© 2012 Plant Management Network.
Impact of Tobacco Thrips on Cowpea
Paul McLeod, Department of Entomology, University of Arkansas, Fayetteville, AR 72701; and Tahir Rashid, Extension/Research Demonstration Farm & Technology Transfer Center, Alcorn State University, Mound Bayou, MS 38762
McLeod, P., and Rashid, T. 2012. Impact of tobacco thrips on cowpea. Online. Plant Health Progress doi:10.1094/PHP-2012-1019-01-RS.
In the southern US, seedling cowpeas are severely injured by the tobacco thrips, Frankliniella fusca (Hinds). This feeding results in discoloration and distortion of the leaf tissue. Although damage from thrips feeding on foliage is common and appears substantial, recent studies have demonstrated little impact on cowpea yield. Caging up to ten F. fusca per cowpea seedling failed to significantly reduce number of pods, seed number, or seed weight when compared to cowpea seedlings caged with no thrips. Also, no significant differences in the days to initial flowering were detected among the treatments, i.e., 0, 1, 2, 5, or 10 thrips per seedling. In field studies, the insecticide, bifenthrin, applied to cowpea seedlings effectively reduced the number of thrips and significantly reduced foliar damage when compared to non-treated seedlings. At harvest however, no significant differences were detected among the other variables tested, i.e., pods per plant, seed per plant, seed per pod, seed weight per plant, nor weight per seed. Thus, data indicate that the impact of F. fusca on cowpea produced in the US is likely less than previously thought and early season thrips management is of little value.
Cowpea, Vigna unguiculata (L.) Walp., is a major vegetable crop in much of the world. Seed contain approximately 25% protein and serve as a chief protein source in much of the developing world (9,10). In recent years, cowpea has become an important vegetable crop in the south-central US. This recent production increase in the south-central US is due to the ability to produce the crop in an area low in cowpea cucurlio, Chalcodermus aeneus (Boheman), populations and an expanded need by the processing industry for cowpea (7). High drought tolerance, nitrogen fixation, and relative ease of cultivation make it an appealing choice for producers as a low cost, last resort crop that can be planted when conditions do not allow for the production of higher value crops. In the US most of the estimated 52,800 to 70,400 acre of cowpea is produced in southern regions and is harvested as dry beans for use by the processing industry (9) and for local fresh markets.
Thrips are a major insect pest of cowpea worldwide with different species attacking either the blooms or foliage (4,10). The most noticeable effect of thrips on cowpea occurs during the initial two or three weeks after seedling plants emerge from the soil (2,8). During the seedling stage tobacco thrips, Frankliniella fusca (Hinds), is the most common thrips species and injures cowpea by feeding on newly developing leaves (11). Thrips possess modified mouth parts capable of puncturing and extracting plant sap from the cells of leaves. This results in leaf distortion and blasting (Fig. 1). Although thrips damage appears substantial during seedling development, after about three weeks the damage becomes less noticeable and subsequent foliage generally appears normal (5). Little information is available on the impact of F. fusca feeding on seedling cowpea. Although Chalfant and Johnson (3) reported a correlation of foliar injury of cowpea seedlings with yield losses, the emphasis of their study was with insecticide use. The insecticides used in their trials have been reported to stimulate plants to increase yield regardless of insect pressure (1,12). Also, use of these insecticides may control damaging insects later in the season and yield increases may not actually be the result of thrips control during the seedling stage. Information on the direct impact of seedling thrips on cowpea is lacking and needed. The objective of the research reported herein was to determine the impact of F. fusca feeding during the seedling stage on cowpea yield.
Greenhouse Cage Studies
A cage study was initiated in a greenhouse at the University of Arkansas Main Experiment Station, Fayetteville, during the spring of 2009. Cowpea seed cv. Early Scarlet were hand planted at weekly intervals in one-gallon black plastic pots filled with potting mix (Miracle-Gro, Miracle-Gro Lawn Products Inc., Marysville, OH). Emerging cowpea seedlings were caged by placing an 8-inch diameter by 12-inch-tall cylinder made from No-Thrip insect screen (Green-Tek, Edgerton, WI) around the top of the pot. The cage was secured to the pot with a large rubber band. Lighting was natural and temperature was maintained at 32°C ± 5°C. Plants were fertilized bi-weekly with 13-13-13 fertilizer.
Thrips were collected from field grown seedling cowpea at the University of Arkansas Main Experimental Station, Fayetteville. Whole plants in the V-1 and V-2 developmental stages were pulled, placed in a cooler and transferred to the laboratory (6). Plants were shaken over a white enamel pan and adult F. fusca were transferred to glass vials with a camelhair brush and vials were capped. The number of thrips per vial were 0, 1, 2, 5, and 10 (Table 1). Cages were opened and a vial containing the thrips was placed on the potting soil. Caps were removed and cages were closed allowing the thrips to move to the plant. At time of infestation, plants were the V-1 (cotyledon) stage.
Table 1. Impact of adult F. fusca on cowpea flowering and yield in greenhouse cages.
Foliar injury level: 0 = no damage, 1 = 1-20%, 2 = 21-40%, 3 = 41-60%, 4 = 61-80%, and 5 = 81-100%.
Within column means followed by the same letter do not significantly differ (P < 0.05).
A total of 20 cages were used for each infestation level. At 20 days after infestation, cages were removed and foliar damage ratings were taken (Table 1). The damage rating was based on percent of leaf surface area showing thrips damage with ratings of: 0 = no damage, 1 = 1-20%, 2 = 21-40%, 3 = 41-60%, 4 = 61-80%, and 5 = 81-100%. To prevent further infestation, imidacloprid (Provado 1.6F) was applied with an All Purpose Hand Sprayer (Gilmour, Robert Bosch Tool Corp., Peoria, IL) calibrated to deliver 0.1 lb ai/acre weekly until pod maturity. Once pods had matured and dried on the plants they were removed, shelled, and seed were weighed. Statistical analysis of data on days to bloom, pod number, seed number, and dry seed weight was performed via one-way ANOVA using the GLM procedure of SAS software (SAS 9.1, SAS Institute Inc., Cary, NC).
Field Studies to Manage Thrips
Field studies utilizing bifenthrin for thrips management on seedling cowpea were conducted at the University of Arkansas Main Experiment Station, Fayetteville, during 2010 and 2011. Cowpea seeds, cv. Early Scarlet, were planted on 8 June 2010 in two rows 200 ft long. Seeding rate was 5 seeds per row ft and rows were spaced 38 inches. Herbicides included imazethapyr and S-metolachlor and rows were furrow irrigated. Plots consisted of a single row 25 ft long. The experimental design was RCB with eight replications. In plots managed for thrips, the insecticide bifenthrin (Capture 2EC, FMC Corp., Philadelphia, PA) was applied at 0.04 lb ai/acre during the V-1, V-2, and V-3 developmental stages with a CO2-powered backpack sprayer equipped with one Delvan HB10 nozzle. Pressure was 25 psi and spray volume was 19.4 gpa. Control plots were not treated. When plants reached the V-3 developmental stage 10 plants were randomly selected from each plot and rated for foliar damage as described above. Plants were inspected daily for blooms and at maturity, pods were harvested from each of 10 randomly selected plants per plot and seed weighted as described in the greenhouse study. The field test was repeated in 2011. The only difference was the planting date of 7 June. Statistical analysis of data on foliar damage rating, days to bloom, pod number, seed number, and dry seed weight was performed via one-way ANOVA using the GLM procedure of SAS software (SAS 9.1, SAS Institute Inc., Cary, NC).
Greenhouse cage study. Caging even a single F. fusca adult on an emerging cowpea seedling produced a significant increase in foliar damage (Table 1). The mean foliar injury level increased form 0.2 for seedlings with no thrips to 1.2 for plants with one F. fusca. Although thrips are minute insects, most attacking cowpea seedlings are found within new leaves prior to the unfolding of the leaf. Jones (5) reported that up to 86% of adult F. fusca on cowpea seedlings were found in newly formed leaves prior to unfolding. Leaf tissue damaged prior to unfolding, becomes much more pronounced after the leaves expand (Fig. 1). Additional increases in thrips numbers on caged cowpea caused significant increases the level of foliar damage. On plants with 10 F. fusca adults, the mean foliar injury level was 4.4 on a scale of 5. On these plants almost all of the foliage was distorted. Although foliar damage was extensive, by three or four weeks after plant emergence, newer leaves had developed with little sign of thrips injury and plants generally appeared normal. The number of days to initial flowering ranged from 27.1 for seedlings with 5 F. fusca to 28.1 for control plants (Table 1). However, this difference was not significant. Also, no significant differences due to the number of caged thrips were detected among the other variables tested, i.e., pods per plant, seed per plant, seed per pod, seed weight per plant, nor weight per seed. In a 2007 field study, Jones (5) reported the highest mean number of F. fusca per seedling cowpea plant was 1.08. During 2008, the number of F. fusca per seedling cowpea ranged from 0.15 to 5.05 (5). Additional thrips samples taken from seedling cowpea in Arkansas and Oklahoma during the past years rarely found more than two or three F. fusca per plant prior to the V-3 developmental stage (P. McLeod, unpublished data). The greenhouse cage study demonstrated no impact of cowpea yield when seedling cowpea were infested at up to 10 F. fusca per plant. Thus, it appears unlikely that F. fusca reaches levels that negatively impact cowpea yield in the south-central US.
Field studies to manage thrips. During 2010 field studies the mean number of F. fusca per non-treated cowpea seedling was 2.4 and the resulting foliar injury level was 2.9 (Table 2). The application of bifenthrin effectively reduced the number of seedling thrips as none were detected. The foliar injury level in plots receiving the insecticide was 0.4, a significant reduction. Thus, the naturally occurring F. fusca population caused foliar damage to greater than 50% of the seedling foliage. However, no significant differences were detected among the other variables tested, i.e., pods per plant, seed per plant, seed per pod, seed weight per plant, nor weight per seed. Similarly during 2011, significantly more thrips were detected on non-treated cowpea seedlings than those receiving bifenthrin and foliar injury level was much greater in unsprayed plots, i.e., 3.7. Again, no significant differences were detected in days to initial flower or among the yield variables measured.
Table 2. Impact of early season F. fusca management on cowpea flowering and yield.
Foliar injury level: 0 = no damage, 1 = 1-20%, 2 = 21-40%, 3 = 41-60%, 4 = 61-80%, and 5 = 81-100%.
Within column means in the same year followed by the same letter do not significantly differ (P < 0.05).
In conclusion, F. fusca severely injures the foliage of seedling cowpea resulting in substantial leaf distortion and blasting. Within three or four weeks after plant emergence, however, plants appear to recover and show little effect of the early feeding. Further, this initial feeding during the seedling stage has no direct impact on cowpea flowering and yield. Although interaction of thrips and other st ressors such as drought or herbicide injury has not been studied with cowpea, it appears that management of thrips on seedling cowpea is of little value to the producer.
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