TOXICITY OF INSECTICIDES ON TRIOXYS PALLIDUS, THE WALNUT APHID AND THE CODLING MOTH. Mary Purcell and Jeffrey Granett

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1 y TOXICITY OF INSECTICIDES ON TRIOXYS PALLIDUS, THE WALNUT APHID AND THE CODLING MOTH Mary Purcell and Jeffrey Granett ABSTRACT The benzoylphenyl urea insect growth regulators (IGRs), diflubenzuron, alsyston, CGA , UC and Thuringiensin, were tested on adult and late third instar Trioxys pallidus and first instar walnut aphids. The organophosphates, phosalone and azinphosmethyl were used as positive controls. The IGRs and Thuringiensin were not acutely toxic to adult and larval Trioxys when contact, topical and ingestion methods of application were used. These chemicals had no contact effects on 1st instar aphids. Azinphosmethyl and phosalone were highly toxic to both the parasite and aphid. However, phosalone when topically applied to mummified aphids did not reduce emergence rates of adult parasites. No detrimental effects on reproduction for both Trioxys and aphids were detected. A colony of codling moth was initiated in August, Bioassays with the above IGRs and Thuringiensin on the codling moth will be completed by July, Eggs, first instar, and adult moths will be used to determine acute and reproductive effects. INTRODUCTION One of the major objectives of the walnut IPM program has been to use selective insecticides on key pests like the codling moth to maintain natural enemies of secondary pests and to prevent outbreaks. Azinphosmethyl, which is used to treat heavy infestations of codling moth is disruptive to natural enemies such as Trioxys and induces aphid outbreaks. The currently used selective compound, phosalone, does not provide satisfactory control for codling moth. Therefore, development of selective insecticides which are effective on codling moth is imperative. OBJECTIVES The overall objective is to identify insecticides that are compatible with the walnut IPM program. The specific goals of the research are as follows: 1. Determine the acute and reproductive effects of currently used and new insecticides on T. pallidus, and the walnut aphid. 2. Determine the relative efficacy of compounds against codling moth which have demonstrated low parasite toxicity. PROCEDURES A. Chemicals The compounds used in the experiments were the regulators diflubenzuron, alsyston, CGA , insect and UC growth 62644, -84-

2 and Thuringiensin, the beta-exotoxin of Bacillus The two most commonly used insecticides against phosalone and azinphosmethyl were included in the thuringiensis. codling moth, tests. B. Insects Aphids and parasites were collected from commercial walnut orchards in Winters, California, May, Aphid and parasite colonies were initiated on English walnut v. Serr seedlings, which were obtained from the DC Davis Pomology department. Egg masses of codling moth were obtained from the insect pathology lab at DC Berkeley, August, Colonies were initiated using a synthetic diet method. C. Bioassays 1. Adult parasite To determine contact toxicity, adult parasites were exposed for 48 hr to a residual of each chemical. The concentrations used were 2000 ppm for the IGRs (10 times the recommended field rate), 22, 555 and 600 ppm for Thuringiensin, phosalone, and azinphosmethyl, respectively (these concentrations correspond to recommended field rates). These concentrations will apply to all other bioassays except where specifically noted. 2. Trioxys larvae To determine if the IGRs disrupt the molting process of the parasite, we selected late 3rd instar parasite larvae just prior to their 3rd molt (the aphid is mummified at this stage). Individual mummies were topically treated. The mummies were held until the time of adult emergence (average duration of mummy stage was 6 days). Dnemerged mummies were dissected to detect morphological abnormalities in parasites. 3. Ingestion and reproduction effects on Trioxys The ingestion of IGRs by adult insects kills developing eggs in several insect species. To determine the effect on parasitism rates of the walnut aphid, adult female parasites were fed treated honey droplets continuously for 48 hr. The parasites were then exposed to aphids for oviposition. The number of parasitized and mummified aphids were counted 9 days later. 4. Walnut aphid To maintain the parasite in walnut orchards, chemicals used in walnut IPM programs should also be non-toxic to its host, the walnut aphid. Residual leaf bioassays were conducted for 1st instar aphids. Mortality and number of offspring produced from treated aphids were assessed, 9 days after exposure. -85-

3 RESULTS 5. Codling moth Acute toxicity tests have not yet been conducted but will be initiated mid-january, Bioassays will be conducted on eggs, 1st instar larvae, and adults. The chemicals will be administered topically to eggs, and in the food of larvae (synthetic diet) and adults (sugar water). Hatch rate of eggs, acute and delayed mortality of treated larvae, and fecundity of treated adults will be determined from these tests. A. Adult parasite. Phosalone and azinphosmethyl killed almost 100% of Trioxys by residual exposure (Table 1). The low mortality values for the other chemicals indicated that under these conditions, diflubenzuron, alsyston, CGA , and Thuringiensin have no contact toxicity to Trioxys. B. Trioxys larvae Emergence rates of Trioxys were significantly reduced only be azinphosmethyl (Table 2). 50% emerged as adults, but died within 24 hr. Only 3% emerged and survived beyond 24 hr. Thus, penetration of azinphosmethyl through the mummy is frequently marginal, which indicates that the mummy protects the developing parasite. Dissections of unemerged mummies showed that 83% died as adults. Thus, the most common cause of death may have been by ingesting a toxic dose of the insecticide when the parasite started to chew out of the mummy (chew holes were evident in most mummies). Phosalone, diflubenzuron, alsyston CGA , UC and Thuringiensin did not significantly reduce emergence rates of parasites. It was surprising that phosalone did not effect emergence rates since it was highly toxic to adults. We must infer that the residual decreased to a non-toxic level by the time of adult emergence. No morphological abnormalities were detected in unemerged mummies of any treatment. All chemicals tested, except azinphosmethyl are apparently unable to penetrate the mummy. In this way, Trioxys is protected between the late 3rd instar and pupal stage from the IGRs, Thuringiensin and phosalone. C. Reproduction effects on Trioxys Ingestion of honey treated with IGRs resulted in no significant differences in per cent parasitism of aphids (Table 3). There was a slight reduction in parasitism rates with adults ingesting Thuringiensin, but this was not significant at the 5% level. Higher than recommended field rates of Thuringiensin may effect reproducti ve performance of Trioxys and should be tested in future work. D. Aphids -86-

4 Phosalone and azinphosmethyl were very toxic to 1st instar aphids. Nearly 100% were killed by the 2nd day after treatment. Two concentrations were used for the IGRs and Thuringiensin: 200 and 2000 ppm. The mean per cent mortality of aphids after 9 days continuous exposure, was not significantly different from the control (Table 4). The mean number of offspring were also not effected. CONCLUSION In these tests we have demonstrated that diflubenzuron, alsyston, CGA , UC and Thuringiensin were not acutely toxic to Trioxys or its host, the walnut aphid. Phosalone and azinphosmethyl were extremely toxic to both the parasite and aphid.0. One exception, was that phosalone did not effect emergence rates of parasites when aphid mummies were treated. No detrimental effects on reproduction for both the parasite and aphids were detected by our methods. Before recommendations can be made for walnut IPM, the efficacy of the IGRs and Thuringiensin against the codling moth must be determined. The methods for testing of these chemicals are currently being developed and the experiments will be completed by July,

5 Table 1. Effect of insecticides on adult -T. pall idus. Rate Treatment (ppm [AI] ) Reps mean % mortality Azinphosmethyl a1... Phosalone a Alsyston b Thuringiensin b Control 0 6 3b Diflubenzuron Ob CGA Ob 1/ Values within a column, followed by different letters are significantly different at P < 0.05, by Tukey's Table 2. Effect of insecticides by topical treatment, on late third ins tar T. pallidus. - Rate Treatment x % succesf' ppm ([AI]) Reps emergence UC a2 Phosalone a Oiflubenzuron a CGA a Control a Thuringiensin a Alsyston a Azinphosmethyl b 1/ The criterion for successful emergence of adults from mummies was survival for at least 24 hr after emergence. 2/ Values within a column, followed by different letters are significa'1tly different at P < 0.05, by Tukey's -88-

6 Table 3. Effect of insecticides added to food on parasitism of -thewalnut aphid by ~ pallidus. Rate Treatment (ppm [AI] ) n mean % parasitism Alsyston a1 Diflubenzuron a Control a CGA a Thuringiensin a 1T-Values within a column, followed by different letters are significantly different at P < 0.05, by Tukey's Table 4. Effect of insecticides on aphid mortality and reproduction 9 days after treatment. Trtmnt Rate Reps X % mort X nymphs/aphid Control a1 12a CGA a 12a CGA a 9a Diflu a 11a Diflu a 7b Thur a 9a Alsys a l1a Alsys a 6b Phos b Ob Azph b Ob l/values within ~-2~fumn, followed by different letteri are significantly different at P < 0.05, by Tukey's