought conditions in Poland in both years, EPN parasitised up to 17% of weevil larvae there.
Effectivity in suppressing pine weevil
In Ireland, both Steinernema carpocapsae and Heterorhabitis downesi were equally effective in suppressing pine weevil populations to below the current, informal thresholds of economic damage. In northern Europe many conifer forests are on deep peat soils. entomopathogenic nematodes were equally efficient in controlling pine weevil in peat and in mineral (lithosols/regosols and acid brown earth/brown podzolics) soils.
Complementary laboratory tests showed that while the peat content of soil affects entomopathogenic nematodes host-finding, the effect depends on soil compaction. Factors affecting patterns of weevil parasitism and suppression in the field include the density and distribution of weevils within pine stumps. Results also suggest that infestation level (number of weevils per stump) can be an important factor in forecasting the success of EPN application, as there is evidence of negative density dependent parasitism (reduced efficacy when weevil densities were high).
Application methods and results
Two application methods were compared: applying the entomopathogenic nematodes suspension along the top edges of the stumps (‘top’) or to the soil around the stump (‘standard’). Results indicate that suppression can be improved by altering the method of the application, but in a manner that depends on the nematode species. For S. carpocapsae, better results can be achieved when the suspension is applied in the ‘standard’ fashion, but for H. downesi, greater weevil suppression can be achieved when the suspension is applied at the top of the stump.
Pictures: Insect traps erected over pine stumps to assess effect of nematodes on number of adult pine weevils emerging (Pictures: NUIM, Ireland)