Jerry Brust, IPM Vegetable Specialist, University of Maryland;jbrust@umd.edu
If you think striped cucumber beetles seemed to be in greater than normal numbers this year you are not alone. This has been a particularly bad year for striped cucumber beetles in squash, cucumber, watermelon and, lately, pumpkins. The beetles have been ravaging cucurbit fields in southern and central Maryland as well as the eastern shore. Some fields have been hit particularly hard with beetles causing 10-15% plant loss due just to their feeding. The cool wet spring we had slowed the emergence of the beetles from their overwintering sites. When you combine the delayed emergence of the beetles with the slow planting schedule of squash and cucumber because of the cooler weather you wind-up getting a massive movement of beetles into some fields as soon as there are any cucurbit plants available.
The beetles tend to mass on small plants where they eat, mate and defecate (Fig. 1). This type of frenzied activity where there are many beetles feeding on a few leaves or a small plant leads to increased chances of bacterial wilt problems (Fig. 2). The bacterium that causes bacterial wilt in cucurbits, Erwinia tracheiphila, is in the cucumber beetle’s feces. As the beetles defecate on the leaves where they are feeding the bacteria can be moved into open (feeding) wounds with water that is in the form of precipitation or dew. The more beetles feeding and opening wounds on susceptible crops like cucumbers and squash the greater the chance of bacterial wilt infection. In a few small cucumber fields I saw as much as 45% of the plants with bacterial wilt.
One additional problem with these pests and why control sprays have not worked as well as they should have under some conditions is that the beetles are consistently hiding at the base of the plant (in the plastic hole) where they are feeding on the stem (Fig. 3). Sprayers are usually set up to cover a lot of leaf canopy and often do not do a very good job of putting chemical down in the plant hole. This stem feeding can be severe enough to cause some wilting. It is hard enough to control cucumber beetles with a good cover spray, but when only small amounts of spray are reaching them down in the plastic hole they will not be controlled. On many of the farms that were hit hard with early beetle populations, beetle numbers seem to be much lower the last week or so.
Figure 1. Feeding frenzy of striped cucumber beetles-eating, mating, defecating
Figure 2. Small and large cucurbit plants infected with bacterial wilt. At first a few leaves or one vine will wilt and eventually the whole plant
Figure 3. Feeding at the base of a young cucumber plant by striped cucumber beetles.