By Beth Teviotdale
Why are there no leaves on some shoots on my redbud tree? This sad sight is not uncommon come late spring and into summer. The villain responsible for this defoliation most likely is the redhumped caterpillar, which gets its name from the reddish hump located on its back a little behind its red head. These caterpillars are voracious eaters and will consume all the leaves on a shoot. In addition to redbud trees, the pest feeds on numerous other deciduous trees and shrubs.
This insect has four stages of growth: egg, larva (caterpillar), pupa, and adult (moth). Moths appear in April and May to mate then the females lay eggs in clusters on the undersides of leaves. Tiny caterpillars hatch from the eggs and stay clustered together during the early stages of growth. As the larvae enlarge, the clusters break into smaller groups or individual caterpillars. In autumn, fully grown caterpillars move to the ground, where they spin a cocoon in which to spend winter. In spring they become pupae and then adult moths. There are two or more generations each year.
Redhumped caterpillars usually are well controlled by spot pruning and biological control. Look for egg or young caterpillar clusters in April and May, remove infested leaves or shoots, and destroy the caterpillars. Several naturally occurring insect predators and parasitic wasps attack and kill the caterpillars. Another biological control agent, Bacillus thuringiensis (Bt), is the active ingredient in several commercial products. Such products should be applied thoroughly to the undersides of leaves when chewing damage first appears and again 7 to 10 days later. To protect these natural biological control agents, avoid using harsh pesticides.
For more information, check this website:
https://ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnredhumpedcaterpillar.pdf