Evaluation of Insect Growth Regulators for Management of Whiteflies in Melons

John C. Palumbo

Abstract

Whitefly populations were assessed under different IGR exposure levels, and compared to Admire. When used alone during the season Applaud, Knack, and Sterling significantly reduced immature colonization similar to the standard Admire application and significantly greater than the untreated melons. Applaud treatments, regardless of spray frequency, showed the most consistent reduction in immature whiteflies. Applaud through its vapor activity also appeared to provide a long residual period of control against nymphs. Single applications of Knack and Sterling were considerably less effective in preventing colonization than applying these material twice during the season. These compounds appeared to have considerably less residual activity, which is consistent with their modes of activity. All of the IGRs had a significant impact on the distribution of nymphs among the leaves on the primary vine. In addition , Applaud provided the best melon quality. We now have a good understanding of how the IGRs influence whitefly population growth, the residual mortality of the IGRs and proper application timing for whitefly management. This information will allow us to develop a simple and reliable method that growers and PCAs can use to assess product performance and time spray applications.

Introduction

The number of effective insecticides currently available to growers for insect control in melons is relatively small compared with other crops. Furthermore, with the uncertainty surrounding the recent passage of the Food Quality Protection Act, as well as increasing environmental concerns, the melon industry in the western U.S. could potentially be facing the loss of a number of important insecticides. A recent example was the voluntary withdrawal of Phosdrin from the vegetable market. Fortunately, this did not have a significant impact on melon production because the loss of Phosdrin for aphid control coincided with the development and registration of Admire. However, as older chemicals are lost, it is critical that replacement chemistries be available that live up to both regulatory and grower standards. There are several new insecticide chemistries currently being field developed that have demonstrated activity on many of the key sucking pests that infest melons. The discovery of these new insecticides could not have come at a better time.

The new insect growth regulators (IGR) being developed have completely different modes of action than Admire and other products used in melons. They offer a great deal of promise for pest management because they are very selective for sucking insect pests, and are environmentally friendly to handlers and consumers. In some cases, these products may only be 1-2 years away from full registration on melons. However, strategies to use these products on melons have not been fully determined. Thus, the development of methods to integrate these new chemicals into our local pest management programs will be very important for the sustainability of western melon production.

While we have determined that these chemicals have activity against the sucking insects that infest melons, Palumbo et al. 1996a, Palumbo et al. 1994, Palumbo et al. 1993) it is important that we gain a better understanding of how these materials will be used in a our local growers management programs. Thus more intensive field development work will be required to determine how to best use these products, such as proper timing of application, residual activity, secondary activity on beneficials and other emerging pests.The objective of this study was to evaluate the impact of the IGRs on the growth and development of whitefly populations and their natural enemies in cantaloupes under spring/summer growing conditions

Materials and Methods

The goals of this research project were to examine and quantify the population dynamics of sucking insects and their natural enemies on melons in response to IGR treatments. Although alate aphids were present early in the study, aphids did not colonize in high enough numbers to collect data. Whitefly populations were assessed under different IGR exposure levels, and in combination with Admire. We now have a good understanding objective involved intensive field sampling and data collection to examine and quantify the population dynamics of immature whiteflies and their natural enemies on melons in response to IGR treatments. The study was designed to provide an assessment of whitefly population development under different IGR exposure levels, and begin to understand residual mortality and optimum timing. Ultimately, this data will allow us to develop a method for assessing product performance and spray timing. Cantaloupe plots ‘Durango’ were established at the Yuma Agricultural Center on March 3, 1997 and managed similarly to local growing practices. Plots consisted of 4-80 inch beds by 60 ft long with a 15 buffer between each plot. The study was designed as a randomized complete block design (4 replicates/treatment) with the IGRs (Table 1) applied to melons using two different approaches: The first consisted of a single application (April 25) coinciding with the first occurrence of nymphs on the crown leaves. The second approach consisted of two applications, the first coinciding with the first occurrence of large nymphs(April 25), and the second applied 17 days later (May 13). The IGRs were applied at their manufactures recommended rates: Knack (8 oz/acre), Applaud (8 oz/acre) and Sterling (0.09 lbs[ai]/acre). Admire (16 oz/acre) was used as a standard and applied preplant, 3" sub-seed line.

Whitefly populations were assessed by making estimates of immature and adult densities bi-weekly (9, 23 April, 5, 20 May and June 9). We estimated the effects of the IGRs on population densities by making intensive whole plant counts. This entailed removing the entire primary vine of 5 plants within each replicate and estimating whitefly densities on each leaf under magnification. These data provide both seasonal growth rates of the population and a within-plant distribution of immatures relative to the activity of each IGR. Natural enemies were evaluated at each sample period by examining nymphs for presence of predator feeding and parasite emergence. Additionally, vacuum samples were made periodically to estimate natural enemy abundance. An assessment of melon quality was made on June 12 to measure sooty mold contamination, foliage quality, sun exposure to melons, and fruit maturiy (% full slip melons) per 10 row feet.

Results and Conclusions

The whitefly population present during the study was moderate-high. In general, each of the IGRs significantly reduced immature colonization similar to the standard Admire application. Table 3 shows the densities of immature whiteflies averaged across all sample dates from both the most infested leaf on the plant (MIL), and estimated from all leaves on the primary vine (Total vine). Both estimates show that among the three IGRs, Applaud provided the most consistent control of nymphs. Although none of the IGRs causes direct mortality to adults, the Applaud treatments had the largest impact on adult populations, presumably by it’s activity of preventing immature colonization (Table 2).

Figures 1 and 2 show the effects of the IGR treatments on seasonal immature population growth. Applaud treatments showed the greatest reduction in most cases, and in general, the single applications of Knack and Sterling were considerably less effective in preventing colonization than applying these material twice during the season. Further, the second application of Applaud did not provide a significant reduction in whitefly densities. This suggests that Applaud through its vapor activity provided a long residual period of control. The moderate temperatures occurring during the spring growing season probably allow for much longer activity than is experienced in cotton, where Applaud has a 14-21 day residual period. The data also suggests that Knack and Sterling have considerable less residual because 2 applications with these compounds were required to provide similar control. This is consistent with their modes of activity which effect primarily require ingestion for whitefly mortality.

Yield data was not recorded upon the request of the CMRB. However, we did make an assessment of melon quality at harvest. Table 4 shows the results of a single assessment made on 12 June. There were no differences among the treatments in the number of netted melons in 10 row ft. However, there were differences among treatments in foliage rating, % sunburned melons, % sooty mold contamination and melon maturity that can be directly associated with whitefly feeding and damage. These data show that Applaud, regardless of spray frequency, provided the best melon quality. This is consistent with the lack of colonization by small and REN nymphs observed in these treatments.

Natural enemy activity in our research plots was minimal during these studies. Several species of predators were recorded in all plots consisting primarily of Geocoris, Orius, Chrysopa, and lady beetles. These counts never exceeded 1 / ft2 and were not different among the IGR treatment and the untreated plot. Estimates of nymphal mortality due to parasitism and predation averaged from 10-20% at the end of the study, but again did not differ among the treatments. These results are consistent with our previous observations that parasitism and predation in untreated plots are generally low under normal desert growing conditions. Further, this data also supports the claims that these IGRs and Admire have minimal impact on natural enemy populations.

Timing of application appears to be important for these products. Our previous experience with these products in cotton and lettuce, suggested that timing applications early in population development was critical for optimal control. The results of our study support this approach, especially for Sterling and Knack which appear to have less residual. Consequently, frequency of application may be more critical for these products. Based on these data, initial timing of applications should be made when RENs first appear on the crown leaves. It is difficult to recommend subsequent applications until we replicate our study and finalize our analysis of nymphal distribution within the vine. However, based on this study, we can say with some confidence that subsequent Applaud sprays may not be necessary under low- moderate whitefly conditions, but Sterling and Knack will probably require more than 2 applications under normal desert growing conditions. Ideally, once these products become registered they should be used only in conjunction with Admire and other foliar alternatives for product sustainability.

Figure1

Figure2


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_5.html
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