Shown here are adults of the eastern subspecies of the spotted cucumber beetle (Diabrotica undecimpunctata howardi), left, and the western subspecies (Diabrotica undecimpunctata undecimpunctata), right. (Left photo by Russ Ottens, University of Georgia; right photo by Ian Grettenberger, Ph.D.)

By Ariela Haber, Ph.D.

Ariela Haber, Ph.D.

The spotted cucumber beetle (Diabrotica undecimpunctata), also known as southern corn rootworm, is a polyphagous leaf beetle species native to North America. It feeds on over 200 plant species across approximately 50 plant families. In late summer fields, “spotties” sometimes seem to be everywhere!

While it causes little or no damage to many plant species, the spotted cucumber beetle inflicts economic damage on several crops, including cucurbits (squash, melon, cucumber), corn, sorghum, beans, peanut, and sweet potato, with larvae feeding primarily below ground on roots and adults feeding on leaves, flower petals, pollen, and fruit. Adults are strong fliers and readily disperse from field to field during the growing season. Spotted cucumber beetle’s broad host range and high mobility make it particularly challenging to study its ecology and to manage it as a pest.

In a paper published in September in the Journal of Integrated Pest Management, a team of researchers from across the U.S. and I review the biology, damage, and current and potential strategies for managing spotted cucumber beetle. We focus on the two subspecies most frequently documented as agricultural pests. The eastern subspecies (Diabrotica undecimpunctata howardi) ranges east of the Rocky Mountains west to southern Nevada and southeastern California in the United States and as far west as Alberta in Canada. The western subspecies (Diabrotica undecimpunctata undecimpunctata) ranges west of the Great Basin in California, Oregon, Washington, Idaho, and Baja California Norte.

Adults overwinter in mild climates and spread seasonally northward, with populations increasing into late summer. Females lay eggs in soil at the base of host plants. After eggs hatch, larvae feed on roots, stem bases, and sometimes fruit rinds in contact with the soil. They pupate in the soil and, when they emerge as adults, disperse by flight to various host plants. The number of generations per year ranges from one in the north to three in warmer regions.

The monthly distribution of iNaturalist research-grade sightings of the spotted cucumber beetle (Diabrotica undecimpunctata) is illustrated in this 12-frame animation, showing a total of 29,072 georeferenced observations from May 1987 through March 2023. The dotted line indicates the division between subspecies D. u. undecimpunctata (to the west) and D. u. howardi (to the east). (Graphic by Daniel Wilczek)

Adults are pests primarily on vegetable crops. In cucurbits, the greatest concern from spotted cucumber beetle is that, in the eastern U.S., it vectors Erwinia tracheiphila, which causes bacterial wilt. Plants infected with bacterial wilt rarely recover or produce marketable fruit. In addition, feeding from both subspecies can stunt or kill seedlings and reduce fruit set. In beans, spotted cucumber beetle’s pest status is greatest in snap (green) beans, where feeding on pods damages the harvested crop and can interfere with pod set. In particular, the western subspecies is described as a primary pest of snap beans in the Pacific Northwest.

The larval stage is the primary cause of damage to corn, peanuts, and sweet potato. In corn, larger larvae bore into the seedling, which destroys the growing point, resulting in stunted plants with no ears. In peanut, below-ground feeding prevents proper pod formation, and feeding on pods creates entry ways for pathogens, resulting in pod decay. In sweet potato, the below-ground feeding causes direct damage to the marketable part of the plant.

Examples of damage by adult spotted cucumber beetles (Diabrotica undecimpunctata) includes  bacterial wilt, caused by the bacterium Erwinia tracheiphila, which is vectored by the beetle, shown at left on winter squash; and feeding damage on snap beans, shown at right. (Left photo by G. Higgins, University of Massachusetts Vegetable Program; right photo by Whitney Cranshaw, invasive.org)

Much of current spotted cucumber beetle management relies on chemical control, which can be unreliable and ecologically detrimental. In addition, concerns about nontarget effects and mammalian neurotoxicity have resulted in withdrawal of some key pesticide registrations in the U.S. However, alternative strategies show promise for more sustainable management. Cultural and physical controls can reduce colonization in some cropping systems, particularly those attacked by the adult stage. In peanut and sweet potato, there are opportunities to breed cultivars with enhanced resistance.

Although biological control for spotted cucumber beetle is not well-studied, especially below ground, a variety of natural enemies are known to attack spotted cucumber beetle. This indicates potential for increasing predation and parasitism using strategies that that conserve and enhance natural enemy populations. Furthermore, recent work demonstrated spotted cucumber beetle cross-attraction to vittatalactone, the aggregation pheromone of the cucurbit specialist striped cucumber beetle (Acalymma vittatum). This offers new opportunities for use of attractive baits in monitoring and management.

Examples of damage by spotted cucumber beetle (Diabrotica undecimpunctata) larvae includes  destruction of the main stem of corn, shown at left, and damage to sweet potato, shown at right. (Left photo via Clemson University – USDA Coop. Extn. Slide Series, invasive.org; right photo by Fred Musser, Ph.D.)

Spotted cucumber beetle’s broad host range, geographic spread, and mobility necessitate accurate evaluation of its population levels and damage in multiple cropping systems. However, its ecology and behavior are relatively understudied, and there is little information available on management strategies specific to this species. Our profile describes damage and various management tactics in several key crops. We suggest that the development and implementation of cultural controls, biological controls, behavioral controls involving attractant baits, crop plant resistance, and judicious chemical control coupled with improved thresholds offer opportunities for integrated pest management specific to each crop that does not rely on scheduled or prophylactic chemical control.

Ariela Haber, Ph.D., is a postdoctoral researcher at the U.S. Department of Agriculture-Agricultural Research Service’s Invasive Insect Biocontrol and Behavior Laboratory, in Beltsville, Maryland. Email: ariela.haber@gmail.com.