Insect identification > Coleoptera > True Coleoptera > Striped cucumber beetle

Striped cucumber beetle

The striped cucumber beetle (Diabrotica vittata Fab.). - The common cucumber beetle is found everywhere in the U.S. (of which it is a native) east of the Rocky Mountains. It is a small beetle about a fifth of an inch long, with a black head, yellow pronotum and three black stripes along its yellow elytra.

The insect passes the winter as the adult beetle in protected places, probably among dense weed growth. It leaves its winter quarters early in the spring, before any of its cultivated food plants are available, and feeds on blossoms of various kinds until cucumbers, squashes and the other cucurbits which are its favorite food plants are available. It then attacks these and may also seriously injure peas, beans, apples and, later in the season, corn.

It lays its eggs either singly or in clusters, in the ground near the stems and roots of the cucurbits, often in crevices of the soil, the total number of eggs per beetle varying from a few hundred to over a thousand. The eggs hatch in a week or two, according to the temperature at that time, and the grubs feed on the stems and roots. They are tiny, white, slender, and resemble maggots more than the usual forms of beetle larvae, and when full grown, after 2 to 5 or more weeks, according to the temperature, are only about three-tenths of an inch long. They then soon change to pupae, still in the ground, in which stage they remain for about a week before the beetles emerge.

The life cycle therefore varies in length according to the temperature, it being perhaps not over 4 weeks in the South and 8 in the more northern states. This gives time for several generations each season; and though in the North there is apparently but one, this number increases farther south until in Texas there may be four.

The destruction caused by these insects when they are abundant is often very great. Their first attacks come just when the young plants are struggling to establish themselves and the feeding of the adult beetles is often sufficient to kill them. Later in the season the beetles continue feeding on the leaves and stems, reducing the vigor of the plant and its productiveness, and they may also feed on the outer surface of the fruit, making it more or less unsalable. They also frequently enter greenhouses and attack cucurbits there. The larvae affect the vitality of the plant by attacking the underground stems and roots but are less injurious than the adults.

The beetles are also injurious by carrying the "bacterial wilt" disease and "cucurbit mosaic" disease, not only from plant to plant but also from one season to the next. As these diseases are serious ones, often destroying plants, this adds to the importance of the insect as a pest.

On the Pacific Coast is a slightly larger species, known as the western striped cucumber beetle (Diabrotica trivittata Mann.), which has much the same habits as the eastern form. In the more southerly portion of this region the adults are more or less active during the cold months. There appear to be at least two generations a year, and the methods given below for the control of the eastern species also apply for this one.