Greenhouse Grower Notes: May 2007 1

January 18, 2008
Written by Gillian Ferguson
The recent registration of Distance™ for control of whiteflies on greenhouse crops is welcome news. This product is called a juvenile hormone mimic, and inhibits egg hatch, directly or through the females, and prevents adult formation by affecting the larval and pupal stages. However, growers have to be keenly aware of the need to manage its use to minimize and delay the development of resistance in whiteflies to this product. The historical ease with which whiteflies have developed resistance to pesticides supports the need for such vigilance.

RESISTANCE IN WHITEFLIES
Both sweet potato (Bemisia tabaci) and greenhouse (Trialeurodes vaporariorum) whiteflies have developed resistance to a wide range of insecticides. Resistance to major older chemical families, such as organophosphates, carbamates, and pyrethroids, is well documented. Relatively recent insecticides with new modes of action, such as neonicotinoids (e.g. imidacloprid or Intercept™) and insect growth regulators (e.g. pyriproxyfen or Distance™), have also encountered resistance where such products have been used without careful consideration and use of resistance management strategies.

Resistance to imidacloprid was first demonstrated in sweet potato whitefly from greenhouses in the Almeria region of southern Spain. Resistance to pyriproxyfen was first detected in Israel during the early 1990s, just two years after its registration. At that time, a high level of resistance was found in sweet potato whiteflies collected in a greenhouse after three successive applications of pyriproxyfen were made. The resistance affected inhibition of egg-hatch much more so than suppression of adult formation. Researchers found that there was more than a 500-fold increase in resistance to pyriproxyfen in the egg-hatch stage, and about a 10-fold increase in resistance at the adult formation stage. Although use of pyriproxyfen had ceased in the greenhouse from which the resistant population was found, a high level of resistance was observed for a further two years.

By contrast, the same researchers did not detect any appreciable reduction in effectiveness of pyriproxyfen against whiteflies following a single treatment of pyriproxyfen in cotton fields. Cotton growers had adopted an integrated resistance management (IRM) strategy whereas the greenhouse grower had not.

FACTORS AFFECTING RESISTANCE DEVELOPMENT IN GREENHOUSES
Possible reasons for the rapid development of resistance to pyriproxyfen in whiteflies in the Israeli greenhouse are as follows:

High selection pressure for resistance – Whiteflies are widely exposed to this compound when it is applied to a crop. It affects all stages of the whitefly, either directly or indirectly; it inhibits egg-hatch and suppresses adult formation after treatment of the larval stage. In addition, because this compound has translaminar activity, it is able to affect larvae present on the lower side of the leaves. These two factors may serve to hasten the reduction of susceptible individuals, and at the same time, increase the numbers of resistant individuals. So, the efficiency of this active ingredient is both its boon and bane at the same time, depending on how it is used.

Closed population – Limited immigration of susceptible individuals from outdoors into the greenhouse, particularly during winter months, provides little opportunity for out-breeding, and therefore dilution of the resistant population. The short life cycle of the whitefly together with minimal out-breeding serve to accelerate the growth of the resistant population in the greenhouse.

RESISTANCE MANAGEMENT STRATEGIES
The guiding principle to minimizing or delaying the development of resistance to pyriproxyfen in greenhouse crops is to use this product sparingly, and as part of an integrated pest management (IPM) program for whiteflies. Such a program would include the following components:

Monitoring of crops – A good monitoring program is essential to any IPM and IRM program. Information so generated provides the basis for timely decisions and implementation of strategies.

Reliance on biological control agents as the major strategy. Parasitoids such as Encarsia formosa and Eretmocerus species are effective biological control agents against whiteflies and should be the first line of defence together with cultural controls such as weed control and good sanitation between crops.

Follow label directions – Label directions instruct that no more than two applications be applied within a six-month period.

Rotate with other chemical families – If two successive applications of a pesticide for whiteflies are needed, then rotate with other products that have a different mode of action such as Intercept or Forbid™. Studies to date indicate that there is no cross-resistance between pyriproxyfen and these two latter products.

Spot-treat whenever possible – Treatment of “hot-spots” before spread of whiteflies to the rest of the crop is always best. This is easier to implement and reduces selection pressure.

In general, these resistance management strategies apply to other greenhouse pests, and products registered for their management.

Gillian Ferguson is the greenhouse vegetable IPM specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs in Harrow.

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