Decline in the proportion of mites resistant to fluvalinate in a population of Varroa destructor not treated with pyrethroids

Abstract

-The reversion of resistance to pyrethroids in Varroa destructor Anderson & Trueman was studied in Friuli (northern Italy), where resistance was detected in 1995 and pyrethroids had not been used since. Mites were sampled in seven localities each year between 1997 and 2000 and assayed in the laboratory for the resistance to fluvalinate by using paraffin coated capsules. Survival at the diagnostic concentration, expected to kill all susceptible mites (200 mg/kg), decreased in all the localities by about ten times in three years, from 19-66% to 1.3-7.8%. Thus, the disadvantage associated with the resistance to pyrethroids in V. destructor is small, as usual when resistance is due to monooxygenases. Its impact on the selection of resistant mites during annual application of treatments is negligible; appreciable effects of reversion can be expected only over many generations of the mite.Varroa destructor / reversion / resistance / pyrethroids INTRODUCTIONIn recent years, populations of Varroa destructor Anderson & Trueman resistant to pyrethroids have been detected in several countries (Lodesani et al., 1995;Trouiller, 1998;Elzen et al., 1998) and increasing attention has been paid to this problem. Several laboratory assays have been developed (e.g., Milani, 1995;Faucon et al., 1996;Colin et al., 1997;Elzen et al., 1998) and used to monitor the spread of resistant populations (Trouiller, 1998).The biochemical mechanism of resistance has been investigated; monooxygenases of the P450 system are involved, at least in the strain of V. destructor that originated in Italy and later spread through the Old World, while esterases do not play a significant role (Hillesheim et al., 1996;Mozes Koch et al., 2000). Fast selection and spread of resistant mites make it unlikely that resistance is polygenic; this is a general rule with insects and mites (Roush and McKenzie, 1987).Resistant genotypes usually are at some fitness disadvantage in the absence of pesticides (Roush and Daly, 1990;Denholm and Rowland, 1992), because of unbalanced or unregulated physiological processes. This makes the frequency of resistant mites decline when the acaricide is not used (a phenomenon usually called reversion). No information is available on the decrease in fitness associated with resistance to pyrethroids in V. destructor. More data on this aspect would be useful to elaborate theoretical models of the development of resistance and to develop integrated resistance 417

A scientific note on reversion of fluvalinate resistance to a degree of susceptibility in Varroa destructor

“…Such reversion in resistance to fluvalinate is also consistent with results found by Milani and Della Vedova (2002) in Italy. They attribute the slow reversion they observed as characteristic of monooxygenases.…”

Toxicity of selected insecticides to Trichogramma chilonis: Assessing their safety in the rice ecosystem

“…Analysis Necessary corrections were made for natural mortality in the control using Abbott's formula (Abbott 1925) and later referred to by Milani & Vedova (2002), Michaud & McKenzie (2004) and Demissie et al (2008). Then the data were subjected to probit analysis as per Finney (1971) and the Log concentration probit mortality curve was obtained.…”

Standard methods for varroa research

“…pyrethroids and some organic acids). The bioassay described has been used to test the activity of several acaricides including tau-fluvalinate, flumethrin (Milani, 1995), perizin and Cekafix (Milani and Della Vedova, 1996), oxalic and citric acids (Milani, 2001), as well as for the study of reversion of resistance (Milani and Della Vedova, 2002). Other acaricides that are widely used for the control of the varroa mite are air-borne and the bioassay above is not suitable.…”