While flea beetles are a common problem of spring vegetable crops, they can be particularly troublesome following mild winters. We often refer to flea beetles as if they were a single pest; however, there are several different species of flea beetles, and on some vegetables there may be three or more different species present at one time. In general, flea beetles cause round holes in leaves (Figure 1), making diagnosis easy. The exception is the corn flea beetle, which just feeds on the upper surface of foliage leaving parallel ‘scratches.’ Early-season flea beetle damage can retard plant establishment and growth. Heavy populations can destroy young plants.
Corn flea beetles (Figure 2) and cutworms are the two primary pests that attack seedling corn. Flea beetles can transmit the bacterium that causes Stewart’s wilt, so wilt-resistant varieties are recommended following mild winters, such as this past winter. Otherwise the threshold for direct feeding by corn flea beetles is 50% of the plants with leaf scars and some leaves turning white.
We have been catching moderate numbers of black cutworm in our pheromone traps and producers should expect some activity in winter annual weeds as they prepare for planting. Use 3% cut plants and active feeding as the guideline for treating cutworms.
Tomatoes, Eggplant & Other Solanaceous Crops
Tobacco and potato flea beetles will attack tomato and other solanaceous plants. Usually, plants will quickly outgrow moderate damage. Occasionally, serious damage can occur to plants less than 6 inches tall or with fewer than five leaves. Use four or more beetles per plant and plants less than 6 inches as the guideline for treating tomato plants.
Eggplant is both very attractive and susceptible to flea beetle damage. Use the same threshold as for tomatoes above. Generally, as plants get some size to them, plants become more tolerant to flea beetle damage and the holes they cause. Because eggplants are so susceptible to flea beetle damage, some growers use row covers to protect plants until plants become established and begin flowering.
There have been a few reports of thrips on greenhouse grown tomatoes and transplants this spring. This is a result of the mild winter. Thrips are a concern as they attack plant buds and cause distorted leaves to emerge. They also vector two common viruses: tomato spotted wilt and impatiens necrotic spot. In addition, thrips can cause a defect on tomato fruit called gold flecking.
Cole Crops & Leaf Greens
Striped and crucifer flea beetles are common on these vegetables. While different leafy greens and cole crops vary considerably with respect to flea beetle susceptibility, some are highly susceptible to damage. Early planting and slow seedling growth in cool weather lessens the plant’s ability to outgrow flea beetle damage.
Managing Flea Beetles
Generally, a few flea beetles is not enough to warrant an insecticide application; however, we want to keep damage at a level where it doesn’t retard seedling growth. This means try to limit damage to less than 10% to 20% defoliation. Young plants should be monitored once or twice a week until they become established. Some seed treatments and at-planting systemic insecticides will provide some control of flea beetles, but they should still be monitored for activity.
Resistance to insecticides continues, and it is becoming a serious problem for flea beetles as many products are not as effective as they once were. While we are fortunate to have several effective insecticides, prolonged use of the same mode of action may result in reduced effectiveness. Because of this, producers should not use insecticides with the same mode of action for consecutive generations of flea beetles or season after season as the sole flea beetle insecticide. Often local populations of this insect may be resistant to one group of insecticides, and in other areas they may be resistant to others. What works well in one county may not work at all in another. The new IRAC codes on the labels should help producers identify products that can be used in rotation.
By Ric Bessin, Extension Entomologist