The responses of Epiphyas postvittana (Walker) and Planotortrix excessana (Walker) (Lepidoptera: Tortricidae) to insecticides

Abstract

Topical application of azinphos-methyl and fenvalerate to lightbrown apple moth (Epiphyas postvittana) and greenheaded leafroller larvae (Planotortrix excessana) showed the latter species to be more susceptible to both insecticides on a mass basis. The intrinsic toxicities of 12 insecticides to 1st instar lightbrown apple moth larvae were compared using the Potter tower direct spray method, with acetone solutions of technical grade insecticides. Pyrethroids were the most toxic group, followed by organophosphorus insecticides, a carbamate (carbaryl), and an organochlorine (DDD). The relative toxicities of six of these insecticides were similar for greenheaded leafroller larvae when compared with azinphos-methyl, although this species was again relatively more susceptible to fenvalerate than lightbrown apple moth.

Insecticide deposits were applied in a calibrated Potter tower on to apple and other leaf surfaces. The residual toxicities of a range of formulated insecticides were, by comparison to azinphos-methyl, generally similar to the relative toxicities measured by direct spray for both species, except that greenheaded leafroller larvae were more susceptible to fenvalerate deposits than lightbrown apple moth larvae. The larger size of 1st instar greenheaded leafroller larvae generally accounted for their higher lethal residue values. The median lethal residue of azinphos-methyl deposits on apple leaves increased with successive instars, with an approximately 15X difference between 1st and final instar larvae.

Azinphos-methyl resistance in lightbrown apple moth was determined by five methods, by comparison of a susceptible strain from Lincoln (Canterbury) with a strain from Mariri (Nelson). Resistance ratios were 5-6X for topical application of adults, 14X for residue exposure of 3rd-4th instar larvae, 5X for residue exposure of 1st instar larvae, 20X for direct spray of 1st instar larvae, and 133X for topical application of 3rd-4th instar larvae. The geographic distribution of resistant individuals was surveyed using two methods. Direct spray of larvae reared from light trapped female moths indicated that resistance was largely confined to the area around Mariri (near Motueka, South Island) in March-April 1982. Survey 2 used a novel method to survey resistance, using topical application of pheromone attracted males in April 1983. The middle of 125 ha of apple orchards at Mariri contained a central zone of resistant individuals, with a mixture of phenotypes near the edges, and susceptible males in the surrounding vegetation (i.e. gorse, broom, blackberry, etc.). Resistant individuals were also present at Moutere Bluffs (>10 km south of Mariri).

Cross resistance was greater to methyl than to ethyl organophosphorus insecticides and carbaryl. No cross resistance to pyrethroids was detected. The fecundity of resistant females was significantly lower than that of susceptible females.

Monitoring of azinphos-methyl residues by gas chromatography on ‘Red Delicious’ and ‘Sturmer’ foliage showed faster decay of residues in spring, compared with late summer, principally due to leaf growth. Two components of leaf growth, expansion of existing leaves and the appearance of new leaves were considered to be important. Lightbrown apple moth larval mortality followed the chemical decay curves, using contemporaneous bioassays of field sprayed apple leaves, and a 16 hr exposure interval. Residues caused over 50% mortality of 1st instar larvae for three weeks in spring, and six weeks in late summer. These residues caused more than 50% mortality of 3rd-4th instar larvae for two weeks in spring, and almost five weeks in late summer. The minimum effective level of azinphos-methyl estimated to be for lightbrown apple moth control was estimated to be between 0.2-0.4 µ/cm² on apple foliage, regard less of season. Longer intervals between sprays would be possible following the cessation of leaf growth in January, due to the slower rate of residue decay in the absence of growth. The use of chemical assays and bioassays could optimize spray recommendations with pest phenology, degradation rates, and dosage-mortality responses, particularly with new insecticides or in new horticultural crops where spray programmes are more likely to benefit from this approach due to less field testing of alternatives.