Toxicity of Insecticides to Cotesia plutellae, a Parasitoid of Diamondback Moth

Toxicity of 17 commonly used insecticides used to control diamondback moth, Plutella xylostella (L.) was evaluated against adults of Cotesia plutellae Kurdjumov. The insecticide concentrations studied were equivalent to those recommended in the field. Adults of the parasitoid were released into exposure kits with fresh dry insecticide film for 24 hours, at which time mortalities were noted. The surviving adults were supplied with larvae of diamondback moth for parasitism. Results revealed that carbofuran, cartap, mevinphos, quinalphos, methomyl, methamidophos and deltamethrin were harmful (mortality >99%) to adults of C. plutellae, while the remaining 10 insecticides proved to be harmless (mortality < 50%). The effects of insecticides in reducing parasitism of C. plutellae in decreasing order were as follows: fenvalerate > acephate > Bacillus thuringiensis subsp. kurstaki strain NRD-12 (SAN 415) > teflubenzuron > permethrin > chlorfluazuron > acephate > B. thuringiensis subsp. kurstaki strain HD-1 (Dipel) > fenvalerate > teflubenzuron. Most of these insecticides were considered harmless to parasitism of C. plutellae on larvae of diamondback moth except fenvalerate, acephate and B. thuringiensis which were classified as slightly harmful. Seven insecticides (quinalphos, deltamethrin, carbofuran, methomyl, cartap, mevinphos, methamidophos) were highly toxic to adults of C. plutellae and were used to test their toxicity to the pupal stage of C. plutellae. Cocoons at different ages (1-, 2-, 3-, 4-dayold) were sprayed with insecticides at recommended concentrations, and emergence of adults and mortality 24 hours after emergence were recorded. All insecticides tested were harmless to pupae of C. plutellae, except quinalphos which was ranked slightly harmful. These results provide further information on the selectivity of insecticides in integrated management of diamondback moth. Introduction The diamondback moth (DBM) Plutella xylostella (L.), (Lepidoptera: Yponomeutidae) is one of the most important pests of cruciferous vegetables in Taiwan. It has several natural enemies ( W u 1968), and inundative releases of the larval parasitoid Cotesia plutellae Kurdjumov apparently exert an effective check of this pest (Chiu et al. 1974). However, it appears unlikely that C. plutellae alone could achieve the desired level of control of DBM (Chua and Ooi 1986). Regular insecticide spray to control DBM therefore seems inevitable. However, there is little information available on the toxicity to C. plutellae of insecticides commonly used in cruciferous vegetable cultivation (Fan and Ho 1971; Chang 1974; Feng and Wang 1984; Mani and Krishnamoorthy 1984). This may cause serious damage to C. plutellae if insecticides applied are not safe for this parasitoid. It will be possible to practice integrated control measures against DBM only if selective insecticides are chosen. Therefore, investigations were undertaken to develop a standard testing 287