Trophic interaction between Sitodiplosis mosellana (Diptera : Cecidomyiidae) and spring wheat: Implications for yield and seed quality

Larvae of the wheat midge, Sitodiplosis mosellana (Gehin), feed on developi ng seeds of common and durum wheats, Triticum aestivum L. and Triticum duru m L. (Graminae). The trophic relationships between insect and plant were qu antified as biomass gains or losses using plants infested artificially in t he laboratory and naturally in pints 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 in fested seeds had 1-3 larvae, but at least 11 larvae could mature on a singl e seed without reducing larval biomass. When larvae finished feeding and se eds 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 increas ed. Specific impact rose to 100 mg/mg as seeds of T. aestivum and a primiti ve wheat, Triticum monococcum L., matured, and higher still for T. durum. W heat plants did not compensate for wheat midge damage, and no indirect dama ge to uninfested seeds was detected. The distribution of biomass for infest ed seeds was bimodal, with over 40% less than 8 mg when hand harvested, whe reas infested seeds harvested mechanically had a unimodal distribution, wit h nearly all of the most severely damaged seeds removed during harvest. A v isual rating system of six damage categories was related to the biomass of the seeds. The germination and early growth mts of infested seeds were redu ced compared with those of uninfested seeds, Based on the biomass relations hips fur the insect-plant interaction and the visual rating of damage, high -protein number 1 grade common and durum wheat and number 1 grade durum whe at can tolerate up to 6% of the seeds being infested by larvae, before down grading is likely. For other grades, the economic threshold is 10% of the s eeds infested, based on yield loss. Seed growers can adopt the threshold fo r number 1 wheat (6% infestation) to prevent downgrading, which would also reduce the effects of infestation on seed germination to an acceptable leve l.