Cucumber beetles are major plant pests. As rootworms (larvae), they feed underground, which affects plant health and increases the risk for pathogens. Above-ground, the adult beetles consume foliage and fruits. The western spotted cucumber beetle (Diabrotica undecimpunctata undecimpunctata M.- better known as 12-spots, 12S) is the most common species we have in this region. This is a geographic variant of southern corn rootworm (common name for the larval stage).

Additionally, there has been increasing concern of western striped cucumber beetle (Acalymma trivittatumSCB), because it can vector bacterial wilt in cucurbits. Similarities and differences are highlighted in TABLE 1.

IDENTIFICATION

Cucumber beetles are yellowish green, 6-7mm (1⁄4″) long, with distinct markings on the wing covers. Mature larvae are about 12mm (1/2″) long and white except for the head and last abdominal segment, which are brown.

rootworm
Diabrotica larvae are called rootworms. They are 3-12mm long and have brown sclerotized plates on the head and at the rear. IMAGE CREDIT: Scott Bauer, USDA Agricultural Research Service, Bugwood.org
A brief highlight of some differences between the two species of cucumber beetles we have in this region.
 Western Spotted Cucumber Beetle (12S)Western Striped Cucumber Beetle (SCB)
Scientific nameDiabrotica undecimpunctata undec.Acalymma trivittatum
Larval host plantscorn, beans, small grains, spinachcucurbits (squash, cucumbers, melons)
Adult host plants & damageFeed on foliage of corn, snap bean, cucurbits, potatoes, tomatoes, and othersFeed on foliage, stems, blossoms and fruits of all plants listed at left (same as 12S)
ID characteristicshead: black
prothorax: yellow
elytra: 12, distinct black spots on a yellow background
head: black
prothorax: orange-yellow
elytra: 3, alternating yellow and black stripes, can be cream-colored
LifecycleAdults overwinter in field borders then migrate to lay eggs and feed on foliage. Larvae feed on roots, pupate in the soil, and emerge again as adults in Jul-Aug

Cucumber Beetles

Lifecycle

There are at least two generations of western spotted cucumber beetles beetles per year in Oregon (FIG.1).

Adults overwinter and begin to disperse during warm periods in April and May. At that time, adult females lay eggs at the base of seedling plants, including sweet corn. Eggs hatch in 7 to 10 days. Larvae feed on roots for about 3 weeks before pupating in the soil.

In early June, the population of adult beetles drops significantly because larvae are developing underground. Roughly 2 weeks later, generally in early July, a new generation of adults emerges and begins feeding on pollen, plant foliage, flowers, and bean pods. As grass seed fields dry down, beetles move into irrigated cover crops, including vegetables.

Beetle populations tend to be relatively high from August through October, when the second summer generation emerges from the soil. A percentage of cucumber beetles from the second generation overwinter as fertilized females, and reemerge the following spring, which can greatly contribute to population buildup over time. The life cycle of the twelve spot beetle has a large impact on our pest control strategies.

12S activity data

FIG. 1 – 12S beetles have been extremely abundant over the past 2 years. Note in the activity graph that timing is similar to the 15 yr-average, but the scale of the trap count values (Y-AXIS) is more than 500% greater than normal.

Monitoring

[NOTE: a more detailed approach to monitoring can be found at this page] Sticky traps are placed just above the crop canopy. Mark the date that you place the trap. Come back in seven to ten days. Calculate the beetles per trap per day. Keep in mind that 12S beetles are very mobile, and trap counts may not reflect actual pressure within the nearby crop. Sweep net sampling is recommended. A tentative treatment threshold for snap bean is 2+ beetles/sweep. Scouting efforts for 12S should concentrate on the week just prior to and during snap bean bloom.

Management

Biological Control: Both 12S and SCB are attacked by a variety of natural enemies, including parasitic flies (Celatoria spp.) and wasps (Centistes spp.). Natural enemies are rarely effective enough, however, to reduce populations below economically damaging levels. Entomopathogenic fungi (Isaria spp.) are available as a formulated product and may have good efficacy when larvae are feeding underground.

Cultural Control: Row covers, trap crops, and other cultural control tactics may help to reduce damage to plants, but efficacy varies depending on setting. Because these beetles have such a wide host range, crop rotation is not a useful strategy. In fact, damage can be very bad if corn follows winter legumes, for instance.

Chemical Control: Cucumber beetles are difficult to control. Products directed at adult beetles are not selective and could be a threat to bees and other beneficial insects. Rescue treatments may be needed for extensive underground feeding by larvae. Consult the  PNW Insect Management Handbook for recommendations.