It’s that time of year and cucumber beetles are once again wreaking havoc on tender cucurbit seedlings. In all stages of life, these beetles do damage to cucumbers, winter and summer squash, melons, pumpkins, and gourds. In addition to inflicting significant damage by feeding, these beetles add insult to injury by transmitting bacterial wilt along the way. While there is no easy solution to this problem, preventative measures and specific cultural practices can limit damage and bring an otherwise unruly pest under control.
Know Thy Enemy
There are six species of cucumber beetle in the United States. Perhaps the most prevalent of these is the striped cucumber beetle, a member of the Acalymma genus. Measuring just one fifth of an inch long, this beetle is yellowish green with a black head and yellow thorax, and can be easily identified by three parallel black stripes running lengthwise on the wings of adults.
Cucumber beetles over-winter as adults in bordering vegetation, plant debris, and nearby forest. As springtime temperatures climb, cucumber beetles are actively feeding on the petals and leaves of flowering plant hosts outside of the garden. When cucurbits are planted out into vegetable fields, migration of the striped cucumber beetle is swift. After feeding on seedlings, these adults mate, with females laying up to 1500 eggs each over the course of several weeks.
The Beetle and the Damage Done
In the southern regions, up to three generations of striped cucumber beetles can be produced in one growing season, while the north generally sees only two. Cucumber beetles cause feeding damage three times during their life cycle. Over-wintered adults feed on the cotyledons and stems of emerging cucurbit seedlings. As the eggs develop, larvae tunnel into the soil and feed on plant roots. As second- and third-generation adults emerge, they feed on foliage, flowers, and stems, and can even damage mature fruit.
Fortunately, fully developed, healthy cucurbit plants can withstand 25-50% defoliation before yields are dramatically affected. Far and away the greater threat posed by cucumber beetle infestation is the potential for transmission of bacterial wilt. Bacterial wilt is caused by the bacterium Erwinia tracheiphila, which is stored in the intestinal tract of adult cucumber beetles. Once a plant is infected, the Erwinia bacterium spreads rapidly through the vascular system of the plant, creating resins which restrict the movement of water and nutrients. This causes the plant to wilt and die, sometimes in as few as seven days. Additionally, research has shown that cucumber beetles can be vectors for squash mosaic virus, as well as lead to an increased incidence of powdery mildew and black rot, and a predisposition to fusarium wilt.
Delayed Planting – Growers can avoid the most significant damage by simply delaying the planting of summer cucurbits by a few weeks. If you’re not set on getting the first cucumbers or summer squash to market, let a neighbor’s crop take the brunt of the spring cucumber beetle migration. This tactic can also allow seedlings extra time to grow into vigorous, mature plants capable of withstanding beetle pressure. Some growers in regions with longer growing seasons opt to skip summer cucurbits altogether, planting cucurbits in time for a fall harvest when beetles are much less of an issue.
Cultivation and Residue Removal – As cucumber beetles can over-winter in crop residues both above and below ground, it is important to practice clean and thorough cultivation after fall harvests. Cornell University suggests deep tillage, compost application, and cover-cropping in the fall to encourage decomposition of residue which may harbor beetles through the winter months. Any diseased plant matter should be burned or otherwise discarded rather than composted for future use.
Mulching – Using straw, hay, plastic, or fabric as mulch can deter cucumber beetles from laying eggs in the ground near the plants. While mulching will not halt egg-laying or feeding, it will limit direct access to the stem, as well as significantly slow larval migration through the soil.
Row Cover – Floating row covers can be a big help by excluding cucumber beetles during the seedling stage of life. This allows plants to mature and develop substantive leaf mass and a strong root system, enabling the plant to withstand a moderate pest attack. Remove row covers at the onset of flowering to allow for adequate pollination. Since row covers foster weed growth too, many producers use weed suppressing mulches in combination with floating row cover.
Kaolin Clay – Here at our production farm in Wolcott, VT, we start our cucurbit season off with a bit of defensive strategy. We’ve seen a growing number of producers using Surround, a type of kaolin clay, as a protective film to ward off early damage from striped cucumber beetles. Surround comes in a powdered form, which is then mixed with water and sprayed on the seedlings. Some growers choose to dip entire flats of seedlings into the mix, which coats the underside better than does a backpack sprayer. When the seedlings are planted out in the field, the clay acts as a sticky barrier to hungry cucumber beetles, causing what some call “excessive grooming”. This keeps the beetles busy when they would otherwise be eating your prized seedlings and searching for mates.
Trap Crops - Striped cucumber beetles have food preferences just like we do. There is a long list of cucurbit varieties that are favored by cucumber beetles, and are thus excellent trap crops. By luring cucumber beetles into a concentrated area, control measures can be focused, localizing the damage and limiting the spread of disease. We recommend using Baby Blue Hubbard Squash as a trap crop, as it is highly attractive to cucumber beetles, has particularly vigorous seedlings, and is less susceptible to bacterial wilt than many other squash varieties. Trap crops should be planted on the perimeter of the field in multiple rows if beetle pressure is particularly severe. We recommend planting trap crops a week or two earlier than your primary cucurbit planting to proactively direct migration.
Sticky Traps – When it’s time to take prisoners, many growers employ yellow sticky lines of tape to trap cucumber beetles en masse. Use these ribbons in tandem with trap crops for the most effective control. Homemade yellow sticky traps can be made by coating a yellow plastic cup with glue available specifically for this use. For added effect, attach a cotton swab soaked in the oil of clove, cinnamon, cassia, allspice or bay leaf, all of which act as a powerful floral attractant.
And finally, there’s always hand-picking, but you’ll have to be stealthy because they’ll fly away when they see you coming! Let us know if you’ve had success with other strategies and best of luck to you with your summer cucurbits.