Jeffrey Hahn, University of Minnesota Asst. Extension Entomologist
Flea beetles are active on crucifers now. These insects are about 1/16 - 1/8 inch long and an iridescent black violet (flea beetles on other plants are the same size and can vary in color). They overwinter as adults and are active in the spring, feeding on the leaves. They chew small, shallow pits and holes into the leaves. A heavily infested plant looks like it got shot with a BB gun.
Plants are most susceptible to damage in spring - seedlings are more vulnerable than transplants. If your plants are suffering 10 % - 30 % damage, you should treat plants to protect them from flea beetle damage. Apply a garden insecticide, such as permethrin, spinosad, or carbaryl. Different flea beetle species also attack potatoes, spinach, beans, squash, corn, and other plants so be on the watch for feeding injury on these plants as well. More information on flea beetles is available at this link (http://www.extension.umn.edu/distribution/horticulture/M1210.html).
Photo 1: Flea beetle on crucifer. Jeff Hahn
An interesting insect was found on rhubarb recently. At first glance, this very small (1/16th inch long), orange insect appeared to be an aphid. However upon closer inspection, the insects were quite mobile and apparently not feeding. The shape was also not quite right, this insect was more globular than the typical pear-shape you find with aphids. In fact the insect we found was a springtail, more specifically a sminthurid springtail.
Springtails are an extremely abundant group of insects but because of their small size and the fact they are most common in the soil and leaf litter, they are infrequently noticed by people. Springtails are considered beneficial insects because they feed on decaying plant material, helping in the decomposing process. However, infrequently they are found on plants and can actually be found chewing on them. No damage was found on the rhubarb that was inspected. At worst we would not expect anything more than minor feeding. Management for springtails is not necessary.
Photo 2: Springtail on rhubarb. Jeff Hahn