Large infestations of tree caterpillars can cause serious damage.

Tree caterpillars are the larval stage of moths or butterflies that feed on the leaves and branches of woody plants. In small numbers, they don’t do much damage and tend to cause only aesthetic problems. But large infestations of caterpillars can remove most or all the leaves on a tree. In some cases, the affected tree may die. Chemical controls for tree caterpillars are often unreliable and can be toxic to beneficial organisms, but there are several natural options you can try.

Mechanical Repellents

You can discourage caterpillars from climbing and feeding on your trees by placing mechanical barriers on the trunks. These include sticky bands that will capture any caterpillars that try to cross them. Remove any branches or leaves with large numbers of feeding caterpillars and dispose of them at a significant distance from the tree. These techniques work best for relatively small infestations.

Predators and Parasites

Encouraging natural predators and parasites is one good way to keep tree caterpillar populations low while encouraging them to find other food sources. The most common caterpillar predators include damsel bugs, ground beetles, spiders, assassin bugs, soldier beetles and green lacewings, notes University of California Statewide Integrated Pest Management Program. The presence of these creatures in and around your trees may encourage caterpillars to leave them alone. Parasites, such as wasps in the Hyposeter genus, can also provide some natural control options.

Soaps and Oils

Horticultural oils and insecticidal soaps work best if you want to kill tree caterpillars, but they can also discourage them from feeding. These substances tend to block the insects’ breathing pores or dry out their bodies when they come into contact with the caterpillars. They may also make foliage less appealing as a food source when applied directly to the leaves. Some insecticidal soaps can damage tree leaves, so test them on a small area of the plant before spraying heavily.

Botanical Repellents

Botanical caterpillar repellents are extracted or derived from plants. These substances tend to degrade quickly when exposed to the air and weather, making them less dangerous to beneficial insects than chemical repellents. This fast breakdown time does mean you have to apply botanical repellents and control products carefully. Some common options include citrus oils from oranges, limes and similar fruits, neem oil, chrysanthemum-based pyrethrins, and capsaicin from hot peppers. To repel caterpillars, these products must be applied over all the leaves you wish to protect, making them more practical for small trees. Some botanical products can also be sprayed directly on the caterpillars to kill them.

Bacterial Controls

Several bacterial products can help you kill tree caterpillars without harming other beneficial insects. These rely on Bacillus thuringiensis or Saccharopolyspora spinosa, microorganisms that primarily attack caterpillars. These substances should be sprayed on tree leaves that caterpillars are likely to attack. The pests will either consume them and die or choose a different food source. Bacterial controls work best on young caterpillars because larger ones tend to eat less and have stronger immune systems.

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