In vitro rearing of Bracon hebetor Say, 1857 (Hym.: Braconidae): nutritional evaluations for improvement of artificial diets

Author(s)

Sandra Regina Magro

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2005-03-09
• Advisor: José Roberto Postali Parra

Abstract

This work is intended to define the nutritional requirements of the larval parasitoid Bracon hebetor Say (Hym.: Braconidae) in order to improve an artificial diet for in vitro rearing with high survivorship rates and normal cocoon spinning behaviour, and several approaches were used to achieve these goals. Therefore, we analyzed the parasitoid development, aspects of its digestive physiology and morphological and chemical changes induced on the salivary glands when B. hebetor was exposed to different rearing conditions. Three instars were verified for the larval development of B. hebetor, similarly to the insect reared in vivo, although with a longer larval stage duration when the insect was reared on artificial diet. Observations of the fine and ultrastructure of the silk glands from in vivo- and in vitro-reared insects was the same under transmission (TEM) and scanning electron microscopy (SEM). Low protein concentrations were found in the silk glands of the "in vitro" reared insects, which justifies the absence of cocoons for those reared on artificial diet, not as protein-rich as the natural host (40% pupal holotissues of Diatraea saccharalis (F.) (Lep.: Pyralidae), 20% egg yolk, 20% semi-skim milk, 10% Neisenheimer salt solution and 10% water). Further analysis of the role of proteins and lipids as diet components indicated that among the protein components tested, the pupal holotissues of D. saccharalis (65%) and serum albumin (3%) provided the best results for rearing B. hebetor. While the fatty acids, the lipid concentrate and the linolenic acid improved survivorship significantly. The association of the pupal holotissues of D. saccharalis (65%) and the linolenic acid (1%) led to a 120% increase in cocoon formation, as compared to those insects reared on the basic diet. Yet, paralization rates and parasitization capacity of females developing from this improved diet were similar from that of in vivo-reared females. B. hebetor larvae developed different trypsin classes when reared on artificial diets (with pupal holotissues of D. saccharalis), than those reared on the natural host; however, trypsin was the main digestive enzyme under both rearing systems (in vivo and in vitro). Since lepidopteran larvae hemolymph can show trypsin inhibitors, artificial diets for B. hebetor must use hemolymph which do not affect the enzyme activity of the parasitoid. Thus, the improved diet for rearing B. hebetor (65% pupal holotissues of D. saccharalis, 8.5% lactalbumin hydrolysate, 8.5% fetal bovine serum, 1% linolenic acid, 18% egg yolk), amended the inadequacies found when the insect was reared on the basic diet, that are: insufficient protein quantities in silk glands, insufficient silk production, reduced survivorship and arrestment of immature development. This improved diet yielded 70% survivorship with 70% of the larvae normally spinning their cocoons. (AU)