This Year's Pyrethroid Affects Next Year's Mites

Work by Dr. Frank Zalom’s group at UC Davis has shown that applications of bifenthrin (Brigade/Bifenture) and λ-cyhalothrin (Warrior with Zeon) applied at hullsplit for NOW control negatively impacted the population of the predatory mite Galendromas occidentalisIn the study, G. occidentalis mites were placed onto ‘Butte’ and ‘Nonpareil’ branches that were sampled from an orchard at 1, 111, and 194 days post hull-split pesticide application. After 72 hours, the number of females that were still living, the number of eggs laid, and the number of eggs that hatched were counted. The group found both short term and long term effects on adult survivorship and the number of eggs laid. With applications of bifenthrin, 30%, 20%, and 14% fewer adults survived and 50%, 33%, and 24% fewer eggs were counted at 1, 111, and 194 days post application, respectively. With applications of  λ-cyhalothrin 50%, 35%, and 14% fewer adults survived and 84%, 58%, and 28% fewer eggs were counted at 1, 111, and 194 days post application, respectively.Seeing these results, they came to several conclusions:1. Even 6 months after the application of the selected pesticide, residues had a negative impact on the survival of G. occidentalis exposed and number of eggs laid. This data did not take into account the immediate/direct effects of the pesticide application (which are known to be severe);2. Insecticide residues remaining on treated surfaces can have a negative impact on the following season’s migrating and overwintering beneficial mite population, indicating that the use of some pyrethroids have a larger, persisting negative impact on the population of predatory mites;3. If possible, pesticides that reduce the negative impacts on beneficial/predatory mites should be selected;4. Consideration should be given to the long term effects and impact on beneficials as pesticides are being developed or applied as it is clear that the effect of pesticides are more complex than previously thought.This work is in support of the field observations of which spider mite control is often difficult the following year(s) after applications of pyrethroids.Work Referenced: Hamby, K.A., J.A. Alifano, and F.G. Zalom. “Total effects of contact and residual exposure of bifenthrin and λ-cyhalothrin on the predatory mite Galendromus occidentalis (Acari:Phytoseiidae). Experimental and Applied Acarology. Feb. 2013.