As of April 2nd, 2015, we have received several reports of large numbers of spring cankerworms in north and central Texas. These insects have "outbreak" years every so often and this year is shaping up to be one of them. Spring cankerworms are generalist feeders on many types of broadleaf trees. One easy identification character is that this insect only has two "prolegs", or legs toward the back of the body. As a result, the caterpillar moves by the characteristic "inchworm" maneuver of seeming to inch along by raising the center part of the body. There are very few other insect species that are present in high numbers this time of year. However, while our photo shows a purple to pink caterpillar, many of the larvae look more grayish. Leaf feeding will continue for 3 - 4 weeks and defoliation can be severe.
People often ask whether they need to control the insects, and the answer is "maybe". Healthy trees can withstand a lot of defoliation, but drought stressed trees or those that are diseased or in some other way unhealthy are less able to tolerate significant defoliation. High value trees should probably be sprayed if defoliation is significant. In response to this 2015 outbreak, Dr. Mike Merchant has just posted a very good blog article on spring cankerworm. The University of Minnesota has control information here. The first option listed on the Minnesota publication is Bacillus thuringiensis (Bt). This insecticide is made from crystalline proteins produced by bacteria, and these are completely harmless to people, pets and livestock. Bt is approved for organic agriculture. Bt is not a contact insecticide; it must be eaten by the caterpillars. Bt is a good choice because it only kills certain kinds of caterpillars and will not kill beneficial insects that prey on other insects. One downside to Bt is that it is broken down quickly by sunlight and repeated applications may be necessary. The Minnesota publication also lists several synthetic pyrethroids as control options. Most synthetic pyrethroids have active ingredient names ending in the letters "..rin", like cyfluthrin or bifenthrin. These are excellent broad spectrum insecticides that kill by both contact and ingestion. The downside to pyrethroids is that they kill many different types of insects and will kill the beneficial insects present on the trees and in the vegetation beneath the trees. (And some pyrethroids are highly toxic to fish; check the label before using a pyrethroid near bodies of water.)