For issue no. 9 (May 25, 2001) of the Bulletin, I wrote an article titled "Will 2001 Be the Year of Lepidoptera?" At that time, armyworms and variegated cutworms were wreaking havoc in most areas of the state, and black cutworms were causing some distress, too. Although European corn borers have not been as aggravating as their brethren, there still is time for them and other Lepidoptera to give us headaches. Therefore, a couple of somewhat unusual situations bear reporting in this article.|
Many of you will recall that we had some minor outbreaks of a pest known as the thistle caterpillar, Vanessa cardui, in soybean fields a few years ago. This year I am aware of one report of large numbers of these striking caterpillars feeding on thistles in and near a soybean field in Ford County. According to the report, the caterpillars were not feeding on the soybeans yet. Matt Montgomery, Extension unit educator in Sangamon and Menard counties, reported that he has seen a large number of painted lady butterflies in his area. As many of you know, the beautiful painted lady butterfly is the adult stage of the same species. One might ask, "How could such beautiful parents produce such ugly offspring?" But then we've all known . . . never mind.
Thistle caterpillars are about 1-1/4 to 1-1/2 inches long when fully grown. They are brown to black, with a yellow stripe along each side of the body. Their most striking features, however, are the branched spines that cover the larva's body. The painted lady butterfly is brown with red and orange mottling and black and white spots. They can be observed easily in the countryside.
Thistle caterpillars from Ford County. (Photo forwarded by Suzanne Bissonnette, Extension IPM Educator, Champaign.)
Thistle caterpillar. (Photo courtesy of Marlin Rice, Iowa State University, Ames.)
Painted lady butterfly. (Photo courtesy of Marlin Rice, Iowa State University.)
The painted lady does not overwinter in the Midwest; rather, it migrates into our area each spring from tropical or subtropical locations. Females lay eggs singly on hosts, and the eggs hatch in about 7 days. The larvae (thistle caterpillars) feed for 2 to 6 weeks on more than 100 species of plants, including Canada thistle, soybean, sunflower, garden vegetables, and ornamentals. When large numbers occur, they can become economic pests of soybeans. They feed on the leaves, defoliating plants and webbing the leaves together. The published threshold is three or more larvae per plant for soybean stages V3 and V4. The threshold should be higher for more mature plants, or you can rely on percentage defoliation (25 to 40% defoliation, depending on the stage of soybean development) as a treatment guide.
The other, even more unusual occurrence was reported by Walt Longo and Bob Bess with Mason County Service Company. They sent me a photo of several hornworms that they had found in a field of sweet corn in Mason County. Although the photograph showed plenty of detail, I had no idea what I was looking at. However, I was able to use an insect key (Field Key for Identification of Caterpillars Found on Field & Vegetable Crops in Colorado from Colorado State University) to identify the species rather easily--whitelined sphinx, Hyles lineata. The "horn" on the rear end of the caterpillar was a dead giveaway that the insect was the larva of a sphinx or hawk moth, but I was not aware that any of these insects had ever been seen feeding in field crops. However, another reference I consulted indicated that this species feeds on many crop and ornamental plants. In addition to the horn, the whitelined sphinx caterpillar has striking coloration--black or yellow stripes the entire length of the body but highly variable in color, ranging from almost entirely black, with a yellow pattern, to almost entirely yellow or green, with a black pattern. Walt indicated that the defoliation injury to leaves was obvious but not serious. However, he and Bob observed very large numbers of these caterpillars crawling from a field of sweet corn to a neighboring field.
Whitelined sphinx caterpillars. (Photo courtesy of Bob Bess, Mason County Service Company.)
So add two more Lepidoptera to our list for 2001. I suspect we will add others before this season is over.--Kevin Steffey