100 F~3lIFAS3A flr//LI 1 North Florida Research and Education Center Research Report NF-90-18 September, 1990 Effects of Systemic Insecticides on Grain Yield of Hessian Fly Susceptible and Resistant Wheat Varieties by J.B. Hartman, R.K. Sprenkel, R.D. Barnett, and A.R. Soffes A fall application of either phorate (Thimet8) or disulfoton (Di-syston) is recommended for wheat growers who plant susceptible varieties in areas where Hessian fly is known to be a problem. This at planting application provides approximately six weeks of protection to the young wheat plants getting them through their most vulnerable stage. However, work done at the University of Georgia during the 1988/89 season showed that winter and spring infestations could reach damaging levels.1 Flaxseed counts of resistant wheat grown at Marianna in 1989 showed low numbers of flaxseed. In order to test the desirability of spring applications and applications for the control of low level infestations, the first year of a 2 year trial was carried out at Marianna. Six varieties were used, three susceptible and three resistant. Florida 301H is a backcross line of Florida 301 with genetic resistance to Hessian fly. Florida 301 and 301H are early maturing varieties and are nearly identical in growth habit and appearance. Similarly, FL85363-G18-14 is a backcross line of Florida 302. They are both late varieties. Coker 9766 and Florida 303 are unrelated. The susceptible varieties were given one of four treatments: No insecticide treatment, an application of Thimet(20G at 7 Ib/A) drilled with the seed at planting, the same application of Thimet plus a spring application of Di-syston (8EC at 1 pt/A), or Thimet at planting followed by two spring applications of Di-syston. The resistant varieties were given one of three treatments: no insecticide, at-planting application, or at planting plus one spring application. The initial (at planting application) was made November 15th, and the side applications were made FebruayJ1,._ 1990 and March 1, 1990. All treatments were replicated six times. Central Science The results showed no positive effect from applying system ic insecticides to resistant cultivars (Table 1). Insecticide applicati ns had no, effect ip, the reduction of flaxseed on resistant wheats. A single spring application ha positive effects on both y eld (Table 1) and test weight (Table 2) of susceptible varieties. Yields ofriFo7iddi8o illustrate this effect most dramatically. Flaxseed counts were reduced from 55 flaxseed/culm for untreaed Flori a 302 to 5 flaxseed/culm for Florida 302 with three applications. These data suggest that it is probably unprofitable to apply systemic insecticides when using resistant varieties or under conditions of low Hessian fly pressure. It also suggests that when growing a susceptible variety in an area heavily infested with the Hessian fly a spring application of Di-syston may be desirable in addition to the in the row fall granular application. 'Buntin, G. David and Paul Raymer. 1989. Susceptibility of winter wheat and triticale to the Hessian fly. Georgia Agri. Exp. Sta. Research Bulletin 389.