Effects of Mancozeb on biocontrol agents

In using biocontrol agents, we need to consider the effects of not just insecticides and miticides, but also fungicides. Most fungicides tend to have less of a negative effect on biocontrol agents. However, there are some old standards that do exert negative effects on some of the commonly used biocontrol agents. One such fungicide is mancozeb, the active ingredient in Manzate 200 DF which is registered by Dupont Canada for use on greenhouse tomato for management of early and late blight, and septoria leaf spot.

Mancozeb is a fungicide that was released for use by Dow AgroSciences since 1961. It is a very useful fungicide particularly since there are no recorded incidences of resistance despite intensive use worldwide. It is broadspectrum in its activity, acts by contact and has high protectant activity. The reason for lack of resistance to this fungicide is attributed to its mode of action. Chemical properties of mancozeb enable it to act on various "sites" in the physiology of fungi, and hence they are called "multisite inhibitors." Because of this multi-pronged attack on disease-causing organisms, it is very difficult for resistance to develop towards fungicides like mancozeb. By contrast, fungicides such as Benlate and Ridomil that act on a single site, result in rapid development of resistance, if used frequently. However, it is the multisite property of fungicides that make them liable to have an effect on organisms other than the target diseases.

Mancozeb is known to have negative side-effects on a family (Phytoseiidae) of predatory mites that include species commonly used in greenhouse crops. These species include Amblyseius cucumeris which is used for thrips control and Amblyseius californicus. The exception in this family is Phytoseiulus persimilis which was not negatively affected in lab and semi-field tests. To further investigate the effects of mancozeb on other biocontrol agents, more recent lab and field studies were undertaken by Dow AgroSciences. Results of these studies were presented at a meeting on "Pesticides and Beneficial Organisms" held in D?be, Poland, September 27-30, 2005. These studies demonstrated that high rates of mancozeb were only slightly harmful to Orius sp., and harmless to Aphidius sp. after seven days. By contrast, comparatively low rates were highly toxic to Amblyseius species after just one application.

The implication of this information is that if predatory mites are being used for management of thrips and spider mites, populations of such predators may be severely reduced if mancozeb is applied for fungal diseases. Repeated applications of this fungicide can therefore result in disruption of biocontrol programs for thrips and spider mites. Careful consideration must therefore be given, not just to insecticides and miticides, but also to the choice of fungicides incorporated in IPM programs.

Support from the Ontario Ministry of Agriculture, Food and Rural Affairs and the Ontario Greenhouse Vegetable Growers to attend the meeting on "Pesticides and Beneficial Organisms" in Poland is gratefully acknowledged.

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