Insect identification > Coleoptera > True Coleoptera > Colorado potato beetle

Colorado potato beetle


The Colorado potato beetle (Leptinotarsa decemlineata Say). This well-known insect was discovered about 1823 by Long's exploring expedition to the Rocky Mountains, in the region of the upper Missouri River. Its food there was the buffalo bur (Solanum rostratum Dunal) and the insect was apparently not very abundant, and certainly of no economic importance, nor did it become so until civilization, and with this the potato, reached that territory.

Then a new and satisfactory food plant, abundant enough to provide all the insects with food, became available and the potato beetle increased in numbers and began to spread to the East. At first its rate of spread was only about 50 miles a year but after crossing the Mississippi River this became more rapid and it reached the Atlantic Coast about 1874.
Since then it has spread both northward and southward until it is now found practically everywhere east of the Rocky Mountains where the potato is grown and it has also reached the Pacific Coast. It does not apparently thrive in the hot climate of the more southerly states.

The adult beetle is somewhat less than half an inch long and about two-thirds this width, its back rather high and rounded. It is clay yellow and has 10 longitudinal black lines on its elytra. The head has a black spot above, and the pronotum has a number of irregular spots. Winter is spent as the adult in the ground but the insects come out quit early in the spring.

As soon as the potatoes are up, they begin to feed and soon lay their eggs, placing these on the under surface of the leaves in small clusters, an individual laying 500 or more in all. They are small yellow eggs which hatch in 4 days to a week or more, according to the
temperature.

The grubs or "slugs," as they are often called, are dull brick red, soft and with fat bodies. They feed from 2 to 3 weeks, then go into the ground where they pupate for a week or two, after which the adults emerge and lay eggs for a second generation, the adults of which appear early in the fall. This second generation of beetles feeds for a time, then in September or October enters the ground to pass the winter.

As the eggs of this insect are not all laid at one time, different ages, and different stages even, may be found together in the same field. And as the adults feed in the spring during their egg-laying period, as do the two generations of adults produced during the season, in addition to the two generations of grubs which also consume the leaves, the plants are being attacked much of the time.

While the potato appears to be the preferred food of this insect, other members of the nightshade family are sometimes attacked, particularly the tomato and eggplant.

Various birds, skunks, snakes and toads feed on the Colorado potato beetle to some extent, and it also has numerous insect enemies.

The history of the development of the Colorado potato beetle, from an unimportant, even probably a rather uncommon, insect, feeding upon a plant of no value to man, into one of the most abundant and widely distributed of our pests, attacking and seriously injuring an important crop, is a suggestive one. In a division of the insects of the United States into those which are injurious as regards man and his various interests; those which are beneficial; and those which are of little or no economic importance either way, we shall find that the last group is by no means a small one.

How many species in this group are there which are potential pests? It is true that the making available of a new food plant to which the Colorado potato beetle could turn was probably the chief factor in this particular case, but any insect which for some reason changes from an unimportant food plant to a crop plant may at once become a pest. Thus another chrysomelid only a little smaller than the Colorado potato beetle and closely related to it, the three-spotted doryphora (Doryphora clivicollis Kirby), which feeds on milkweed, is now of practically no importance. But if it should change its food to some valuable crop plant, it would at once become an important addition to the list of insect foes man has to combat.

Several such cases are already known. How many others may appear as the changing conditions which always accompany an increasing population and the consequent changes in plant population take place, no one can predict. Some species of plants once common are rapidly disappearing. As they go, will the insects feeding on them go too, or will they be able to find another food plant, and will this one be of value to man? The appearance of new pests in such ways may come at any time, and the fact that an insect is not now a pest should not lead to its being ignored, for it may have great potential importance.

The murky ground beetle (Harpalus caliginosus Fab.) is now mainly a carnivorous beetle but sometimes, though rarely, attacks the strawberry. If it should turn to this latter plant entirely for its food, another important pest would be added to our list and lost from among our friends.

Such facts call for as complete a knowledge as possible of the life and habits of all insects whether now beneficial or only of no economic importance, in order that we may have the knowledge of them and their ways which is necessary in case they should become injurious.