Neonicotinoids (Assail, Admire): Mites and Resistance

Assail 70 WP (acetamiprid) has been registered for use on various crops since 2002. On grapes, it can be used for management of leafhoppers; on pome fruit (apples, crabapples and pears) it is registered for use against aphids, tentiform leafminer, leafhoppers, codling moth, and pear psylla. Assail is in the same chemical family as Admire (imidacloprid): the neonicotinoids (sometimes referred to as chloronicotinoids).

Mites

There is evidence in some studies in the U.S. and in British Columbia of increased numbers of mites with the use of neonicotinoid pesticides. Although I have not completed an exhaustive literature review yet, I would be happy to provide a review of what I have gathered or the references themselves to anyone interested. I think it is important to point out that negative results are sometimes not reported (or not noticed). Researchers at Agriculture and Agri-Food Canada in Vineland have not yet documented increased mite egg production or predatory mite decreases in small plot field trials of Assail.

Probably the most sensible reaction to the mite issue is to recognize that there is some need for concern but that should go along with the caution that there are many possible reasons for mite outbreaks, so one should be careful in assigning blame for mite problems to any one cause. In all cases, keep a close eye on mite numbers through regular monitoring as a normal part of vineyard and orchard management.

When mite increases are attributable to the effects of neonicotinoids, they seem to be as a result of two different causes. Many chemicals cause decreases in predatory mites which are the most important control (biological or otherwise) of phytophagous (plant feeding) mites. This effect has been shown in some studies but is not always seen, either because of low numbers of predatory mites throughout the trial plots or because it wasn't occurring in a particular study.

The other way that neonicotinoids may affect pest mite numbers is by causing the female mites to lay more eggs. Entomologists love to coin new and difficult to pronounce words; this increased mite egg laying is called 'hormoligosis'. This effect is NOT analogous to "Viagra for mites" as some people have suggested.

Higher numbers?

Determining if a particular product is causing pest mite increases, one really needs to do properly replicated, statistically analyzed, scientific studies of the situation. If you do see mite increases, take a look at management practices over the last few years and environmental factors as well.

Many pesticides can reduce predatory mite numbers, allowing pest mites to rapidly increase. The most important management practice to consider is how frequently you have used products that are harsh on beneficial predatory mites; these include carbamates like Sevin, pyrethroids (Cymbush, Pounce, etc.) and even some fungicides like Dikar. Predatory mite populations rebound slowly compared to pest mites, so review pesticide use for at least the last few years. Another question to ask is how often have you used a miticide. Perhaps several seasons have gone by as pest mites slowly built up, but remained below damage threshold until conditions were ideal for their development. Of course, miticides should only be used when necessary, but it is another factor to consider before assigning blame for a mite outbreak.

Environmental factors are extremely important in regulating pest mite populations; mites, especially European red mite (ERM) and twospotted spider mite (TSSM) reproduce exceptionally quickly in hot, dry weather. Areas along dusty roadways can have amazingly high populations of ERM and TSSM in August so don't base mite management on drive-by monitoring. Thorough, regular, systematic monitoring by qualified crop scouts is really required. Experienced crop consultants will also be able to tell you how your beneficial predatory mites are doing. Conserving the predatory mites is the single best way to keep pest mites under control in the long run.

Resistance

A final word or two on resistance development. There are few members of the neonicotinoid family currently registered but there are more in development, so it is important to use Assail judiciously so as not to develop resistance. New neonicotinoid products would be less efficacious if resistance to Assail develops. Follow all recommended resistance management strategies with this product: rotate to other chemical families, keep below the maximum number of uses allowed per season, use recommended rates and timings, ensure thorough coverage, etc. Articles on resistance management have appeared in previous issues of this newsletter and we are always happy to reprint them for you if needed. OMAF publication 360 has several sections on resistance management strategies and many pesticide labels now include similar information. Development of resistance to this or any product affects everyone in the grape and tender fruit industries and in other crops as well.

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