Developing Integrated Fruitworm Control Strategies for Blueberry in Preparation for Pesticide Restrictions

To prevent infestation of highbush blueberries by fruitworms, growers employ pheromone-baited traps to monitor adult moths and apply insecticides to fields at risk from infestation by the larvae of these pests. However, there are increasing restrictions on availability of broad-spectrum insecticides and monitoring traps have not been used to predict optimal timing of controls for these species due to the lack of a degree-day model. We are developing an IPM program that integrates monitoring traps with degree-day models to provide optimal life-stage timing of reduced-risk insecticides for control of fruitworms while maintaining biological control activity. Bioassays with biological control agents provide support for the role of conservation strategies to maintain beneficial insects. This project will help achieve the long-term goal of improving yield and quality while reducing environmental impact and worker risk in blueberry production. INTRODUCTION Blueberries produced in eastern North America are at risk of contamination by two lepidopteran insects that infest developing fruit. The cranberry fruitworm (Acrobasis vaccinii Riley) and cherry fruitworm (Grapholita packardii Zeller) lay their eggs on the young green fruit beginning at petal fall (Beckwith, 1941; Hutchinson, 1954). Young larvae then penetrate the berries causing a risk of contamination to harvested fruit and also yield loss if the population is high enough (Mallampalli and Isaacs, 2002). Cherry fruitworm larvae tend to infest single berries, which often drop prematurely before harvest, thus are not a high risk as a post-harvest contaminant. Larvae of cranberry fruitworm typically infest multiple berries in a cluster, leaving frass in the silken webbing between berries. This is more apparent to inspectors than cherry fruitworm, so earlyseason insect management programs tend to focus mainly on preventing infestation by cranberry fruitworm. In response to the risk of crop rejection if larvae are discovered in harvested fruit, broad-spectrum insecticides with long residual control have become the dominant insecticides used to protect blueberries from fruitworm infestation. For example, Guthion (azinphosmethyl), Sevin (carbaryl), and Lannate (methomyl) are used on 58, 33, and 23% of Michigan’s blueberry acreage, respectively (USDA-NASS, 2006). Because these compounds can only be used after bloom due to bee safety concerns, the lepidopteranspecific insecticide Confirm (tebufenozide) has increased in use to 34% since its recent registration (USDA-NASS, 2006). Despite the reliance of this industry on broad-spectrum insecticides, government restrictions on these pest management tools continue to increase, mandating the search for alternative management tactics. With a phase-out of azinphosmethyl proposed by the United States Environmental Protection Agency by 2012, there is increasing urgency for development of an integrated pest management program for fruitworms in blueberry that is not dependent on such insecticides. Cranberry fruitworm eggs are infested by Trichogramma minutum in field conditions in Michigan, and this parasitoid may provide biological control to suppress Proc. IX IS on Vaccinium Eds.: K.E. Hummer et al. Acta Hort. 810, ISHS 2009 260 populations. Since parasitoids tend to be susceptible to insecticides, it is important to document their relative sensitivity to new pesticides being considered for crop protection against fruitworms. Here we report recent studies to improve the prediction of adult moth emergence and egg-laying by cranberry fruitworm, to improve control of this insect using degreeday-driven timing of insecticide applications and to understand the relative risk of new insecticides to biological control agents. MATERIALS AND METHODS To develop a degree-day-driven phenology model, cranberry fruitworm adult emergence and egg distribution were monitored at eight blueberry farms from 2003 to 2007. Adult emergence was monitored using pheromone traps. Weather data to calculate growing-degree-day (GDD) accumulation were collected from weather stations located in the vicinity of the sampling sites. We used GDD to determine the relationships between heat accumulation, emergence of male moths and deposition of eggs on fruit. We employed a base temperature of 50°F because previous analysis had shown this to provide lowest variability among sites and years (C. Garcia-Salazar, unpublished). A non-linear Weibull distribution model was fitted to the moth and egg abundance data to describe the moth emergence and egg-laying dynamics as a function of heat accumulation. Trapping dates were transformed to their GDD equivalents, and cumulative weekly moth catches from all sampling sites were pooled to estimate the corresponding Weibull parameters. The resulting phenology model for predicting adult cranberry fruitworm emergence was: CumCBFW=1-exp(CumGDD50-biofix/422.02). In 2007, to validate the accuracy of this model, cumulative captures of cranberry fruitworm from seven blueberry farms were compared to those predicted by the phenology model. GDD accumulation (i.e., CumCBFW) began on March 1, and the biofix was set as the cumulative GDD base 50°F at which the first sustained captures of moths are found. Temperature data were collected in the proximity of four trapping sites and from nearby automated weather stations close to the other three sites. Predicted and observed cumulative moth captures were compared using paired t-tests on data from each site. To determine the effect of spray timing on the performance of Confirm against cranberry fruitworm in blueberry, an experiment was conducted at the Trevor Nichols Research Complex in Fennville, Michigan in 2007. A mature planting of Vaccinium corymbosum cv. Rubel was used, and treatments were applied to twelve bush plots (1.43 m long) arranged in a randomized complete block design with guard rows between each treated plot. We compared the fruitworm infestation in plots that were untreated, treated with Guthion 50 WP (1.12 kg/Ha) immediately post-bloom and 14 days later, or treated with Confirm 2F (1169 ml/Ha) at 100, 200, or 300 GDD post-biofix (first sustained moth catch), with each treatment followed by another application seven days later. Cranberry fruitworm adults were monitored with four traps checked three times per week. Degree days above 50°F were calculated each day after seven moths were trapped on May 21 (set as biofix) and treatments were applied when the target GDD were reached. All treatments were applied using an airblast sprayer, in 467.7 L L/Ha of water. Plots were sampled on 29 June, 2007, by counting the number of singleand multiple-berry infestations on blueberry clusters in each plot indicative of cherry and cranberry fruitworm infestations, respectively. To determine the potential of new reduced-risk insecticides for control of cranberry fruitworm, a replicated field trial was conducted with four replicate plots of twelve bushes of V. corymbosum cv. Rubel receiving each of the treatments listed in Table 1. Insecticides were applied as described above and infestation was assessed on July 3, 2007. The numbers of clusters infested with singleor multiple-berry infestation, and the total, were analyzed using analysis of variance and means separation by Duncan’s New Multiple Range Test at P = 0.05. To measure parasitoid sensitivity to registered fruitworm insecticides, ten blueberries, each with a cranberry fruitworm egg parasitized by T. minutum were placed