Synthetic pesticides show little efficacy against Balaustium medicagoense (Prostigmata: Erythraeidae) in the field compared with other pest mites

The Balaustium mite (Balaustium medicagoense Meyer and Ryke) is an important pest of winter crops and pastures in southern Australia. Laboratory studies have shown B. medicagoense has a high natural tolerance to various pesticides and field chemical control failures are common. There is limited information on the biology of B. medicagoense and there are currently no pesticides registered for its control in Australia. Here the field efficacy of several pesticide treatments (lambda-cyhalothrin, bifenthrin, gamma-cyhalothrin and chlorpyrifos) that have recently shown some promise for the control of B. medicagoense in laboratory bioassays were examined. For comparison, the efficacy of these pesticides was also assessed against the redlegged earth mite (Halotydeus destructor Tucker) and blue oat mite (Penthaleus major Duges). The majority of chemical treatments were effective in controlling H. destructor and P. major, but had limited effect against B. medicagoense. Bifenthrin applied at the rate of 40 g a.i./ha had the highest efficacy against B. medicagoense, while high rates of lambda-cyhalothrin (5 g a.i./ha) and gamma-cyhalothrin (2.4 g a.i./ha) provided a low level of control in some trials. This study has further shown that B. medicagoense has a high natural tolerance to a range of agricultural chemicals. As pesticides are likely to be ineffective in many field situations, management strategies that do not solely rely on chemicals need to be considered for B. medicagoense.