Comparison of Alternative Management Approaches for Lepidopterous Larvae
in Fall Lettuce

John C. Palumbo

Abstract

For the second year, a large block experiment was conducted at the Yuma Ag Center to compare the field performance of three lettuce management programs for control of lepidopterous larvae. Conventional, experimental and biorational insecticides were sprayed to control beet armyworm, cabbage looper and Heliothis species throughout the growing season. Differences in populations of total larvae among the four treatments, relative to insecticide treatments and timing of application were observed throughout the season. In general, the standard and experimental treatments provided the most consistent control of lepidopterous larvae following each application. Harvest data showed that the spray regimes had a significant influence of head lettuce yield or quality. Maturity and quality were significantly reduced in the untreated control. An economic analysis shows that net returns varied widely among the management programs at different market prices. In conclusion, this study provides preliminary data to support the need for more development of experimental and biorational insecticide products as alternatives to conventional management programs in desert lettuce production.

Introduction

The beet armyworm, cabbage looper and Heliothis species are the major lepidopterous pests of lettuce in desert growing areas of Arizona (Kerns and Palumbo 1997) . Standard insecticides such as methomyl (Lannate) and thiodicarb (Larvin) combined with pyrethroids have been successfully used in controlling this pest complex over the past several years (Palumbo et al. 1993, Palumbo et al. 1994, Palumbo 1996). These products are used frequently during the season, and many speculate that there field effectiveness will soon be reduced under these use patterns. Unfortunately, there are no alternatives insecticides which offer comparable control of lepidopterous larvae. Furthermore, with the recent passage of the Food Quality Protection Act of 1996 (Schreiber 1996), there is speculation that some of the more broadly toxic compounds may be removed from the market in the next few years.The organophosphate and carbamate insecticides (ie. Lannate, Orthene, Larvin, Endosulfan, Monitor, Diazinon) are being targeted as prime candidates for reduced usage.

There are several alternative products available to lettuce growers for management of the lepidopterous complex. However, these consist of biological (Bts), botanical (neem pyrethrum, rotenone) and inorganic (cryolite) insecticides that have been demonstrated to have only marginal activity on beet armyworm and cabbage looper in lettuce (Palumbo et al. 1992, Palumbo 1995). There activity against Heliothis is not known. Several new compounds are currently being developed which offer not only excellent efficacy against these pests, but are also new insecticide chemistries (Palumbo and Kerns 1996). However, we are uncertain how these alternatives will perform in the absence of the conventional insecticide products under desert growing conditions. Therefore a large- block field study was conducted to compare worm management in head lettuce using conventional, experimental and biorational compounds.

Materials and Methods

Lettuce, Lactuca sativa L., 'Early Queen' was direct seeded into large blocks on double-row beds on 9 Sep at the Yuma Valley Agricultural Center, Yuma, Az. Each plot consisted of twelve beds, 200 ft long beds spaced 42 inches apart and bordered on each side by two untreated beds. Plots were thinned to a stand on 30Sep. Plots were arranged in a completely randomized block design with 3 replicates. Treatments consisted of experimental, biorational and conventional lettuce management programs compared to an untreated control (Table 1).

Applications were made on an as needed basis (Table 1). All chemicals were applied by a tractor-mounted boom sprayer operated at 60 psi and 30 gpa. Three, disc-type cone nozzles were used per bed. All chemicals included spreader sticker (Latron CS-7) at a rate of 0.125% of the total volume. Evaluation of lepidopterous larvae control was based the number of live larvae per plant sampled from the center 8 rows of each replicate at 1-3 times per week. The number of samples per replicate varied throughout the season, decreasing as plant size increased (Table 2). The sample unit consisted of examination of whole plants for presence of beet armyworm (BAW), Spodoptera exigua, Cabbage looper (CL) , Trichoplusia ni , and Tobacco budworm (HEL), Heliothis virescens larvae. Each species was characterized as large or small larvae. For BAW and HEL, larvae were considered small if <5 mm in length, large >5mm. For CL, larvae were considered small if <10 mm, large if > 10 mm. The number of Cabbage looper and Heliothis eggs and beet armyworm egg masses per plants were also recorded during the first 50 days of the study.

The effects of larval feeding on plant stand, plant size and growth for each treatment were estimated at pre-thinning (29 Sep), post-thinning (14 Oct), and at pre-harvest (18 Nov) stages. Plant stand was taken by measuring the number of plants within 100 ft on 2 beds within each plot. Plant size and growth was estimated by measuring dry weights (g). Yields were taken when >75% of all heads in the experimental block were considered ready for harvest on 19November (71 days after planting). Yields was taken by weighing all heads within 30 ft of bed in 3 locations within each replicate. Quality was measured by estimating the % marketable heads based on shape and firmness for each head harvested. If heads maintained a reasonable shape and firmness they were considered marketable (#24s). Each head was also evaluated for the presence of worm feeding damage and contamination.A partial budget analysis was used to conduct an economic assessment of each lettuce management

Results and Conclusions

Differences in populations of total larvae among the four treatments were observed throughout the season (Figure 1). In general, the conventional and experimental programs provided the most consistent control of lepidopterous larvae following each application. The seasonal population fluctuations for each Lepidopterous species relative to management program is shown in Fig 2-4. Cabbage looper was most abundant during the study peaking after thinning and again near harvest (Figure 2). All three management programs provided similar control of large larvae, but the conventional program appeared to have the greatest residual control of small larvae. At 7-d pre-harvest, both the experimental and biorational programs had significantly higher larvae numbers than the conventional. As expected, beet armyworm numbers were high during stand establishment (pre-thinning). The experimental management program provided excellent control during this period (Figure 3). Unexpectedly, beet armyworm numbers peaked during the pre-harvest period. Similarly, the biorational program did not prevent larvae from infesting heads at harvest when compared with the experimental and conventional management programs (Table 3). Heliothis number were surprising high in 1997, especially prior to heading (Figure 4). Again the biorational program was inconsistent at holding larval numbers below damaging levels after plants began to head.

Lepidopterous larvae had a significant impact on plant stand and plant growth (Table 4). Prior to thinning the plant stand, plant densities in the conventional plots were most consistent. Plant densities in the biorational plots were significantly lower and larval numbers were higher than the conventional program. As a result of high larval densities, plant stand and plant size were significantly reduced in the untreated plots. After the stand was thinned, all programs had higher plant densities and dry weights when compared to the untreated plots. Larval numbers in the biorational plots were not different from the untreated plots, but this did not effect plant density or plant size. At harvest, lettuce plants in the biorational plots were significantly lighter than in the conventional program.

Harvest data showed that the management programs had a significant influence of head lettuce yield and quality (Table 4). Maturity and quality were significantly reduced in the untreated control. The conventional management program yielded significantly greatest number of cartons than the other management programs when based on average heads/plot, % marketable heads, % worm damage and % head contamination (Table 5). The biorational management program was by far the most expensive, relative to net carton production. Consequently , the economic analysis shows that net returns varied widely among the management programs at different market prices (Table 6.). At a low market price ($5.00/cart) none of the treatments were profitable. At a more common market price ($7.50/cart) the biorational treatment was not profitable, whereas at a market of $10.00 all spray treatments were profitable. In conclusion, this study provides preliminary data to support the need for more development of experimental and biorational insecticide products as alternatives to conventional lettuce management programs under desert growing conditions.

References

1. Kerns, D.L. and J.C. Palumbo. 1997. Lettuce IPM: Southwestern USA. In E.B. Radcliffe and W.D. Hutchinson [eds.], Radcliffe’s IPM World Textbook, URL:http://www.ent.agri.umn.edu/academics/classses/ipm/impsite.htm. University of Minnesota, St. Paul, MN.
2. Palumbo, J.C. 1995. Efficacy and temporal mortality of selective insecticides on beet armyworm larvae in lettuce, pp 130-136. In N.F. Oebker (ed) 1994-1995 Vegetable Report. University of Arizona, College of Agriculture Series P-100.
3. Palumbo, J.C. 1996. Efficacy of Pyrethroids and lannate on lepidopterous larvae in lettuce. Arthropod Management Tests, 21: 124.
4. >Palumbo, J.C. and D.L. Kerns. 1996. Temporal activity of new insecticides chemistries against beet armyworm in lettuce,143-146. In N.F. Oebker (ed) 1994-1995 Vegetable Report.University of Arizona, College of Agriculture Series P-104.
5. Palumbo, J.C., C.H. Mullis, and F. Reyes. 1992. Evaluation of biological control of beet armyworm and cabbage looper in lettuce, 1991. Insecticide and Acaricide Tests 17:115-116.
6. Palumbo, J.C., C.H. Mullis, and F. Reyes. 1993. Evaluation of insecticides for control of lepidopterous larvae inlettuce. Insecticide and Acaricide Tests, 18:133-134.
7. Palumbo, J.C., C.H. Mullis, and F. Reyes. 1994. Evaluation of insecticides for control of lepidopterous larvae in lettuce. Arthropod Management Tests 19: 101-102.
8. Schreiber, A. 1996. Food Quality Protection Act, Part I. Agrichemical and Environmental News, Issue No.130, Dec 1996, p.2-4.

Table 1. Insecticides, rates and dates applied for each lettuce management program, YAC, Fall 1997.

Table 2. Dates, plant stages, and sample numbers on each collection date, YAC, Fall 1997.

Table 3. Numbers of larvae found on lettuce wrapper leaves and heads at harvest, YAC, Fall 1997

Table 6. Yields per acre and costs associated with various spray regimes for management lepidopterous pests on lettuce, YAC, Fall 1996.