THE MAIZE BILLBUG AND DIATRAEA SPP.: INSECTS AFFECTING THE SEED PRODUCTION OF EASTERN GAMAGRASS
The maize billbug, Sphenophorus maidis (Chittenden), southern cornstalk borer, Diatraea crambidoides (Grote), and southwestern corn borer, Diatraea grandiosella Dyar, reduce seed production of eastern gamagrass, Tripsacum dactyloides (L.) L., at Woodward, Oklahoma, USA. Adult maize billbugs feed on shoots leaving a transverse row of holes across the leaf blade. This feeding pattern commonly occurs in maize, Zea mays L., as well. Female billbugs chew small holes into the base of culms where they deposit eggs. The culm slowly dies as larvae begin to feed and complete their life cycle. Larvae of the southern cornstalk borer and southwestern corn borer cause damage by feeding on leaves and boring into the culms. Adults of these insects lay their eggs in the upper canopy of the plant. Once eggs hatch, the larvae feed in the local area where eggs were laid creating a “windowpane” effect on the leaf. Larvae move down the culm feeding on lateral inflorescences and eventually bore into the culm where they develop and complete their life cycle. As with the billbug, culms slowly die as the larvae develop, thus reducing over all plant vigor and seed production.
Eastern gamagrass is a highly productive and palatable perennial, warm-season grass used for pasture, hay, and conservation purposes (Dewald et al., 1996; Eaheart, 1992; Rechenthin, 1951). Its wide use, however, has been limited by its high seed costs due to low seed production and slow establishment from seed (Ahring & Frank, 1968; Dewald & Kindiger, 2000). As the number of hectares of eastern gamagrass increases in the United States the incidence of disease and insect pests has become more evident. Plant diseases and insects commonly found in maize are causing serious problems in eastern gamagrass. Cultural practices used to control many of these insects in maize will not work for eastern gamagrass due to its perennial habit. Chemical control measures generally are not labelled for gamagrass and if labelled may only drive seed costs higher. Knowledge of the life cycles of the maize billbug, southern cornstalk borer, and southwestern corn borer in maize may be useful for developing cultural practices for eastern gamagrass.
The USDA-ARS Southern Plains Rangeland Research Station maintains the largest collection of temperate eastern gamagrass germplasm in the United States. We surveyed our collection and found no resistance to these insects. Eastern gamagrass, a close relative of maize, has been used as a gene source for developing disease and insect resistant maize lines (Bergquist, 1981; de Wet, 1979; Moellenbeck et al., 1995). Maize lines with limited resistance to the southwestern corn borer are available and it is possible to move genes from maize into eastern gamagrass. Another alternative is to develop Bt gamagrass (Krizek et al., 2003).
The utilization of eastern gamagrass for pasture, hay, and soil stabilization is increasing every year and other uses such as grain for human consumption or pharmaceuticals from this amazing plant are being explored. As eastern gamagrass hectares increase, it will be very important to develop strategies to minimize the economic impact imposed on the crop by injurious insects and other organisms. We anticipate that these problems will be solved through an integrated approach of plant breeding and cultural practices.