Two years of efficacy trials with different Bacillus thuringiensis sv kurstaki formulations against Lymantria dispar and Malacosoma neustria

Lymantria dispar L. (Lepidoptera: Erebidae) and Malacosoma neustria L. (Lepidoptera: Lasiocampidae) are the main cork oak forest defoliators in Sardinia. To contain their infestations over large areas, the use of entomopathogenic microorganisms such as Bacillus thuringiensis serovar kurstaki (Btk) is the presently available option. However, the features of the microbial control agent formulations represent a key factor for the success of application programs and a continuous management of insect resistance can be achieved through the rotation or combination of different bioinsecticidal formulations based on diverse microbial strains. The results of two years efficacy trials with different formulations of Btk, conducted in 2012 and 2013 in a cork oak forest in North-Western Sardinia, are reported. In the first year, trials were carried out on a M. neustria population, while in the second year a mixed population of L. dispar and M. neustria was involved. Trials included two formulations of Btk strain EG 2348 (Rapax® and Rapax Experimental) in comparison with two other commercial formulations (Foray 48B® and Delfin®). Both formulations of Btk strain EG 2348 proved to be effective in controlling the two pest species, showing a forest protection potential comparable to that of the reference products, Foray 48B® and Delfin®, containing spores and insecticidal Cry proteins of strains HD-1 and SA-11, respectively. The defoliation levels, 2 weeks after treatment applications, were significantly higher in untreated control trees than in treated ones. Mean percent defoliation, according to the Guidelines for Evaluation of Crown in the Mediterranean Region, in the untreated trees reached 75%, while it was below 10% for Rapax® at both rates, Foray 48B® and Delfin® in both years. For Rapax Experimental, the percent defoliation exceeded 10% reaching 17.5% in 2013 when applied at 1.0 l/ha.