Effects of Conunonly Used Insecticides on the Potato Tuherworm and Its Associated Parasites and Predators in Potatoes

J. Econ. Enlomol. 74: 30}-308 (1981) Larvae and adults of Phthorimaea operculella (Zeller) in potato foliage were effectively controlled by commercial applications of azinphosmethyl and a combination of methomyl and methamidophos, but eggs and pupae were not significantly affected. Azinphosmethyl, methomyl, and methamidophos provided adequate control of foliar larval populations, but larval parasitism was markedly reduced in treated plots. Predator populations and predation of potato tuberworm eggs and pupae were high in untreated potato plantings and were greatly reduced by scheduled applications of azinphosmethyJ. After azinphosmethyl applications, adult populations of potato tuberworm resurged rapidly, but more time was necessary for larval resurgence. In commercial potato plantings in California, control of the potato tuberworm, Phthorimaea operculella (Zeller), is based largely on the frequent use of insecticides. In southern California, treatments are typically applied at 10to 14-day intervals beginning shortly after the vines close in the rows (Kennedy 1975). Several insecticides effectively reduce populations of potato tuberworm larvae mining in the foliage (Shorey et al. 1967, Bacon et aJ. 1972, Hofmaster and Waterfield 1972), but tuber infestation is largely determined by cultural practices affecting tuber accessibility (Shelton and Wyman 1979a,c), and insecticides may fail to prevent economic levels of tuber damage (Bacon 1960, Foot 1974). Immigration of potato tuberworm moths may also contribute to the inefficiency of insecticidal control (Bacon 1960). In the absence of insecticidal control, the potato tuberworm is attacked by numerous natural enemies (Graf 1917). In southern California, Oatman and Platner (1974) recovered 13 species of primary parasites from potato tuberworm larvae, with parasitism being higher in late summer plantings than in spring plantings. Predation also may be an important mortality factor in helping to suppress potato tuberworm populations, as in South Africa where Findlay (1975) recommended limited use of insecticides to preserve the abundance of predators and parasites. This study was conducted to determine the efficacy of standard commercial insecticide treatments for controlling vllfious developmental stages of the potato tuberworm and to quantify the effects of such control practices on parasite and predator populations occurring in potato plantings in southern California. Materials and Methods Insecticide Effects on Potato Tuberworm Developmental Stages Potato tuberworm eggs, larvae, pupae, and adults were removed from an insectary culture (Platner and , Lepidoptera: GeJechiidae. • Received for publication 9 September 1980. 3 Present address: Dcpartmentof Entomo)ogy, Cornell University. Geneva, NY 14456. 4 Present address: Department of Entomology, University of Wisconsin, Madison. WI 53706. Oatman 1968) and exposed to insecticide applications in commercial potato plantings in Riverside County, Calif. during 1978. Exposure and recovery were accomplished as follows. Ten swatches of muslin cloth, each containing 10 freshly laid eggs, were pinned to the soil surface near the base of potato plants where eggs are typically laid (Traynier 1975). After exposure, eggs were returned to the laboratory, counted to ascertain predation, and attached to potato tubers to complete development until adult emergence. Six potted potato plants (ca. 60 days old), each infested with 40 late larval instars were placed in the potato plantings. After exposure, plants were cut off at soil level and individualy placed in sealed emergence cartons with five potato tubers. Surviving larvae infested the tubers when the foliage desiccated and were allowed to complete development before emerged adults were counted to assess larval survival. Five pupae, which were washed to remove silk, were buried 0.5 cm under field soil in each of 10 plastic petri dishes (8.5 cm diameter). Petri dishes were placed on top of plant beds underneath the plant canopy and, after exposure, dishes were removed and placed in plastic bags until adult emergence. Groups of five unsexed adult moths (2 days old) were caged in each of 10 square lumite screen cages (50-mesh, 1,000 cm3) and placed at the base of potato plants where adult potato tuberworm were frequently observed during daylight hours. After exposure, cages were returned to the laboratory where adult mortality was assessed after 24 h. All potato tuberworm developmental stages were placed in the field, as described above, I h before insecticide application and retrieved 2 h after application. Four tests, utilizing the most commonly used insecticide treatments for control of potato tuberworm in this area, were conducted in spring potato plantings. In tests I and 2, a combination of methamidophos (0.83 kg of AI/ha) and methomyl (0.50 kg of AI/ha) was applied by air at 90 Iiterslha. In tests 3 and 4, azinphosmethyl (0.83 kg of AI/ha) was applied by ground rig (420 liters/ha) and by air (90 liters/ha), respectively. No larvae were exposed in tests 3 and 4. Each test was duplicated at the same time in an adjacent untreated field and survival was transformed to account for natural mortality by Abbott's Formula. Percent mortality due to insecticide ap©1981 Entomological Society of America 303 0022-0493/81/0303-0306$02.00/0 304 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 74, no. 3 plication was compared statistically with survival in untreated fields by using a t test. Effects of Insecticide Application on Potato Tuherworm Larval Populations and Associated Parasites A 0.31-ha late-summer planting (16 August) of Norgold potatoes was established on the University of California's Moreno Field Station in Riverside County in 1977. The experimental area (38 rows, 101 m long) was subdivided into a randomized complete block design with five treatments, including the untreated control, replicated five times. Plots (four rows, 15.2 m long) were buffered by three untreated rows on each side and by 3 m of untreated row on each end. Insecticide treatmenls were azinphosmethyl (0.83 kg of AUha), methamidophos (1.1 kg of AIlha), methomyl (1.0 kg of All ha). Treatments were applied 71 days into the growing season with a tractor-mounted boom sprayer with three nozzles per row delivering 420 liters/ha. Potato tuberworm larval populations were assessed at 70 days (preapplication) and 89 days (18 days posttreatment) after planting by random removal of five plants per plot. The lower six lateral branches and the main stem were dissected to determine potato tuberworm larval infestation. To assess parasitism, excised larvae were reared individually in 35-ml plastic cups containing a section of potato tuber. Emerged insects were counted after 4 weeks. Effect of Foliar and Systemic Insecticides on Predation and Predator Numbers A 0.74-ha late-summer planting (9 August) of White Rose potatoes was established on the Moreno Field Station in 1977. The experimental area (105 rows, 87 m long) was subdivided lengthwise into three 35-row plots. Plots were treated as follows: a single application of aldicarb (15G, 3.35 kg of AIlha) applied at planting as a band in the seed furrow; four weekly applications (16, 23, and 30 October and 6 November) of azinphosmethyl (0.83 kg of AUha) applied with a tractor-mounted boom sprayer delivering 420 liters/ha; and untreated control. These insecticides were used because aldicarb has no significant effect on potato tuberworm larvae in potato foliage (Bacon et al. 1972) but is used for leafhopper control in this area, whereas azinphosmethyl is used extensively for foliar potato tuberworm control. Predator populations were estimated with 36 pitfall traps per plot arranged in four groups of 9. Traps (200ml plastic cups) were buried to the rim on the surface of potato beds and filled with a water-detergent mixture. The effects of predation on potato tuberworm immature stages were estimated by placing laboratory-reared eggs and puape in each plot. Egg predation was measured by pinning 40 swatches of muslin cloth, each containing 10 fresh potato tuberworm eggs, to the soil surface at the base of plants. Pupal predation was measured by burying 40 groups of five pupae 0.5 cm deep in the bed surface beneath potato plants. Predation was estimated in the center five beds of each plot, ensuring adequate buffering (15 rows) between treatments. The pitfall traps and immature stages were placed in the field weekly (14,21, and 28 October and 4 and 11 November) and retrieved after 48 h. Predaceous arthropods captured in pitfall traps were counted and identified. Predation of eggs and pupae was calculated by subtracting the number of normal eggs and pupae recovered from the total placed in each plot. Effect of Azinphosmethyl on Potato Tuberworm Adults and Larvae and on Parasitism and Predation of Their Immature Stages Six 0.35-ha spring plantings (28 April) of Kennebec potatoes, separated by 13 m of unplanted buffer area, were established on the Moreno Field Station in 1978 for several research activities. For the tests reported here, three applications on azinphosmethyl (0.83 kg of AIIha) were applied to three plantings on 6 and 25 July and 13 August, with the other three plantings left untreated. Treatments coincided with the period of highest potato tuberworm activity in the area (Shelton and Wyman 1979b,c). Treatments 1 and 3 were made with a tractor-mounted boom sprayer with three nozzles per row delivering 420 liters/ha, and treatment 2 was applied by air at 90 liters/ha. Soil type, cultural practices, and irrigation were similar in the treated and untreated plantings. Adult populations were monitored with 2 UC/Davis water pan traps (Bacon et al. 1976) placed at ground level in each planting. Traps were baited with a rubber septum, impregnated with potato tuberworm sex attractant (trans-4, cis-7-tridecadienyl acetate, Zoecon Corp., Palo Alto, Calif.). Trapping was initiated on 3 July, and trap catches were monitored twice weekly until harvest. To assess larval populations and parasitism, 20 plants were removed at random from each of two plantings on 6, 15, and 24 July and 3, 13, and 22 August. Each planting had similar cultural practices, but one was treated with azinphosmethyl, as described above, whereas the other was left untreated. Potato tuberworm larvae were excised from the plants, and 30 larvae per planting were placed individually on potato tuber sections in plastic cups. After 4 weeks, emerged potato tuberworm adults and parasites were counted. Predation was assessed by placing potato tuberworm eggs and pupae in th field as described previously. Eggs and pupae were placed in the field 48 h before the dates that plants were removed to assess parasitization.