Trap Cropping in Management of Cucumber Beetles
Commercial Horticulture Newsletter, July-August 1998
John S. Caldwell
Department of Horticulture
Virginia Tech (0327)
Blacksburg, VA 24061
Stephanie Stockton
Virginia Cooperative Extension
101 South West Street
Culpeper, VA 22701
Introduction. Striped and spotted cucumber beetles [striped, Acalymma vittata (Fabricius); spotted, Diabrotica undecimpunctata howardi Barber; Coleoptera: Chrysomelidae] are major pests of vine crops (cucurbits) in Virginia (Caldwell et al., 1995), due to feeding damage on young plants, transmission of bacterial wilt, and feeding damage on fruit. Cucumber and melon are most susceptible to bacterial wilt, while most squash and Jack-O-Lantern pumpkin types are not. However, Hubbard and butternut squash and processing pumpkin varieties with butternut squash in their breeding history can also be susceptible to bacterial wilt (Foster and Flood, 1995).
Cucumber beetles show preferential attraction to squash and pumpkin (Cucurbita pepo and C. maxima) over cucumber and melon (Cucumis sativa and C. melo), and to certain squash varieties in particular, for several reasons. Cucumber beetles tend to aggregate in small areas, rather than spread out uniformly over a field. This makes it easier to concentrate them on a more attractive crop if the field has more than one cucurbit crop. They will also fly over one type of cucurbit to reach a preferred type. This preference in crop seeking appears to be due to differences in volatile compounds emitted in the air by the plants. In addition, the amount of a compound called cucurbitacin B in the variety may cause beetles to stay longer on a preferred crop after they have found it (Radin, 1992).
Squash cultivar NK530 has shown preferential attraction to cucumber beetles, compared with cucumber and other squash cultivars in Maine (Radin, 1992), and Virginia (Caldwell et al., 1998). Pair (1997), using a different squash trap crop cultivar Lemondrop, found that the trap crop planted to less than 1% of total crop area attracted 32-66% of cucumber beetles in canteloupe, squash, and watermelon. Cucumber beetles are believed to overwinter in wooded areas (Houser and Balduf, 1925). Radin (1992) suggested that density would be expected to decrease as distance from the trap crop increased. This trial had as its objective to assess the attractiveness of this trap crop cultivar to cucumber beetles in a pumpkin field, and the extent of change in population as distance increased from the trap crop.
In addition, we monitored one known and one possible natural enemy of cucumber beetles. The known enemy is the Pennsylvania leatherwing, Chauliognathus pennsylvanicus DeG. (Coleoptera: Cantharidae). It looks like a firefly, to which it is related. Its larvae (wormlike young) are predators. They grasp and eat adult beetles when they are on the ground (Houser and Balduf, 1925). We also monitored lady beetles (Coleoptera: Coccinellidae). While many lady beetle species feed on aphids, some are more general in their feeding habits. One of the best examples of a generalist lady beetle is Coleomegilla maculata (AC. mac@). AC. mac@ has been shown to significantly reduce populations of Colorado potato beetle eggs and small larvae (Groden et al., 1990; Hazzard and Ferro, 1991). Although its potential for similar predation on cucumber beetle eggs and small larvae is unknown, such predation would be consistent with its feeding habits. Understanding its distribution in a cucurbit field is one part of research to determine to what extent it may have a role in predation of cucumber beetle eggs and larvae and thereby could affect populations of the next generation of adult cucumber beetles.
Materials and Methods. Seed of >NK530' trap crop was planted on June 10-11, 1997, at the forest edge of a pumpkin field. The pumpkin was planted on June 13-14, 1997, using cultivar Howden. The field had dimensions of 490 ft by 200 ft (Figure 1). Along the 490 ft edge next to the forest (top horizontal dimension in Figure 1), six transects were established perpendicular to the forest (vertical rows of plots going down in Figure 1). Each transect consisted of five 40 ft x 25 ft plots separated by 25 ft. These plots were labeled as sampling stations 0, 50, 100, 150, and 200 ft. In three transects, the 0 ft station was planted in the trap crop, with 80 plants/plot. Trap crop plots were separated by 180 ft. In between each trap crop transect, a second transect of five stations was established at 50 ft rom the trap crop transect edge. At each sampling station (trap crop or pumpkin), visual counts of the above pest and beneficial insects were made on five plants in the fourth row. A yellow sticky card was placed between the fifth and sixth plant in the fifth row, and counts were taken from the card after it had been one week in the field. Counts were taken on six dates: July 3, 22, and 30, and August 7, 15, and 22.
Differences between treatments in numbers of each species of cucumber beetle were analyzed by analysis of variance and orthogonal contrasts (PROC GLM of the Statistical Analysis System). Orthogonal contrasts examined the differences in counts for two variables: distance, comparing the edge (0 ft station) and subsequent stations for both crops together; and the interaction between crops (trap crop vs. pumpkin) and distance. Insect counts were transformed using _ (x + 0.5) prior to analysis.
Results and Discussion. Populations of both species were low during July, but increased in August (Figure 2). The mean count of both species on August 21 (11.0 insects) was 3.6 times the average of mean count for July (2.3 beetles). This is consistent with the life cycle of cucumber beetles, which produce the first true generation in August after adults of the overwintering population mate in June and die in July. In August, as populations increased, spotted cucumber beetle populations were approximately 5 times greater than striped cucumber beetle populations. Populations of both species together exceeded 6 beetles / sticky card on August 15, and spotted cucumber beetles alone exceeded this count on August 22. A count of 6 beetles / sticky card was found by correlation analysis of 135 paired visual and sticky card observations to correspond to a visual count of 5 beetles on 5 plants in 1997 (Caldwell et al, 1998). A count of 1 beetle per plant (corresponding to 5 beetles on 5 plants) has been used as a threshold for transmission of bacterial wilt in melons in Indiana (Brust et al., 1996; Foster and Flood, 1995).
Both edge effects (Foster and Flood, 1995) and trap crop effects on spotted cucumber beetle counts were observed on the last two dates, August 15 and 22 (Table 1; figures 3 and 4). The edge effect was evidenced in spotted cucumber beetle counts 8 times higher at the 0 ft station than in the average of the subsequent stations on both dates; there were no significant differences in counts at subsequent stations. The trap crop effect was evidenced in spotted cucumber beetle counts 42% higher on sticky cards in the trap crop than in the pumpkin next to the forest (0 ft station) on August 15, and 81% higher on August 22. The difference between counts in the two crops (trap crop vs. pumpkin) at the 0 ft station compared to their counts at other stations was significant on August 22 but non-significant on August 15. There were no significant differences in the counts between the trap crop and pumpkin in subsequent stations on either date. These results suggest that spotted cucumber beetles became relatively more concentrated in the trap crop as their population built up, but that this effect was confounded with an edge effect.
Visual counts were lower than counts on sticky traps on the above two August dates, and only the edge effect was apparent and significant on both dates (figures 3 and 4). Spotted cucumber beetle counts were 2 times higher at the 0 ft station than in the average of the subsequent stations on August 15, and 5 times higher on August 22, but there were no significant differences in counts at subsequent stations. Visual counts are sensitive to the time of day they are done, and reflect the situation in the field on only one day and point of time of monitoring. However, sticky cards are left in the field for a week, and so they provide a less-time dependent assessment of the insect population over a longer period. These results suggest that sticky cards may be a more effective method to detect the effect of a trap crop.
Lady beetles were more prevalent than Pennsylvania leatherwings. Populations of the lady beetle AC. mac@ reached their peak in the first half of August, while other lady beetles declined after July 31 (Figure 5). Significant differences in AC. mac@ counts due to distance from the forest were found on August 7 and 15, but greater populations were not observed in or near the trap crop, where greater cucumber beetle egg and small larvae prey would have been expected. Rather, there appeared to be a population buildup in the center of the field (Figure 6). Thus, no relationship in the distribution in the field of AC. mac@ and cucumber beetles was apparent.
These results indicate that >NK530' shows preferential attraction to the spotted cucumber beetle. This attractiveness might be used to greater potential benefit in an early spring planting. At the stage of the crop when the trap crop attracted cucumber beetles in this trial, economic damage from the beetles was likely to be minimal. In contrast, planting the trap crop in the early spring, prior to planting the economic crop, could attract the overwintering population. A localized application of pesticide on the trap crop could then be made to prevent feeding damage on small seedlings, a major cause of damage caused by the beetles. This reduction in the overwintering population could also reduce the number of mating adults, and thereby reduce the subsequent August generation and population buildup in the field in the following year.
Go to Figure 3 Effect of Distance on Spotted Cucumber Beetle Counts, August 15, 1997.
Go to Figure 4 Effect of Distance on Spotted Cucumber Beetle Counts, August 22, 1997.
Acknowledgment
We thank Dr. Brian Nault, Eastern Shore Agricultural Research and Extension Center, Painter, Virginia, for helpful comments on an earlier draft of this article.
Literature cited
Brust, G. E., R. E. Foster, and W. G. Buhler. 1996. Comparison of insecticide use programs for managing the striped cucumber beetle (Coleoptera: Chrysomelidae) in muskmelon. Journal of Economic Entomology 89(4):981-986.
Caldwell, J.S., J-P. Amirault, and A.H. Christian. 1995. Insect pests, beneficial insects, and cover crops of biological farmers. HortScience 30(4):806.
Caldwell, J.S., S. Johnson, M. Lachance, and S. Stockton. 1998. Threshold monitoring, trap cropping, and aluminum mulch repulsion for management of cucumber beetles on cucurbits. HortScience 33(2):475.
Foster, R., and B. Flood (eds.). 1995. Vegetable insect management. Meister Publishing Co., Willoughby, Ohio.
Groden, E., F.A. Drummond, R.A. Casagrande, and D.L. Haynes. 1990. Coleomegilla maculata (Coleoptera:Coccinellidae): its predation upon the Colorado potato beetle (Coleoptera:Chrysomelidae) and its incidence in potatoes and surrounding crops. Journal of Economic Entomology 83(4):1306-1315.
Hazzard, R.V., and D.N. Ferro. 1991. Feeding responses of adult Coleomegilla maculata (Coleoptera:Coccinellidae) to eggs of Colorado potato beetle (Coleoptera:Chrysomelidae) and green peach aphids (Homoptera:Aphididae). Environmental Entomology 20(2):644-651.
Houser, J.S., and W.V. Balduf. 1925. The striped cucumber beetle. Bulletin 368. The Ohio Agricultural Experiment Station, Columbus, Ohio.
Pair, S.D. 1997. Evaluation of systemically treated squash trap plants and attracticidal baits for early-season control of striped and spotted cucumber beetles (Coleoptera:Chrysomelidae) and squash bug (Hemiptera:Coreidae) in cucurbit crops. Journal.of Economic Entomology 90(5):1307-1314.
Radin, A.M. 1992. Colonization biology of the striped cucumber beetle, Acalymma vittata (F.) (Coleoptera: Chrysomelidae) and the potential for its control on cucumber, Cucumis sativa (L.), using squash, Cucurbita maxima (Duch.), as a trap crop. M.S. Thesis, University of Maine, Maine.
Table 1. Effects of Trap Crop and Distance on Populations of Spotted Cucumber Beetles, Orange County, 1997
| Effect | F value and significance | |
| 8/15 | 8/22 | |
| Interval | 45.4 ** | 30.6 ** |
| edge (0 vs. 50-200) | 180.2 ** | 121.2 ** |
| remaining intervals (50-200) | <1.0 NS | <1.0 NS |
| Crop*Interval | 1.55 NS | 2.88 + |
| crop*edge (0 vs. 50-200) | 1.17 NS | 8.89 ** |
| crop*remaining intervals (50-200) | 2.46 NS | 2.25 NS |
NS, +, ** Differences in counts nonsignificant at p > 0.10, trend at 0.05 _ p < 0.10, or highly significant at p_ 0.01.
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