Rearing the wheat stem sawfly on an artificial diet

Department of Entomology, 333 Leon Johnson Hall, Montana State University, Bozeman, Montana 59717, United States of America Macedo et al. 500 The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is an insect pest in dryland wheat cropping systems in the southern Canadian Prairies and the northern Great Plains of the United States (Morrill 1997). Yield losses caused by C. cinctus are due to reduced head weight (Holmes 1977; Morrill et al. 1992) and lodging, which decreases harvest efficiency. Estimates of yield losses in Montana alone are about US$25 million per year. The biology of C. cinctus is well known. It has a wide host range including nearly all large-stemmed grasses except oats, Avena sativa L. (Farstad 1940). Adults emerge from fallow fields and migrate into crop fields as wheat stems elongate, from mid-May to late July (Morrill and Kushnak 1999). Eggs are laid in stems, and larvae feed on the parenchyma (Ainslie 1920) of growing wheat plants. Plants are susceptible from stem elongation through flowering. Mature larvae move to the base of the plant and cut notches around the inner perimeter of the stem near ground level, which usually causes lodging and loss of wheat heads at harvest (Ainslie 1920). The larvae overwinter in diapause within the lower section of the cut stem (Ainslie 1920). Diapause is completed after about 3 months at low temperature (Salt 1947), and pupation occurs in the spring. New basic research studies would be possible if C. cinctus larvae could be reared on artificial media. Currently, larvae can be reared only in plant stems, which must be reared in a glasshouse or be collected from the field. Both topical and consumption dose-response studies are difficult to quantify with insects that are feeding within plant stems. Off-season adults can be obtained by refrigerating field-collected diapausing larvae. Pupation and adult eclosion will occur following incubation at room temperature. Adults readily mate in the laboratory, and artificial oviposition sites can be provided to obtain eggs. The first reported attempt to rear C. cinctus larvae on artificial diet used natural foods such as wheat juice and cellulose powder (Kasting and McGinnis 1958). Although C. cinctus were reared on a diet mixture (Kasting and McGinnis 1958), larval survival was limited to 14 days, and it was necessary to provide fresh diet every 24–48 h (McGinnis and Kasting 1962). Identified problems included mold contamination and the configuration of the diet (McGinnis and Kasting 1962). The legless C. cinctus larvae are adapted to feeding within an enclosed tube, or plant stem. To simulate these conditions, McGinnis and Kasting (1962) improved larval survival by placing evenly spaced plastic plates with grooves in the media. Since then, there have been no reports of improved methods for rearing C. cinctus larvae. Development of a satisfactory artificial medium, based on a better understanding of nutritional requirements and presentation of diet, would open new opportunities for our current research on larval antibiosis and fungal pathogens. This paper presents a new diet medium for the wheat stem sawfly that was successfully used to rear neonate larvae to maturation. In the first experiment, a commercial diet for fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), consisting of sucrose, cholesterol, methylparaben, sorbic acid, vitamin mix (Vanderzant), ascorbic acid, stabilized wheat germ, casein, sterile corn cob grits, Wesson salt mix, and raw linseed oil (diet F9179B, BioServ, Frenchtown, New Jersey), was tested. The dry mixture was combined with 1 L of boiling agar (A-7002, SIGMA Chemical Co., St. Louis, Missouri) and blended for 2 min. The final mixture was dispensed immediately into 60 mm × 15 mm sterilized disposable plastic petri dishes (Fisher brand, Fisher