Field Evaluations of Novel Chloronicotinyls and Insect Growth Regulators against the Greenhouse Whitefly on Strawberry

The efficacy of two novel chloronicotinyls and two novel insect growth regulators against the greenhouse whitefly [Trialeurodes vaporariorum (Westwood)] on summerplanted strawberries was evaluated in field experiments. Imidacloprid applied in soil 3 weeks after planting decreased whitefly adult numbers by 58% to 90%, first and second instars by 78% to 93% up to 56 days postapplication, and third and fourth instars by 42% to 86% up to 77 days postapplication, whereas thiamethoxam applied similarly reduced adults by 58% to 80%, first and second instars by 78% to 93% up to 6 weeks posttreatment, and third and fourth instars by 48% to 80% up to 10 weeks after initial application, compared to nontreated controls. Imidacloprid applied in soil immediately prior to planting further suppressed numbers of whiteflies by 71% to 83% (adults), 58% to 74% (first and second instars), and 52% to 74% (third and fourth instars), in comparison with the same compound applied through drip irrigation lines 4 weeks after planting. Buprofezin and pyriproxifen applied 6 weeks after planting reduced numbers of adult whiteflies by 25% to 81% and 40% to 73%, respectively; first and second instars by 61% to 92% and 51% to 100%, respectively; and third and fourth instars by 45% to 100% and 37% to 87%, respectively, on most sampling dates up to 7 weeks postapplication. The potential roles of these insecticides in integrated greenhouse whitefly management programs are discussed. Received for publication 13 Nov. 2001. Accepted for publication 27 Feb. 2002. We thank Frank Byrne and Steve Castle for critical review of an earlier version of the manuscript. This research was funded in part by the California Strawberry Commission. E-mail address: [email protected] The greenhouse whitefly is a worldwide pest of many horticultural crops. Large populations of this insect can ingest sufficient quantities of plant phloem sap to cause yield reduction (Johnson et al., 1992). In addition, this insect produces large amounts of honeydew and associated sooty mold, which can decrease the market value of crop products (Liu et al., 1993). Furthermore, this insect may transmit plant virus diseases (Muniyappa, 1980). Strawberry [Fragaria ananassa (L.)] is an economically important horticultural crop in California, with annual values exceeding $200 million since 1980 (McGregor 1981). In recent years, this crop has been increasingly under attack by the greenhouse whitefly, especially in the coastal area of southern California. Prevention of economic losses requires development of an integrated greenhouse whitefly management program. Insecticides are important components of integrated pest management systems designed to suppress whitefly populations on various crops (Liu et al., 1993; Toscano et al., 1998). Intensive research has been carried out in recent years for evaluating insecticides with novel modes of action against whiteflies. Imiand the crop is terminated in late December, whereas fall plantings are made in late September to late October and the crop is terminated the following summer (late June). We previously investigated efficacy of chloronicotinyls and insect growth regulators against greenhouse whiteflies on fall-planted strawberries when these chemicals were applied in midseason (April). We demonstrated that all these chemicals were effective in suppressing the whitefly populations except for foliarapplied thiamethoxam (Bi et al., 2001). The present study was initiated to test the efficacy of two chloronicotinoids and two insect growth regulators against greenhouse whitefly on summer-planted strawberry when applied in early season under field conditions. Materials and Methods Experimental plots and insecticides. The efficacies of imidacloprid, thiamethoxam, buprofezin, and pyriproxyfen were evaluated against the greenhouse whitefly in a commercial strawberry field in Oxnard, Calif. Strawberry plants of ‘E6’ were planted in late July 2000 on four-row beds. Each bed was 1.3 m wide × 70 m long. The test was arranged in a randomized complete-block design with four replicates. Plot size was 8 m long × 1.3 m wide with a 1-m buffering area between plots. There were ≈106 plants in each plot. The following insecticides were obtained from their respective manufacturers: imidacloprid (Admire 2F, from Bayer Crop Protection, Kansas City, Mo.), thiamethoxam (Platinum 2SC, from Syngenta, Inc., Greensboro, N.C.), buprofezin (Applaud 70 WP, from AgrEvo USA Co., Pikeville, N.C.), and pyriproxyfen (Knack 0.86 EC, from Valent USA Corp., Walnut Creek, Calif.). Insecticide concentrations and applications. Insecticides applied at the label-recommended concentrations were as follows: imidacloprid at 0.5556 kg·ha a.i., thiamethoxam at 0.1758 kg·ha a.i., buprofezin at 0.3888 kg·ha a.i., and pyriproxyfen at 0.06 kg·ha a.i. Imidacloprid and thiamethoxam were delivered into the soil through the drip irrigation lines on 24 Aug. The effect of application timing (preplanting vs. postplanting) on efficacy of imidacloprid was also investigated. For the preplanting applications, a 10-mL solution was applied on 28 July to each of the planting holes, immediately prior to planting. For postplanting applications, imidacloprid was injected into the drip irrigation lines 4 weeks after planting. Buprofezin and pyriproxyfen were applied on 14 Sept. at a volume of 15323.9 L of water per hectare with an ECHO air-assisted sprayer. Control plots were left untreated. Whitefly sampling methods. Sampling of whitefly adults and immatures was initiated just prior to application of all the insecticides, except for sampling of immatures from chloronicotinyl-treated plants, which was initiated 3 to 6 weeks after applications. The sampling was conducted on a weekly basis. The number of whitefly adults was deterdacloprid and thiamethoxam are chloronicotinyl insecticides that act as nicotinic acetylcholine receptor agonists and are used to control homopteran pests (Elbert et al., 1998; Leicht, 1993). Buprofezin is an insect growth regulator that inhibits chitin synthesis in several homopteran pests, including whiteflies (De Cock et al., 1990). Pyriproxyfen is a juvenile hormone mimic affecting the hormonal balance in insects and resulting in strong suppression of embryogenesis and adult formation (Ishaaya and Horowitz, 1994). The separate modes of action of these compounds, together with their selectivity against targeted insect pests and relative safety to beneficial insects and other organisms, present an exciting opportunity for their effective integration into pest management strategies (Darvas and Polgar, 1998; Ishaaya and Horowitz, 1998). The availability of such chemical diversity enables the development of a management strategy that minimizes the threat of insecticide resistance (Denholm et al., 1998). Agronomic factors influence insecticide efficacy against sucking insects. Slosser et al. (1997) showed that cotton plant maturity and application timing of dicrotophos affected control of the cotton aphid Aphis gossypii (Glover). Fuson et al. (1995) demonstrated that efficacy of chlorpyrifos and endosulfan against cotton aphid was better on late-planted than early-planted cotton. For commercial strawberry production in southern California, two planting seasons exist. Summer plantings are usually made in late July to mid-August 7114, p. 914-918 10/1/02, 1:23 PM 914 915 HORTSCIENCE, VOL. 37(6), OCTOBER 2002 mined using a leaf turn method. Adults were counted on 20 of the youngest and fully expanded middle leaflets of trifoliates, each from randomly selected plants in each of the plots. Ten older leaflets from 10 randomly chosen plants in each plot were excised and transported to the laboratory to count nymphs and pupae within a 4.5-cm disk area in the center of the leaflet using a stereo dissecting microscope. Statistics. A least significant difference (LSD) test in one-way randomized completeblock design of analysis of variance (ANOVA) in SAS (SAS Institute Inc., 1989) was used in this study to analyze the data and separate the means. Before the ANOVA, numbers of whitefly adults and immatures were transformed using the formula log (y + 1) to normalize the data.