Mating Disruption of Beet Armyworm in Vegetables by Synthetic Pheromone
David L. Kerns, Tony Tellez and Jeff Nigh
The beet armyworm pheromone dispenser, Yotoh-con-S, was evaluated for its ability to inhibit mate location and subsequent population growth of beet armyworm in head lettuce. Pheromone dispensers were ineffective at reducing beet armyworm populations the under high population pressure. Under low beet armyworm pressure, pheromone dispenser did significantly reduce beet armyworm populations, but not to the point where insecticide applications could be eliminated.
Beet armyworm, Spodoptera exiqua (Hübner), is one of the most serious lepidopterous pest attacking lettuce in Arizona. Presently, pest control advisors (PCAs) and growers rely primarily on Lannate (methomyl) or Larvin (thiodicarb) to control this pest. However, these products have limited utility. In recent years, high level of resistance to Lannate has led to control failures in portions of Yuma County, and at normal use rates, Larvin is limited to two applications. Although, several new insecticide chemistries are been developed for control of beet armyworm in lettuce, there is much interest in non-insecticide biorational approaches to pest control in lettuce.
The objective of this study was to collect data on the ability of Yotoh-con-S beet armyworm phermone dispenser (Shin-Etsu Chemical Co,. Ltd.) to disrupt mate location and copulating of beet armyworm in head lettuce, and to determine if mating disruption results in a decrease in beet armyworm populations.
Materials and Methods
Experimental Sites. Tests were conducted at four locations, one in 1996 and three in 1997. In 1996, the test site consisted of two ca. 50 acre pheromone treated fields and four ca. 50 acre untreated fields of head lettuce grown by the Barkley Company, located on the Holst Ranch in the Gila River Valley of Yuma. Each pheromone treated field was split to form two replicates, for a total of four replicates. The untreated check fields were located approximately 1.0 mile north, south, eats and west of the pheromone treated fields.
In 1997, test sites were located near Somerton, near Roll and near Texas Hill. The Somerton site, located on Ranch 12 managed by Amigo Farms, consisted of a 30 acre field of broccoli treated with pheromone and a 30 acre field of untreated broccoli located approximately 1.0 mile to the west. The Roll site was managed by Amigo Farms and was located at Avenue 37E and County 5th Street. This site consisted of 30 acres of pheromone-treated lettuce and 30 acres of untreated lettuce located 0.25 mile to the west. The Texas Hill site was managed by the Braden Family Partnership and was located near Avenue 51E and County 2nd Street South. This site consisted of 30 acres of pheromone-treated lettuce and 30 acres of untreated lettuce located 0.25 mile to the west. Winds in Yuma County during the fall are predominately westerly, thus untreated lettuce was located to the west of the pheromone-treated fields to avoid pheromone drift into the untreated fields. Pheromone-treated and untreated fields at the Somerton, Roll and Texas Hill site were divided into four quadrants for sampling and replication purposes.
Pheromone treatments. The Yotoh-con-S pheromone is a brown 20-cm long "twist tie" dispenser, consisting of a polyethylene tube with an aluminum wire. The tube contains approximately 160 mg of a 70:30 blend of (Z,E)-9, 12-tetradecadien-1-ol acetate and (Z)-9-tetradecen-1-ol acetate. To prevent the pheromone dispensers from coming in contact with the ground or the lettuce, they were attached to 18-in long by 0.25-in wide bamboo splits. The pheromone dispensers were attached by twisting them around the top 4-in of the bamboo split. The pheromones were applied approximately at thinning on 17 Oct, 1996 at the Gila River site, on 09 Sept, 1997 shortly after plant emergence at the Somerton site, on follwing thinning on 07 Oct, 1997 at the Roll and Texas Hill sites. Phermones were placed by inserting the bamboo split approximately 5-inches into the soil in the seed line. The bamboo splits with pheromone dispensers were uniformly distributed in the fields at a rate of 100 pheromone dispensers per acre.
Evaluation of mate location. Pheromone traps were used to determine whether or not the male beet armyworm moths could locate a pheromone point source in the pheromone treated fields and in untreated check areas. Yellow/white/green Unitrap pheromone bucket traps (Agrisense/Biosys) were used to trap the male moths. Two bucket traps was placed in each replicate of the the pheromone-treated and untreated fields. Traps were placed inside the field approximately 166 ft. from the edge and were uniformly spaced within each replicate.
The bucket traps were baited with beet armyworm pheromone (rubber septa, Trece, Inc.). Bucket lures were replaced every 2 weeks throughout the evaluation period. Each bucket trap contained a 1 by 3-inch Vapona strip to kill captured moths. The bucket traps were supported by 4 ft long 0.25-in thick metal rods with a hook on the end. The bar was inserted approximately 1 foot into the soil, so that the bucket was suspended about 3 feet off the ground. Moths were collected from the bucket traps every 5 to 7 days.
To further determine if synthetic pheromone inhibited the ability of male beet armyworm moths from locating virgin female beet armyworm moths, on several occasions in 1997, virgin female moths were place in the pheromone retention cage of bucket pheromone traps. One bucket, with one virgin female moth, was centrally placed in each replicate of each field late in the afternoon. Each bucket trap contained a 1 by 3-inch Vapona strip to kill captured mle moths. These moths in these buckets were collected the following afternoon.
Evaluation of population reduction. Beet armyworm populations were estimated within each replicate of pheromone-treated and untreated lettuce by counting the numbers of egg masses and small (1st and 2nd instar), and large (3rd, 4th and 5th instar) larvae from 100 plants. Count were taken on a weekly basis.
Statistical analysis. All data were transformed using a log10 transformation for analysis (non-transformed values presented). These data were analyzed using pairwise T-tests.
Results and Conclusions
Trap Counts. Trap counts from bucket traps containing pheromone lures within the pheromone treated fields were significantly lower than from traps located outside the treated area on all collection dates at each test site (Table 1). indicating that the pheromone dispensers were effective in preventing male BAW moths from finding a pheromone point source. Thus, suggesting that mating within the pheromone treated field would be disrupted.
Similarily to the bucket traps containing synthetic pheromone, trap counts from traps containing virgin female moths were always significantly lower in the pheromone-treated fields than in the untreated fields (Table 2). However, since some male moths were retrieved from traps located in the pheromone-treated fields, suggests that some low levels of mating may occur within these fields.
Beet Armyworm Population Counts. Beet armyworm populations were low at all test locations except the Somerton site. At the Gila River site, populations were highest on 28 Oct when 4.75 larvae per 100 plants were estimated infesting the untreated fields (Table 3). Consequently, 28 Oct was the only date at the Gila River site where a significant reduction in beet armyworm larvae in the pheromone-treated fields relative to the untreated could be detected. At the Somerton site, beet armyworm were high early in the trial averaging over 20 larvae per 100 plants in the untreated plots on 19 Sept, but dropped by early October (Table 4). The Pheromone-treated field contained fewer armyworm larvae than the untreated on 03, 10 and 16 Oct, and fewer small larvae on 13 Nov. At the Roll and Texas Hill sites, armyworm populations were low, never exceeding 4.50 and 1.25 larvae per 100 plants, repectively (Table 5 and 6). Since the pheromone dispensers did effectively disrupt mate location, but eggs and larvae were still being found in the pheromone-treated fields, it was evident that significant mating was taking place outside the pheromone-treated areas and then the moths were moving into the field to oviposit.
Insecticide applications no doubt contributed to the low population densities at these test sites, and made discerning differences difficult (Table 7). However, the difference observed at the Gila River site on 28 Oct did not appear to be influenced by insecticides since the pheromone-treated field had not received any insecticides for more than 10 days. At the high beet armyworm population densities experienced in September at the Somerton site, the pheromone dispensers did not appear to offer significant beet armyworm control (Table 4). Differences between pheromone-treated and untreated fields were observed at the Somerton site, 4 days after insecticide treatment on the 03 Oct evaluation, and 5 days afterwards on the 10 and 16 Oct evaluations, hence the reason small larvae appeared to contribute the most to the total population estimates (Table 4). By 4 to 5 days after treatment, these small larvae were probably not greatly affected by the insecticide treatments, thus suggesting that the pheromone-dispenser did significantly contribute to a reduction in the beet armyworm population at low densities. However, because both pheromone-treated and untreated fields required insecticide treatment, the pheromone dispensers did not offer commercially acceptable control.
Although estimating egg masses would appear to reduce the variability experienced when estimating larval densities compounds by insecticide applications, egg masses were difficult to find and reliably sample (Tables 3, 4, 5 and 6).
Overall, the Yotoh-con-S beet armyworm phermone dispenser did appear to greatly disrupt beet armyworm mating, and resulted in a decrease in beet armyworm population densities under low armyworm pressure. However, the dispensers did not appear to be effective under high armyworm pressure, and under low pressure did not provide the level of control necessary to eliminate insecticide applications targeting armyworms. Additionally, beet armyworm are rarely the only lepidopterous pest present in lettuce in Arizona at an one time. Insecticide applications targeting beet armyworm are almost always also targeting cabbage looper, Trichoplusia ni (Hübner) and sometimes Heliothinae. Thus, the utility of the Yotoh-con-S pheromone in vegetable production in Arizona does not have a good fit.
This is a part of publication az1101:
"1998 Vegetable Report," College of Agriculture, The University of Arizona,
Tucson, Arizona, 85721.