Insects with a Complete Life Cycle

Insects with a Complete Life Cycle

These insects have 4 life stages, the egg, larva, pupa and adult. Generally, the larval (worm) stage does most of the damage. Insects in this category possess chewing mouthparts in the larval stage.

  1. Butterflies and Moths

    The caterpillar (worm) stage of this group does all of the damage to Kentucky crops. When hatched from the egg, they are very small and eat very little and early stages of infestation may be overlooked. As the caterpillar gets larger, it is harder to control with insecticides. Therefore, early detection of economic infestations is important.

    Caterpillars vary widely in color and markings. Experience and training are essential in proper identification.

    Also, while a few caterpillars feed on one plant, many feed on a wide variety of crops.

    1. Foliage and Fruit Feeders

      This group contains more different species of damaging caterpillars than any other. Some feed only on the foliage, or only on fruit, while others feed on both. For the purposes of this manual, it is not practical to cover in detail all of the more important damaging species. The corn earworm has been selected as an example.

      The corn earworm is also known as the cotton bollworm, tomato fruitworm, false tobacco budworm, and soybean podworm. It varies from light green to almost black and has a yellow stripe along each side of the body.

      The caterpillar (worm) stage can last from 14 to 21 days but during the summer it will be nearer to 14 days. The entire life cycle (egg to adult) lasts approximately 30 days so there may be 2 to 3 complete generations each year.

      Corn earworms feed on both the foliage and the fruit. The more important crops it feeds on and the parts damaged are as follows:

      • Corn - ear and whorl
      • Small grains and sorghum - head
      • Soybeans - pod and foliage
      • Tomatoes - fruit
      • Tobacco - bud and other foliage
      • Pepper - fruit
      • Okra - pod
      • Other legumes - foliage and pods

      There are many other foliage and fruit feeding caterpillars. Some of the most important species are fall armyworm, "true" armyworm, tobacco budworm, loopers, black cutworm and tobacco and tomato hornworms.

    2. Boring and Tunneling Insects

      This group contains fewer species than the preceding group but some are very important pests of crops in Kentucky.

      The length of time that these pests feed on the exterior of the plant after eggs hatch and before they enter the plant is relatively short. A properly timed insecticide application is essential to control the pest while it remains outside the plant and before it begins to tunnel.

      A few species are controlled by insecticides, called "systemic insecticides", that are absorbed by the plant being attacked.

      The European corn borer is a good example of this group. Egg masses containing about 30 eggs are laid by the adult moth on the undersurface of the corn leaf. Eggs hatch and caterpillars feed from 10 to 14 days on the exterior parts of the plant. Then they bore into the stalk, where they remain until they mature, about one inch long. The head is dark brown or black and the upper part of the body is gray to pink with rows of dark spots running lengthwise. The underside of the body is cream colored. Tassels or grain heads of damaged plants often break over. In heavy infestations, borer feeding may cause the plant to collapse or the corn ear to fall off.

      European corn borers are important pests of corn and closely related grass crops such as sorghum and millet. European corn borers have been found on nearly all herbaceous plants large enough for the worms to enter. Their hosts include over 200 different plants such as field peas, peppers and Irish potatoes.

      Controlling this pest with insecticides is difficult. Insecticidal sprays must be applied before the insect tunnels into the plant.

      Other boring and tunneling caterpillars include the Southwestern corn borer, peachtree borer and several types of leaf miners.

  2. Beetles and Beetle Larvae

    Beetles and their larvae are important pests of agricultural crops. In most instances, both the adult and larval stages damage plants. The larvae are often called grubs or worms and the life cycles of these pests vary from less than 30 days to one or more years in length for different kinds of beetles.

    1. Beetles

      Beetles usually feed on some exterior part of the plant such as leaves, stems or flowers, while larvae may feed inside the plant, on the roots, or on the outside of the plant.

      The bean leaf beetle is a good example of this category. These widely-distributed leaf feeding beetles are generally of minor importance. However, with high populations they may damage soybeans by leaf, stem and pod feeding. Diseases such as bean pod mottle virus can be spread by these beetles.

      Adults are approximately << inch long and quite variable in color. They range from reddish to yellowish, with or without three or four black spots at the inner edge of each wing cover. The outer margin of the wing cover is banded in black. However, all color forms have a black triangle on the central front part of the wing covers

      The adult overwinters in or near crop remnants. Adults feed on the undersurface of the soybean leaf, producing rounded holes. When disturbed, they generally fall to the soil surface. Each female lays more than 40 eggs, deposited in groups of 12 to 14 in the soil near the base of soybean plants.

    2. Beetle Larvae

      Beetle larvae are usually referred to as grubs or "worms". Larvae feed on underground plant parts, a practice which may cause plant stunting or death.

      The lengths of the life cycles of these pests are variable. Most require one or more years to complete their life cycle, while others require no more than one month under ideal conditions. The bean leaf beetle is a good example of a pest that damages a plant both in the larval and adult stages.

      Bean leaf beetle eggs hatch and the larvae begin to feed on the roots and nodules within two weeks. Feeding on the stem just below the surface can also girdle the plant. In about five weeks the full grown (% inch) larva forms an earthen cell and the resulting pupal stage lasts about one week. In Kentucky there are two generations per year.

    3. Leaf Feeders

      Many leaf feeding beetles are the adults of soil feeding larvae. Often the adult feeding is not considered to be of economic importance.

      The Mexican bean beetle is an important leaf feeder. Both the adult and the larva feed on foliage. All four stages of the life cycle are found on the leaves of plants. This insect is primarily a pest of legumes.

      Control of leaf feeding beetles requires a foliar application of a recommended insecticide.