Review: Johnson et al. (2009). Journal of Economic Entomology, 102: 474-479

Written: May 30, 2009

Posted: 06/01/09

Word count: 892

Question: Is there a downside to rotating miticides for varroa control?

Answer: Yes! Combinations of chemicals can make them more toxic to bees

Integrated pest management, or IPM for short, has been a central tenet in sustainable agriculture for controlling problem arthropods, weeds, and other pests. There are several key components to properly implementing an effective IPM strategy. First, one must routinely monitor and measure pest levels. This is necessary since there is no point in treating for a pest if it isn’t there! And if it is present, it still may not be necessary to treat if the pest levels are sufficiently low. Following this rule, therefore, helps to decrease to usage of pesticides, which helps decrease costs and increases sustainability. Second, and based on the latter point, one must treat only when necessary. If a pest is present, but it is at a low enough level, it may not be necessary to treat. That theoretical level is termed the “economic threshold”, since pest levels (even if they are non-zero) below that level does not do sufficient harm to the crop in question where it will impact the bottom line.

A third component to IPM is to rotate pesticide use if and when it is used. This is an important aspect of IPM, since the continued use of a single chemical can quickly select the pest to become resistant to it. Kill 99% of a pest with chemical X, then that 1% remaining will pass on their genes to the next generation. If those survivors carried genes that helped them survive exposure to that particular chemical, then the subsequent population will be less susceptible to the chemical. Over time, chemical X becomes less effect and, ultimately, useless. Rotating chemicals, therefore, don’t give the pest a chance to evolve resistance to any single one pesticide, increasing the duration that they can be used.

Most apiculturists have therefore adopted an IPM approach in their recommendations for beekeepers to control varroa mites. During the first years that they were introduced to the US in the mid-1980s, most beekeepers used the chemical fluvalinate (trade name Apistan^®) to control mites. This worked for a time, but the mites started to become resistant to it (see above). Several other products were then introduced to the market to control varroa mites, with the most commonly used being the chemical coumaphos (trade name CheckMite+^®). Following proper IPM protocol, then, most apiculturists recommend rotating among the available chemical treatments (see Beekeeping Note 2.01), and thus many beekeepers have been alternating between fluvalinate and coumaphos. Their use is so widespread in the US that a recent finding from the CCD Working Group showed that 100% of all beehives in the country have detectable levels. Both of these compounds are lipophillic (Lating for “fat loving”), meaning that they become absorbed and remain in wax comb long after the plastic strips have been removed from the hive.

But is there another downside to cycling different control measures? A new study by a team at the University of Illinois—a study that will likely, with time, become a seminal paper in apicultural research—has clearly shown that there is one. A BIG one.

It is important to remember that chemicals good at killing mites, by and large, are pretty good at killing bees, too (both are arthropods, after all, second cousins on the evolutionary tree). The products are therefore designed very carefully so that the delivered dose is sufficient to kill the mites but not enough to kill the bees (which is why it is critical NEVER to abuse or misuse these pesticides—if you do, shame on you!). One way the bees are able to withstand sublethal doses of these pesticides is because they are simply larger than mites, but another important mechanism is that the bees have special enzymes (referred to as P450s) that detoxify the chemicals and help protect the bees.

The new study, led by Reed Johnson, has shown that fluvalinate exhibits a much higher toxicity to adult bees that had been previously exposed to coumaphos, and that coumaphos exhibits a moderate increase in toxicity to adult bees that had been previously exposed to fluvalinate. The two chemicals, therefore, synergize so that the combination is more toxic than either chemical independently. The researchers propose that this may result from competition for access to detoxicative P450s; bees that are busy detoxifying one chemical may be much less equipped to detoxify a second.

Clearly, this finding—while not the end of the story but rather the beginning—points to a major problem in the current IPM recommendations for varroa control. Specifically, it questions the validity of rotating fluvalinate and coumaphos, since the combination may be too much for the bees to handle. Therefore, if you need to use miticides to control for varroa, you should avoid this combination if at all possible; use fluvalinate or coumaphos, but not both. If you wish to alternate mite treatments, use alternatives to these synthetic miticides. Doing so will likely improve the health of the bees while controlling varroa for the long term.

Reference

Johnson, R. M., H. S. Pollock, and M. R. Berenbaum. (2009). Synergistic interactions between in-hive miticides in Apis mellifera. Journal of Economic Entomology, 102: 474-479.