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Caterpillars are the larval stage of moths and butterflies. Though the adult insects do not inflict serious damage to trees, caterpillars are leaf feeders and have the potential to defoliate entire trees if they are found in large numbers. There are as many different types of caterpillars as there are moths and butterflies. Even in small numbers, the pests eat through the foliage, flowers and young shoots. Badly damaged trees lose their vigor and are poor in growth. You can use a number of strategies to kill tree caterpillars.
Look for the mass of caterpillar eggs on trees during the dormant season. Wear gloves and press these eggs with your fingers. Throw the residue in the trash.
Prepare a bucket of soapy water by adding about 1/3 cup of dishwashing detergent to a bucket of water. Handpick any caterpillars you see and drop them in the bucket to kill them.
Spray Bt, or Bacillus thuringiensis, on all infested tree areas and directly on the pests. The chemical kills caterpillars on direct contact as well as through the ingestion of treated foliage but does not harm beneficial insects. Once infected, the caterpillars stop feeding and die within the week. The recommended time to use the spray is during warm, sunny days when the caterpillars are feeding actively.
Bt works best on young caterpillars with thin skins that are easily punctured by the toxins in the spray. Use Bt on caterpillars as soon as all the eggs have hatched. You can also use broad-spectrum insecticides such as azadirachtin, pyrethroids or carbamates to kill caterpillars. However, these insecticides will also harm the natural caterpillar predators found in trees.