Insect identification > Coleoptera > True Coleoptera > Corn-root worms

Corn-root worms


The corn-root worms. - There are several species of the genus Diabrotica which as larvae appear to make a specialty of feeding upon either the base of the stem or the roots of corn.

The southern corn-root worm or spotted cucumber beetle (Diabrotica duodecimpunctata Fab.) is found practically everywhere in the United States east of the Rocky Mountains, but is usually a serious pest only from Maryland to Florida and as far west as southern Ohio, Indiana and Illinois, Alabama, Louisiana and Texas.

The insect generally winters as the adult beetle under rubbish or in other protected places, except in the far South where it is more or less active during this period. In spring it lays its eggs just below ground, on or near the young corn plants, and the tiny grubs which hatch, attack the corn, feeding on the roots and drilling into the stem just above them, boring out the crown and killing the bud. From this habit the insect is often called the "budworm" or "drillworm." Small plants injured in this way break off at the crowns when pulled, and larger ones become dwarfed and yellowish.

Other plants such as wheat, millet and alfalfa are also attacked by the larvae. The adult beetle is about a quarter of an inch long, yellowish green with black head and legs and 12 black spots on its back. It feeds on squashes, cucumbers and many other plants. The number of generations appears to vary from one in the North to three and a partial fourth in the South, but most of the injury is caused by the first generation.

Burning over waste places, where there is rubbish, during the cold months will destroy many of the beetles seeking protection there. It is desirable to avoid following a legume crop with corn, and cotton, not being attacked by this insect, is a good crop to follow corn.
The insect is most serious in wet seasons and on low land. Corn is often more thickly
planted on low places on this account, to increase the chance of getting a stand.
Fertilization and cultivation increase the vigor and resistance of the plants to attack. In the far South corn planted during April is more likely to be injured than that planted before this time or after the tenth of May.

Another species (Diabrotica vergifera Lee.) having similar habits and similarly controlled is often destructively abundant in Colorado. West of the Rocky Mountains the western spotted cucumber beetle (Diabrotica soror Lee.) largely replaces the spotted cucumber beetle. It appears to have the same general habits as its eastern relatives, but observations thus far indicate that the grubs are injurious mainly to alfalfa, beet and pea roots, while the adults sometimes appear in enormous numbers and feed on the foliage of nearly all kinds of plants except that of conifers.

The winter appears to be spent in the adult stage and the eggs are laid from March to May in different latitudes. There are probably two generations each year. The adult is one-fifth to one-fourth of an inch long. The head, antennae, legs and body are black; the pronotum and elytra green or yellowish, the latter with 12 black spots often partly fused.