M.M. MATTER
Pest and Plant Protection Dept., National Research Center , Dokki, Cairo , Egypt .

ABSTRACT

Third instar Pieris rapae larvae were parasitized by the solitary parasitoid Hyposoter ebeninus before or after being subjected to Bacillus-treated cabbage leaves. Unparasitized larvae were treated as well. Results obtained, seven days after bacterial treatment, revealed that the LC50 value for parasitized larvae reached about 4.1 folds higher or 0.49 fold lower than that for unparasitized larvae when parasitism preceded or followed bacterial treatment respectively. Parasitoid progeny in hosts which have been treated after parasitism, were mainly depended on age of the parasitoid at the time of infection. Parasitization of previously treated larvae was detrimental to parasitoid production as it increased premature host death. Late third instar host larvae surviving low concentration of the pathogen and held for seven days were more responded to parasitism than similarly held nontreated hosts of the same age. Delay in development of sublethally infected hosts was advantageous for the parasitoid as it extended the exposure period of the preferable ovipositional instar. Nevertheless, fewer parasitoid females and malformed adults appeared among parasitoids emerged from treated medium. It was concluded that B. thuringiensis should be restricted or cautiously used when the parasitoid was efficiently acting in cabbage fields. However, it could be recommended at lower or moderate rates when the parasitoid was not able to control heavy infestations in the event that the late stages of the parasitoid are not affected by the bacterium and could retain its population again.

Key Words: P. rapae, H. ebninus, B. thuringiensis.