Potential demographic effects of various bioinsecticides on Bracon nigricans (Hym: Braconidae) a new parasitoid of Tuta absoluta (Lep.: Gelechiidae)

Integrated Pest Management (IPM) programs usually include pesticide applications. Therefore assessing their potential side effects on biocontrol agents is of primary importance for implementing IPM efficiently. New biopesticides frequently cause no short-term effects to natural enemies but they may induce physiological and behavioural sublethal effects which may lead to important decrease in population growth. The invasion of Europe by the South American tomato pinworm, Tuta absoluta (Meyrick) (Lep.: Gelechiidae), has prompted farmers to use insecticides extensively, thus likely disrupting the pre-existing IPM programs already implemented in tomato crops. The parasitoid Bracon nigricans Szépligeti (Hym.: Braconidae) is an indigenous ectoparasitoid of Lepidopteran larvae which has been found parasitizing T. absoluta in Italy. Its efficacy as biocontrol agent of the pest is currently under evaluation and preliminary results show promising potential of biological control of T. absoluta. We assessed the effects of six bioinsecticides (abamectin, azadirachtin, Bacillus turinghiensis var kurstaki, borax salt plus citrus oil, emamectin benzoate and spinosad) used to control T. absoluta on various physiological traits of B. nigricans adults, exposed either at the pupal (direct spray) or adult (exposure to dry pesticide residues on tomato sprouts) stage. The persistency of the bioinsecticides was also studied during the assays. Bracon nigricans survival was checked daily and the number of eggs laid and the rate of emerged adults were recorded for three days after exposure to the various pesticides. In addition, B. nigricans’ biocontrol activity, i.e. paralyzed and/or parasitized host larvae, of the survived females on T. absoluta was recorded daily. Using the demographic data (life-history elements, survivorship and fecundity data) a delay model based on an age-structured Leslie matrix for estimating the potential intrinsic rate of increase (rm) and the instantaneous rate of increase (ri) was set. The results showed that the various products tested differed greatly in their toxicity and in their persistence. These findings stress the need for careful selection of bioinsecticides to be used in tomato crops for effective and sustainable IPM programs. Our data also indicate that some of these insecticides are incompatible with the parasitoid of T. absoluta, B. nigricans, and more broadly pose a potential threat to natural enemies.