Recent IPM practices have advised the use of
IGRs (*1) diflubenzuron (Dimilin) at bloom time to control PTB larvae. This timing has been shown to be highly effective. Work by several scientists, including researchers from USDA-Tuscon Haydeen Bee Research Lab, Ohio State University, Oregon State University, BASF, and UC Davis (*2), however, has suggested that this may not be the best timing for bee health. Below is a email sent by Dr. Mussen for distribution. It basically concludes that the bloom timing for IGRs may harm bees, and should be avoided if possible. Delayed dormant and May spray applications of these products can and should be considered for PTB control.
Email from Eric Mussen, UC Entomologist/Apiculturist:
“I am writing about an interesting finding that occurred in almonds in Glenn County. Beekeepers were having problems with dying brood and failure to be able to rear queen honey bees, if there was pollen in the hives that had been contaminated with the fungicide, Pristine. They noticed that pupae could not molt and emerge as adult bees beginning 17 days after forager exposure to the applications.
BASF spent a lot of time and money in Germany to test Pristine under ideal conditions. They found it to be not toxic to adult or immature honey bees. So, what was going on here?
If you follow my newsletters, the next suspected culprits in the sprays were the organosilicone adjuvants. Perhaps they were causing otherwise non-toxic chemicals to cross tissue barriers in the brood and cause lethal effects.
Eventually, and this may be the end of the mystery, BASF was here in California collecting pollen samples where the bees were having problems. They found significant amounts of diflubenzuron (Dimilin and other trade names) in the pollen.
We usually think that insect growth regulators (IGRs) tend to be non-problematic to honey bees when they are used to treat field and row crops, although we know that chitin is essential in the exoskeleton of all stages of honey bees and that honey bee larvae fed enough of the compound in the lab will kill them. I guess in the summer, the bees are collecting enough other pollens to dilute the IGR effects and we don’t have problems.
However, in almonds and other early blossoming trees, the bees may be pretty well depending upon just that single bloom to get most or nearly all of its food. Then, the concentration of the IGR consumed by the larvae may be more concentrated. According to one of the labels on Dimilin, “Because of its mode of action, which results in a disruption of the normal molting process of the insect larvae, the action of DIMILIN is slow and several days may elapse before the full effect is seen. Because of its specificity, DIMILIN does not effect (sic.) bees or other beneficial insects when applied at labeled rates and is therefore an excellent product to use in IPM programs.” I would guess that their toxicity tests on honey bees dealt only with exposure of adult bees, in which chitin no longer is being generated.
As a general statement, I would recommend that growers try NOT to make applications of ANY pesticides when either pollen or pollen-collecting bees are observed in the orchards or fields. Unless the formulation of the pesticide is acutely toxic to adult foraging bees, when pollen is contaminated, it will be returned to the hive and placed in storage for immediate or future consumption. Despite the toxicity test results on adult bees (and conveyed to the product labels), most of the products have not been tested to determine what effects occur with prolonged exposure to subacute dosages of contaminated pollen being consumed in the hive… ”
1. It is unknown if other insect growth regulators have the same negative impact on bees as diflubenzuron. Research has shown that diflubenzuron at high rates can negatively impact bees, and therefore further research and understanding of in orchard concentrations needs to be determined. Furthermore, the various modes of action/specificity of various IGRs on bees is be needed. Studies with methoxyfenocide (Intrepid) have been shown to have very limited activity on bees. I am unsure of the impact of calcium disruptors on bee health.
2. I regretfully admit that I accidentally failed to include the large amount of researchers who have performed research on this matter. I am sorry if I offended any of the participating collaborators by my omission prior to editing.