19 July 2011

Newly discovered interaction has farmers buzzing

This article was prepared by postgraduate student Sam Read as part of the ECOL 608 Research Methods in Ecology course.
Nature is full of wonderful and surprising phenomena. Organisms can often be linked directly or indirectly in amazing and unpredictable ways. It came as somewhat of a surprise when honeybees were discovered to have an impact on caterpillars. Common sense would suggest honeybees (Apis mellifera) and caterpillars would have little to do with one another, as honeybees increase plant fitness by pollination and caterpillars decrease plant fitness by herbivory.
Other relatives of the bees are not so benign. Wasps (Vespula spp.) are generalist predators and are natural enemies of many caterpillar species worldwide. Rapid wing movement while flying creates vibrations in the air, heard as a buzz. Previous research indicated that these vibrations stimulate special sensory hairs on caterpillars and pre-warn them that predators are present. The caterpillar will then stop moving and damage to the plant will cease, until the predator has gone. Occasionally, the caterpillars may even drop off the plant, if the predator gets too close.
Wasps could therefore be effective biological control agents to reduce pests. However wasps themselves are considered pests by many. Wasps are one of the most invasive insect pests in the world. They cause problems to agriculture, horticulture, wildlife and are a nuisance in urban environments due to their nesting and aggressive behaviour. Instead, a similar alternative buzzing insect is required. As honeybees are of high economic value and also create a buzz, they were thought to be a potential candidate. A new study was published in Current Biology by Jürgen Tautz and Michael Rostas (Lincoln UNiversity), in which they carried out an experiment to test whether particular herbivores would show the same behavioural reaction with honeybees, as occurs with wasps.

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A field cage experiment was carried out using the beet armyworm caterpillars (Spodoptera exigua), as they are a generalist pest that feeds on at least 50 plant species. The caterpillars were applied to either bell pepper (Capsicum annuum), with or without fruit, or soybean (Glycine max). Honeybee hives were applied to half of the treatments, with the remaining half with no hives as controls.
When the honeybees were present, there was a significant reduction in leaf damage (60.6-69.3%). The caterpillars’ behavioural reaction to the honeybee buzz was similar to the reaction triggered by a wasp buzz. Honeybees created very similar air vibrations, at an almost identical frequency (Hz) to the wasps, which the caterpillars could not distinguish between.
These findings indicate that other caterpillar species with sensory hairs may also react to the presence of honeybees. Some smaller experiments were then carried out by Michael Rostás. The cabbage moth (Mamestra brassicae), a common European moth was then tested and did behave in a similar way in the presence of honeybees. The large white butterfly (Pieris brassicae) is another species which may potentially respond in a similar manner, as it responds to the air particle movement of hand clapping. Numerous other species worldwide could also be candidates, but future success may depend on honeybee densities in the area.
Honeybees can therefore not only perform pollination by transporting pollen from flower to flower, but also they may contribute to the reduction of plant damage by some herbivores. This unexpected interaction may prove to be a useful tool in the development of future integrated pest management programs.

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