TROPHIC INTERACTION BETWEEN SITODIPLOSIS MOSELLANA (DIPTERA: CECIDOMYIIDAE) AND SPRING WHEAT: IMPLICATIONS FOR YIELD AND SEED QUALITY

Larvae of the wheat midge, Sitodiplosis mosellana (Géhin), feed on developing seeds of common and durum wheats, Triticum aestivum L. and Triticum durum L. (Graminae). The trophic relationships between insect and plant were quantified as biomass gains or losses using plants infested artificially in the laboratory and naturally in plots and commercial fields. The biomass of seeds from different parts of a wheat spike varied, but seeds in all parts of a spike were infested, independently of their potential biomass. Most infested seeds had 1–3 larvae, but at least 11 larvae could mature on a single seed without reducing larval biomass. When larvae finished feeding and seeds attained about one third of their biomass, specific impact varied from 4.1 to 8.5 mg of seed biomass lost for each milligram of biomass gained by a larva, with the impact declining as the number of larvae per seed increased. Specific impact rose to 100 mg/mg as seeds of T. aestivum and a primitive wheat, Triticum monococcum L., matured, and higher still for T. durum. Wheat plants did not compensate for wheat midge damage, and no indirect damage to uninfested seeds was detected. The distribution of biomass for infested seeds was bimodal, with over 40% less than 8 mg when hand harvested, whereas infested seeds harvested mechanically had a unimodal distribution, with nearly all of the most severely damaged seeds removed during harvest. A visual rating system of six damage categories was related to the biomass of the seeds. The germination and early growth rate of infested seeds were reduced compared with those of uninfested seeds. Based on the biomass relationships for the insect–plant interaction and the visual rating of damage, high-protein number 1 grade common and durum wheat and number 1 grade durum wheat can tolerate up to 6% of the seeds being infested by larvae, before downgrading is likely. For other grades, the economic threshold is 10% of the seeds infested, based on yield loss. Seed growers can adopt the threshold for number 1 wheat (6% infestation) to prevent downgrading, which would also reduce the effects of infestation on seed germination to an acceptable level.