Plants produce a variety of physical defenses against herbivores. Defenses include tiny hairs called trichomes (with or without glandular secretions), thorns, spines, granules of calcium, silica or other minerals, and sharp or rough leaf edges.
Plant breeders of tall fescue grass are interested in varieties that have smooth leaf edges instead of sharp leaf edges. The varieties with smooth leaf edges are more palatable to grazing livestock than standard varieties with siliceous barbs or edge spines. Entomologists and plant breeders want to know, “Will varieties with smooth edges be more susceptible to feeding damage by caterpillars?”
The short answer is, No. The cells of the digestive system of caterpillars are protected from puncture by sharp objects. The foregut and hindgut of caterpillars are lined with cuticle. These areas are as well protected as the outside of the insect. The cells of the midgut are not lined with cuticle. That would interfere with digestion. Instead the cells of the caterpillar midgut secrete a protein matrix, the peritrophic membrane, that encloses the food and prevents direct contact between the food and the midgut cells. Enzymes, nutrients and digestive secretions can freely pass through the peritrophic membrane. However, the membrane prevents larger particles, such as viruses, bacteria and other pathogens form contacting the cells of the digestive system. The peritrophic membrane is reinforced with chitin, a tough fiber that is resistant to tears and punctures.
A recent study in Biologcical Control*, found that the armyworm digestive system was plenty tough enough to handle the edge spines of tall fescue. Pampering the armyworms by trimming the edges did not make the armyworms healthier or less susceptible to viruses. Therefore, a switch from sharp edge to smooth edge varieties is not expected to lead to an increase in damage from armyworms.
Cells of a Caterpillar Midgut
Image: VRussell/JNeal
*Baculovirus infection of the armyworm (Lepidoptera: Noctuidae) feeding on spiny- or smooth-edged grass (Festuca spp.) leaf blades. Craig P. Keathley, Robert L. Harrisonb, Daniel A. Potter. (Online January 12, 2012).
