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Crop pests are resistant to certain insecticides

Winter oilseed rape will often receive uninvited guests in the form of pests such as the cabbage stem flea beetle and the pollen beetle. Studies at Aarhus University show that there are significant differences in the sensitivity of the pests to insecticides.

[Translate to English:] Forskere fra Aarhus Universitet har undersøgt forekomsten af resistens i en række populationer af rapsjordlopper og glimmerbøsser i rapsmarker. Foto: Janne Hansen.

Pests are the farmer's enemy. Fortunately, the relatively cool climate in Denmark means that the extent of pest infestations is limited, which in turn means that the problem with insects that have become resistant to pesticides is relatively small.


One of the places where pests often strike is in oilseed rape. In Denmark there are more than 100,000 hectares of winter rape where the primary enemy is the cabbage stem flea beetle and the pollen beetle. Other pests rarely occur at a scale that requires control measures. But how well do the insecticides work against these pests?


Scientists from Aarhus University have looked at this by examining the presence of resistance in a number of populations of cabbage stem flea beetle and pollen beetle in Denmark. At the 2015 Plant Congress, Associate Professor in the Department of Agroecology at Aarhus University, Michael Kristensen, divulged more details about the experiments.


When screening for insecticide resistance, the sensitivity of the insect to various concentrations of a given insecticide is tested in the laboratory. The beetles are collected from a number of randomly selected fields or from fields where there are known to be problems. The effect of the chemicals is then tested on the insects to be controlled. The results can provide a relatively quick response to which substances are effective and which are not, and the concentration needed for it to be effective.


Regional differences

The scientists examined 10 Danish populations of cabbage stem flea beetle, all of which proved to be sensitive to pyrethroid insecticides. For the pollen beetles the picture was somewhat different. Pollen beetles from more than 50 localities in Denmark were examined: On Zealand, they found populations that were primarily resistant to pyrethroids, whereas about only half of the populations of Jutland and Funen were sensitive to these chemicals.


The scientists also tested for resistance to neonicotinoids in the 50 populations. In general, the pollen beetles were sensitive to these substances, but there were sporadically surviving insects despite a high concentration. It is therefore important to be heedful that resistance to neonicotinoids may be developing.


Over the past year several new remedies against pollen beetles have been registered in Denmark and pollen beetles can still be controlled, but it is important to alternate between approved remedies in order to use chemicals with different modes of action.


As has been made clear from the experiments, different species of insects, and likewise populations within the same species, exhibit a natural variability in their sensitivity or tolerance to insecticides, which is reflected in the efficacy of the product used in the field.


For further information please contact: Associate Professor Michael Kristensen, Department of Agroecology, e-mail: mikr@agro.au.dk, telephone: +45 87158116