Phosphorus Fertilization Reduced Hessian Fly Infestation of Spring Wheat S. E. Petrie and K. E. Rhinhart Columbia Basin Agricultural Research Center, Oregon State University Introduction Hessian fly is a relative newcomer to northeastern Oregon with the first observations reported in 1980, although it has been found in western Oregon since the late 19th century. Hessian fly has traditionally been a pest in Oregon in areas where the average annual precipitation exceeded 30 inches. Changing cropping patterns, such as increasing irrigation and no-tillage production of cereals, have led to increased concern about Hessian fly. Yield reductions from Hessian fly have become more common in recent years. Several Hessian fly bio-types present in low proportions of the total population are virulent to resistance genes currently deployed in resistant spring wheat varieties. A shift in dominance of biotypes, or the entry into the region by new biotypes, could defeat the genetic resistance currently utilized. Deployment of new resistance genes is required to maintain levels of genetic resistance. Wheat breeders and entomologists are working to identify biotypes and employ genes with resistance to current and newly emerging threats to the wheat industry. Improved plant nutrition has been shown to reduce the incidence and severity of some diseases but there is no information on the effect of P fertilizer on Hessian fly infestation of spring wheat. The objective of this research was to determine if P fertilizer had an effect on Hessian fly infestation in spring wheat. Materials and Methods These trials were established at the Columbia Basin Agricultural Research Center near Pendleton in northeastern Oregon. The average annual precipitation is about 400 mm and 75% of the precipitation occurrs between November 1 and May 31. Winter precip-itation plus growing season precipitation (Sept. through June) was 475 mm in 2006 and 370 mm in We seeded ‘Alpowa’ and ‘Zak’ spring wheat into a Walla Walla silt loam soil (coarse, silty, mixed, mesic, Typic Haploxeroll) in early April. ‘Alpowa’ is susceptible to Hessian fly while ‘Zak’ is resistant to Hessian fly. No insecticidal seed treatments were applied. The seed was sown 2.5 cm deep at 270 seeds/m -2. The previous crop was winter wheat both years. Sodium acetate soil test P values were 20 ppm in 2006 and 19 ppm in 2007; Oregon State University fertilizer guide for spring wheat does not recommend P fertilizer when the soil test P is greater than 15 ppm. Phosphorus was applied at 0 or 45 kg ha -1 in 2006 and 0 or 22 kg ha -1 in In 2006 the trial was seeded using a John Deere 1560 drill; N was applied as urea-ammonium nitrate solution (32-0-0) and P was applied as between the rows. In 2007 the trial was seeded using a Fabro no-till drill. The N was applied as urea between the rows and the P was applied as monocalcium phosphate (0-45-0) with the seed. Individual plots were 3 x m in 2006 and 2.4 x 9.1 m in The treatments were arranged in completely randomized block design with four replications. When the plants were in the jointing stage of development, samples consisting of all the plants from 1 m of row were collected and the plants were separated into fertile and non-fertile tillers and the number of pupae per tiller were counted. The plots were harvested using a Wintersteiger plot combine and yields estimated; subsamples of the harvested grain were used to determine test weight. Results and Discussion 2006 P fertilization had no effect on the number of fertile tillers or the number of total tillers/meter of row for either ‘Alpowa’ or ‘Zak’ spring wheat (Table 1). P fertiliza- tion resulted in a significant reduction in the number of infested tillers of ‘Alpowa’ spring wheat. There were few infested tillers of ‘Zak’ spring wheat; there were no infested tillers when P fertilizer was applied. There were 277 pupae/meter of row in ‘Alpowa’ when P was not applied. There were 93 pupae /meter of row in ‘Alpowa’ when P was applied which was not significantly dif-ferent from the number of pupae in ‘Zak’, the resistant variety. P fertilization also reduced the number of pupae/infested tiller in ‘Alpowa’. The number of pupae/infested tiller was not significantly different in ‘Alpowa’ fertil-ized with P and ‘Zak’. P fertilization increased the yield of ‘Alpowa’ but not ‘Zak’ spring wheat. The grain yield of ‘Alpowa’ fertilized with P was not significantly different than the yield of ‘Zak’ spring wheat. The test weight of ‘Alpowa’ wheat was heavier than the test weight of ‘Zak’ spring wheat and P fertilizer resulted in small but significant reduction of ‘Alpowa’ test weight. VarietyHessian fly response P RateTillersInfested tillers m row -1 Pupae m row -1 Pupae infested tiller -1 Grain Yield Test Wt. FertileTotal 2006kg ha # % # kg ha -1 Lb/bu AlpowaSusc.090.0a111.5a41a277a4.9a2700a60.6a a99.0a24b93b2.3b3235b59.9b ZakResist.090.0a112.0a4c6b0.7b3220b57.3c a91.0a0c0b0.0c3515b57.7c 2007 AlpowaSusc.057.0a141.0a19.2a73.8a2.6a2130a57.5a b129.0a6.8b15.8b1.3ab2720b59.1b ZakResist.082.5ab150.8a0.4b4.3b0.8b2495b58.2ab b152.8a2.3b0.8b1.3ab2570b57.8a Summary and Conclusions P fertilization had no effect on total tiller numbers P fertilization increased the number of fertile tillers on ‘Alpowa’ in 2007 P fertilization reduced the percentage of tillers infested with Hessian fly pupae, the number of pupae per meter of row, and the number of pupae per infested tiller for ‘Alpowa’, a susceptible variety, but not for ‘Zak’, a resistant variety. P fertilization increased the yield of ‘Alpowa’ spring wheat, which is susceptible to Hessian fly but did not increase the yield of ‘Zak’ spring wheat, which is resistant to Hessian fly. P fertilization may be affecting the ability of the Hessian fly to infest the plants or the ability of the eggs or pupae to survive. The mechanism responsible for these observations is not known. ‘ Alpowa’ spring wheat with whiteheads caused by Hessian fly Table 1. Effect of P fertilization on Hessian fly infestation, grain yield and test weight of spring wheat, 2006 and Hessian fly infestation in 2007 was less than in 2006 so the magnitude of the responses was smaller in 2007 than in However, the same pattern of responses was observed. P fertilization had no effect on the total number of tillers per meter of row of either ‘Alpowa’ or ‘Zak’; P fertilization increased the number of fertile tillers on ‘Alpowa’. P fertilization significantly reduced the percentage of infested tillers of ‘Alpowa’ but not ‘Zak’. There were 74 pupae per meter of row in ‘Alpowa’ when P was omitted but only 16 when P was applied which was not significantly different than the number of pupae in ‘Zak’. P fertilization reduced the num- ber of pupae per tiller in ‘Alpowa’, although the reduction was not significant. P fertiliza- tion increased the yield of ‘Alpowa’ but not ‘Zak’. The grain yield of ‘Alpowa’ fertilized with P was not significantly different than the yield of ‘Zak’ spring wheat. P fertilizer increased the test weight of ‘Alpowa’ but not ‘Zak.