Forest Huval, Reagan, Thomas E., Carlton, Christopher E.
Left: Adult cucumber beetle. (David Cappaert, Bugwood.org).
Right: Cucumber beetle on Brassica leaf. (Russet Ottens, University of Georgia, Bugwood.org).
The spotted cucumber beetle is a significant pest of the Cucurbitaceae, or cucurbit, family composed of melon and squash vegetables and other crops in North America. Adult spotted cucumber beetles are between 3/16 and ¼ inch (5 to 6 mm) in body length. They can be distinguished from most similar species by the black head, green thorax and wing covers (elytra). The latter possess 12 oval black spots. Larvae are approximately 3/8 inch (9 mm) in length and hatch from orange-yellow eggs. Their bodies are elongate and wormlike and are cream colored with three pairs of long, brown legs. The head and tip of the abdomen are dark brown.
At least one other species of Diabrotica, the banded cucumber beetle (D. balteata), occurs in Louisiana. It is similar in overall appearance but possesses a reddish head and eight yellow spots on the elytra. Eggs and larvae are similar to those of the spotted cucumber beetle. Many species within the family Chrysomelidae (leaf beetles) are superficially similar to these two species.
Adults emerge during midspring when the soil temperature exceeds 55 degrees Fahrenheit (12.78 degrees Celsius). They feed on pollen and the leaves of a wide variety of herbaceous plants. In Louisiana, soybeans and corn are the predominant agronomic hosts, followed by tomatoes and alfalfa. Once cucurbit seedlings emerge from the soil or are transplanted into the ground, the beetles move to their preferred host to feed and mate. Shortly thereafter, females deposit eggs at the base of the cucurbit plants. After hatching, larvae remain under the soil and feed on roots and stems for roughly two to four weeks, depending on temperature, then mature larvae pupate. Several generations of cucumber beetles can emerge throughout the growing season. Adults feed on foliage of cucurbits and various other plants until fall, when they switch to pollen on various fall flowers, such as asters and goldenrod, until winter. The unmated adults overwinter under leaves and plant debris around fields, along hedgerows and in woodlands.
Cucumber beetles pose a threat to cucurbits in home gardens and various other field crops, including corn and sweet potato. Cucumber beetles feed on seedlings, roots, stems, leaves, flowers and fruits. Seedlings, from the cotyledon to the three-leaf stage, are especially susceptible to plant damage while their roots are not yet fully developed. Damage to mature plants can include plant wilt, decreased yield and severe leaf defoliation. Damage to the flowers can cause a decrease in pollination resulting in reduced crop yield. Cosmetic damage, such as scarring, can occur on the outside of fruits. Although scarring does not affect fruit edibility, it decreases the marketability to both home gardeners and commercial growers. Hemp (Cannabis sativa: Cannabaceae) is an emerging crop in Louisiana, and large numbers of spotted cucumber beetles have been observed on plants during the vegetative stage. Damage to plant foliage is insignificant unless beetle abundance is exceedingly high. The long-term impact of the species on hemp in Louisiana is unclear and the subject of ongoing research. Banded and striped cucumber beetles, Diabrotica balteata and Acalymma vittata, respectively, pose similar problems, although they are not as prevalent in Louisiana.
In addition to direct feeding, spotted cucumber beetles may act as vectors of disease-causing bacteria and viruses that may result in permanent plant damage. Cucumber beetles vector Erwinia tracheiphia, causing bacterial wilt disease. The bacteria overwinter in the gut of the beetle and are then transmitted to the following season’s crop when the beetle defecates on plant leaves. It can also be a vector of squash mosaic virus, cucumber mosaic virus, bean mosaic virus, and maize chlorotic leaf virus, which stunt plant growth and cause deformed fruits, decreased yield and distorted leaves. Fusarium wilt is easily transmitted by larvae feeding on roots, while adults can also spread powdery mildew spores and increase susceptibility to black rot.
Cucumber beetle damage on squash. (Kiki Fontenot, Louisiana State University).
Several control strategies may decrease spotted cucumber beetle infestations. Cultural control methods include delayed planting and using a heavy seeding rate. Delayed plantings allow the beetles to find an alternate food source upon emerging in the spring, while heavy seeding increases likelihood of establishment if beetle damage is to occur. Crop rotation is an effective management strategy, but only if rotation is more than one-half mile (0.8 km) away because of the mobility of the beetle. Floating row covers can be used in smaller plantings or home gardens, but must be removed during the flowering stage to allow pollination of crops. Companion planting with radish, tansy and nasturtium can reduce beetle populations through repellency. Natural enemies of the spotted cucumber beetle can be encouraged to reduce pest populations by minimizing chemical use. Tachinid flies (Diptera: Tachinidae: Celatoria diabrotica), dragonflies (Odonata), daddy long legs (Arachnida: Opiliones), spiders and mites (Arachnida) are useful natural enemies that attack the beetles during various stages of the life cycle. For commercial operations, fungal pathogens and nematodes can be introduced for control of beetle larvae. Mulching and using drip tape irrigation can reduce soil moisture under fruit, thus reducing adult and larval feeding. Planting resistant varieties when available is also recommended. Removing alternate hosts, especially plants in the aster family (Asteraceae) and other wild plants, can minimize pollen sources for beetles in the spring and cause them to move away from the field. Chemical control, organic or conventional, is another effective strategy if implemented before beetle populations reach a high level and in strict accordance with label instructions.