Lethal and sublethal effects of single and double applications of Bacillus thuringiensis variety kurstaki on spruce budworm (Lepidoptera: Tortricidae) larvae.
We conducted laboratory experiments to examine the effects of single versus double exposures of spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae) female larvae to various concentrations of a Bacillus thuringiensis variety kurstaki (Btk) commercial formulation (Foray 48B). Our main objective was to document the vulnerability to Btk and the sublethal responses of fifth-instar larvae that survived from a first ingestion of Btk during their fourth stadium and to compare them with insects treated either during their fifth or fourth stadium only. As reported in the literature, fifth-instar larvae were more vulnerable than fourth-instar larvae, but only at low and medium concentrations. Fifth-instar larvae that had survived Btk ingestion during their fourth stadium were more vulnerable to a high concentration of Btk and had a shorter feeding inhibition period than those that had not been exposed during their fourth stadium. Compared with a single treatment at the fourth stadium, a double exposure to Btk further reduced the population by 20-30%, depending on the concentration applied. The second treatment also induced another feeding inhibition period and increased larval development time by 14%. The impact of the different treatments on pupal weight depended on whether treated insects exhibited supernumerary instars. In the absence of developmental polymorphism, a higher concentration, a late, or a double exposure to Btk significantly reduced pupal weight.
Centre de Recherche en Biologie Forestière, Département des Sciences du Bois et de la Forêt, Faculté de Foresterie et de Géomatique, Université Laval, Sainte-Foy, Québec, Canada G1K 7P4.
Gaétan
G
Moreau
Eric
E
Bauce
D000818:Animals; D001413:Bacillus thuringiensis / Q000502:physiology*; D007814:Larva / Q000254:growth & development; D009036:Moths / Q000254:growth & development; D010572:Pest Control, Biological / Q000379:methods*; D011679:Pupa / Q000254:growth & development