Introduction:

1. Development of an Action Threshold for Spodoptera exigua in TomatoesJames E. Taylor and David G. Riley. University of Georgia, Department of Entomology, P.O. Box 748, S. Entomology Dr., Rainwater Rd. Tifton, GA 31793 Abstract: Action thresholds were evaluated for beet armyworm, Spodoptera exigua (Hübner) in tomato. Beet armyworm is a major fruit feeder in the southeastern tomato growing regions. Insecticide efficacy data was used to estimate new thresholds to compare to current standards. Commercially used thresholds are 1 beet armyworm larvae per 6 plants before fruit set and presence after fruit formation. Data taken from summer 2004 field trials at the Coastal Plains Experiment Station in Tifton, GA suggest an action threshold of 1 larvae per 40 plants for the entire season would be economically viable. This threshold was based on an estimated armyworm seasonal average associated with 5% yield loss. In the Summer of 2005 this new proposed threshold was tested against the commercial standards of calendar sprays. Scouting included whole-plant inspections for all Lepidopteran larvae. Yield was based on weight and number of marketable fruit. Results and Discussion: Based on data collected from the summer of 2004 (Fig 1) we estimated that a season average of 0.15 beet armyworm larvae per plant can cause significant yield loss (P<0.05). This leads to an evaluation of previous commercial EIL that are 1 larvae per 6 plants pre-fruit and presence after fruit set. We proposed from Fig 1 that 1 larvae per 20 plants could be used as an action threshold for treatment for beet armyworm in tomato. We then conducted an artificial infestation field trial in the absence of insecticide applications to attempt to evaluate numbers of beet armyworm larvae to yield loss. Unfortunately, we were unable to obtain yield data as Tomato Spotted Wilt was extremely high (>90%) in the field and yield was variable. We were able to show that low level larval populations were possible through artificial infestation with armyworm egg masses in the spring season, when armyworm pressure is usually low (Fig 2). To evaluate the proposed action thresholds we conducted a field trial using BAS 320I 240 SC. Weekly scouting was conducted to determine if thresholds had been reached and were based on total Lepidopteran larvae per week. Action thresholds of 1 beet armyworm larvae/40 plants and 1 larvae/20 plants weren’t significantly different from the currently used calendar sprays in terms of armyworms (Table 1). At the current time it is unclear if the originally estimated 1/20 plant threshold would be economically viable in tomato because it only provided an intermediate reduction of damage to fruit even though beet armyworms were significantly controlled. Specifically, the action threshold would likely have to be closer to 1 larvae/40 plants. Unfortunately in this field trial, whitefly transmitted Geminivirus (>90% infection) introduced variability into the yield data unrelated to the insecticide treatments and so this test will have to be repeated. Fig 1: An average of 0.15 larvae per 6 plants results in 5% tomato yield loss Introduction: Beet armyworm, Spodoptera exigua (Hubner), is a polyphagous pest on many important commodities in the tropical and subtropical regions. Beet armyworm and other lepidopteran insects are contributors of economic damage of tomato in south Georgia. Estimated losses for the fall tomato and pepper crop in Georgia due to beet armyworm are over $2 million. The beet armyworm is capable of producing economic damage and can be considered a major pest of tomato in the region. Beet armyworm has also shown cross resistance to insecticides (Moulton J. K. et. al. 2000, Meinke and Ware 1978, Brewer and Trumble 1989, 1994). Damage by the beet armyworm can be either foliage or fruit damage (see photos). Both loss in foliage and fruit damage can be correlated into yield loss at harvest with more importance placed on the fruit damage. The economic threshold of beet armyworm is dynamic throughout the growing season with the likelihood of increased damage in the late season (Zalom, F. G. et al,. 1983). Fruit damage can be more detrimental to economic loss due to effect on marketable yield due to low tolerance of fruit damage. Fig 2: A relationship of number of egg masses to number of larvae within the artificial infestation trial per 30 plants. Materials and Methods: The summer 2004 field trials were conducted at the Coastal Plains Experiment Station in Tifton Ga. on Lycopersicon esculentum hyb. Solar Set. They were transplanted in the field on 16 June with 2-ft spacing into 1 row 6-ft wide white plastic mulch. The experimental design was a randomized complete block with four replicates. A total of nineteen insecticide treatments were used to evaluate treatment efficacy and relate that to marketable yields. Scouting included whole plant inspections of 6 plants/plot. Tomatoes were harvested on 2 and 11 of September from 10-ft of row (5 plants) and were categorized as marketable or unmarketable (primarily Lepidoptera larval damage) and the average weight was measured. An artificial infestation trial in the spring of 2005 at the Coastal Plains Experiment station on Lycopersicon esculentum hyb. FL 47 was conducted to attempt to achieve natural field populations present in the 2004 summer season. This trial was conducted in the absence of insecticide application. They were transplanted on black plastic mulch. The experimental design was four treatments within a split randomized complete block with four replicates. The four treatments were; control (no egg masses), 2 (1/4) egg mass, 5 (1/4) egg mass, and 25 (1/4) egg masses per plot (1/4 egg masses equaled 15-30 eggs) per 30 plants. Scouting included whole plant inspections per plot for weekly evaluation of infestation efficacy. Tomatoes were harvested on 14, 21, and 30 of June with 50 fruit/plot per harvest and were categorized as marketable or unmarketable and the average weight was measured. The summer of 2005 field trials were conducted at the Coastal Plains Experiment Station to evaluate new proposed economic injury levels (EIL). Within an insecticide efficacy field trial 2 economic injury levels were compared to a current commercial control standard. The insecticide used to evaluate the proposed EIL was BAS 320I 240 SC. This trial was conducted within a randomized complete block with four replicates on Lycopersicon esculentum hyb. BellaRose on white plastic mulch. They were harvested on 20 of Sept. with 50 fruit/plot per harvest and 10-ft of row (5 plants) and were categorized as marketable or unmarketable and the average weight was measured. Data was analyzed using ANOVA and LSD tests for separation of means (SAS Institute 1991). Table 1: Data from summer 2005 of evaluated action threshold levels. * Means within columns followed by the same letter are not significantly different (P>0.05, LSD test). References Brewer, M. J., J. T. Trumble. 1989. Field monitoring for insecticide resitance in beet armyworm (Lepidoptera: Noctuidae). J. Econ. Entomol. 82: 1520-1526. Brewer, M. J., J. T. Trumble. 1994. Beet armyworm resistance to fenvalerate and methomyl: resistance variation and insecticide synergism. J. Agric. Entomol. 11: 291-300. Meinke, L. J., G. W. Ware. 1978. Tolerance of three beet armyworm strains in Arizona to methomyl. J. Econ. Entomol. 71: 645-646. Moulton, J.K., D. A. Pepper & T. H. Dennehy. 2000. Beet armyworm (Spodoptera exigua) resistance to spinosad. Pest Management Science. 56: 842-848. Zalom, F. G., L. T. Wilson, M. P. Hoffmann, W. H. Lange & C. V. Weakley. 1983. Monitoring lepidopterous pest damage to processing tomatoes. California Agriculture. University of California.