Bug identification

Asked July 12, 2020, 1:29 PM EDT

The person sending this picture said there are very many of these and has seen lots of them on creeping jenny. Are they beneficial or harmful?

Pennington County South Dakota

2 Responses

It appears to be a Prickly Leaf Beetle, and it eats leaves of plants. People probably don't mind it eating creeping Jenny, but it might also eat the leaves of plants and shrubs that people like. They do have other beneficial insects as predators so can try to control by using methods that are not generally harmful to the beneficials such as lady bug larvae:
1) Filing a 1-gallon bucket with warm water and a tablespoon of dish soap. Put on gardening gloves and hold the bucket underneath the leaves of your infested plant. Pick the beetles off and drop them in the bucket. You may be able to tap the plant and watch some of the beetles fall off into the bucket. Extension recommends picking the beetles off the plant leaves either in the early morning hours or late in the evening. These are the times when the beetles are less active.

2) Perhaps Horticultural oils (generally 2 to 3 percent) and/or insecticidal soaps will be useful for control, especially of the eggs. Thorough coverage of plant foliage is needed as these pesticides only kill by contacting the insects. There is some evidence that oils and soaps can also kill the adults, but since there is no residual activity of these products, numerous reapplications will be needed in order to prevent adults from laying eggs.

The insects in the photo are four beetle pupae and one larva that is likely to be of the same species. They are probably in the same subfamily as the prickly leaf beetle (Cassidinae, within the family Chrysomelidae), but all of the common species in the midwest do not have spines as adults. The common name for them is tortoise beetles.

There are also a few species of predatory lady beetles that have larvae and pupae like this, but it would be very uncommon to find them in small groups.