New Insecticide Alternatives for Aphid Management in Head Lettuce

John Palumbo, Clayton Mullis, Jr., Francisco Reyes and Andreas Amaya

Abstract

Several new insecticide was compared to Admire and Provado combinations for management of aphids in head lettuce in Yuma in two trials conducted in 1998. Foliar applications of Provado, Fulllfill, Aphistar and Acetamiprid appear to provide an alternative method of controlling aphids on lettuce comparable to prophylactic applications of Admire. In addition, at planting and side dress soil applications of thimethoxam provided aphid control comparable to Admire. The prevention of aphid colonization in lettuce heads with the foliar alternatives may depend greatly on the timing and frequency of applications before harvest occurs. Residual activity of the new foliar alternatives ranged from at least 7-14 days. These studies suggest that more than one application of the foliar products will be necessary to adequately suppress aphid contamination in heads. Evaluations of thiamethoxam suggest that it is more mobile in the soil than Admire and may be a candidate for side dress applications for aphid management.

Introduction

Green peach aphid, Myzus persicae (Sulzer), is a common pest of iceberg lettuce, Lactuca sativa, in the western United States. This polyphagous aphid species causes economic damage to lettuce through direct injury, virus transmission, and contamination of heads. Because most Western iceberg lettuce is packed in the field at harvest, it must be relatively free of contamination by aphids and other pests. Consequently, lettuce is routinely treated with insecticides throughout the season to prevent aphid colonization. Over the past two years aphid populations in the Yuma area have occurred at manageable levels that growers can tolerate. These declines in pest populations can largely be attributed to the use of Admire 2F (imidacloprid) soil treatments which provide season long protection against green peach aphids. However, an important development in the past year has been the development of several new insecticides. We conducted two studies in the spring of 1998 to evaluate the effects of several foliar alternative chemistries and a new , soil applied chlornicotinyl insecticide compared with Admire. Our objective in these studies was to determine the proper timing and frequency of application of these chemicals for aphid management in head lettuce.

Materials and Methods

The Admire and thiamethoxam at plant treatments were applied as a preplant injection at a depth of 1.5" below the seed line at bed shaping in 10 gpa final dilution. The Admire and thiamethoxam side dress treatments were applied similar to fertilizer side dressing and the materials were placed in the same location as N in 10 gpa final dilution. The foliar applications were applied at 60 PSI in 30 gpa total volume. Insecticide rates and application dates for all treatments are shown in Table 1. Insect data were collected only from the inner two beds of each plot at approximately 2-week intervals, varying with plant size (see Table 2). Ten plants were randomly selected in each plot and carefully placed inside 33 liter collection cans made of galvanized steel. Wire mesh was placed near the bottom of the can to suspend the plants above a steel funnel which emptied into 0.5 liter cardboard containers. Several large cotton wicks were saturated with methyl ethyl ketone and placed inside each can. After 10 to 15 minutes, all leaves were shaken to dislodge aphids into the cans and collected in the cardboard containers. Leaves were further examined to record any aphids which had not been dislodged. Aphids were then transported to laboratory and placed at -10 C for 24 hrs. The number of alate(winged) and apterous aphids were counted with the aid of a stereo microscope. Two yellow sticky traps were placed in the test area to capture alate aphids throughout the season. Traps were replaced twice weekly and aphids were identified to species. Infestation levels of apterous aphids were also estimated at harvest (21 March) by randomly selecting 10 plants within each replicate and visually counting the number of aphids on wrapper leaves and heads.

Results and Conclusions

Trial I

The aphid species in this study were comprised of green peach aphid, Myzus persicae. Aphid populations were relatively light in the first planting. The first noticable increase apterous aphids were at about 70 days after planting (19 Jan) (Figure 1a). Significant reduction in aphid numbers among treatments was first detected on 3 Feb, where minimal colonization had occurred in the Admire and thiamethoxam at planting treatments. At the final pre-harvest sample (18 Feb), all treatments had significantly less aphid colonization than the untreated control. Of the insecticide treatments, the Admire side dress appeared to provide the weakest control. This was also evident in the head contamination assessment made at harvest (Figure 1b). It should be noted that aphid densities in the untreated control at harvest averaged less than 3/head and may not have resulted in significant yield reductions.

Comparisons of Admire and Thiamethoxam efficacy following applications are shown in Fig 2a and 2b. As has been experienced in previous trials, the Admire at planting application provided consistent control of aphids throughout the 110 d growing period (Figure 2a). Thiamethoxam was more variable, but provided >90% reduction of aphid colonization at harvest when compared to the control. The Admire side dress application did not provide significant aphid control at harvest (Table 2b). However, thiamethoxam applied as a side dress provided much better control. This product is very similar to admire (imidacloprid) in chemistry and activity, but appears to be more mobile in the soil.

Comparison of efficacy among the new foliar alternatives is shown in Figure 3. Acetamiprid appeared to show the best activity following the first application. However, following the second application at both 7 and 14 days, all the new insecticides showed similar efficacy. This activity was sufficient to prevent significant head contamination at harvest (Figure 1b).

Trial II

The aphid species in this study were again comprised of green peach aphid, Myzus persicae. Aphid populations were much heavier in this planting. Similar to the first study, the first noticeable increase of apterous aphids was at about 70 days after planting (26 Jan) (Figure 4a). The occurrence of aphid colonization at this time in both studies is associated with a sharp increase in winged aphid captures on sticky traps. Significant reduction in aphid numbers among treatments was first detected on 25 Feb, where all the foliar alternatives and the Admire at plant application had significantly less colonization. At the pre-harvest sample, numbers in the untreated control had exceeded 350 aphids/plant. This resulted in sginificant head contamination (Figure 4b). Both the Admire at plant and conventional foliar management approaches maintained aphid numbers below 10 aphids/plant. This resulted in no detectable head contamination at harvest. All other treatments maintained aphid numbers below 20 aphids/plant and contamination varied from 0.5-1.5 aphids/head.

Comparisons of Admire and Thiamethoxam efficacy following applications are shown in Fig 5a and 5b. The Admire at planting application again provided consistent control of aphids throughout the growing period (Figure 5a). Although thiamethoxam provided >80% control at pre-harvest (Figure 5a), this resulted in significantly high plant colonization than seen in the Admire (Figure 4a). The Admire side dress application showed much better activity in this trial (Table 5b). However, control was variable, especially at harvest where heads were found to be contaminated. Thiamethoxam provided excellent activity as a side dress application. It did not differ significantly from the Admire at planting treatment.

Comparison of efficacy among the new foliar alternatives is shown in Figure 6. Again similar to Trial I, acetamiprid provided the best activity among the new products following the first application. Surprisingly, the conventional Dimethoate and Orthene provided similar efficacy to acetamiprid. Following the 2nd and 3rd applications, Fulfill, acetamiprd and the Provado combinations provided >90 % control for at least 14 days. Aphistar appeared to have less residual activity.

Conclusion. The Admire-at plant application continues to provide excellent aphid control throughout the season. In addition, when timed properly, the rotation of Provado with other conventional products continues to be a viable alternative to Admire. However, there is a need for alternative chemistries to help managment resistance imidacloprid, consequently, the results of these studies are very encouraging. Although, thiamethoxam and acetamiprid are of the same chemical class of Admire and Provado (chlornicotinyls ), they showed to provide good aphid control. Further development of their use patterns will be necessary in the future to avoid exposing the chloronicitinyl chemistry to excessive aphid selection pressure. The other foliar new chemistries (Fulfill and Aphistar) showed good activity, especially when used in rotation with acetamiprid.

Table 1. Description of treatment regimes used in spring aphid trials, Yuma Ag Center, 1997-1998.

Table 2. Sample dates and associated plant stages of lettuce.

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This is a part of publication AZ1101: "1998 Vegetable Report," College of Agriculture, The University of Arizona, Tucson, Arizona, 85721.
This document located at http://ag.arizona.edu/pubs/crops/az1101/az1101_1 .html
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