Tritrophic Effects of Leaf Nitrogen onLiriomyza trifolii(Burgess) and an Associated ParasitoidChrysocharis oscinidis(Ashmead) on Bean

Significant differences in developmental times ofLiriomyza trifolii(Burgess) males andChrysocharis oscinidis(Ashmead) males and females were found in response to percent nitrogen levels in bean foliage. These changes represented differences in developmental times of 4.5% inL. trifoliimales and 2.7% inC. oscinidisfemales and occurred over the ranges of nitrogen examined (L. trifolii2.76–6.27% leaf N;C. oscinidis2.67–6.52% leaf N). Although pupal weights ofL. trifoliilarvae reared on leaves with various nitrogen levels increased significantly as leaf nitrogen increased,L. trifoliifecundity was not significantly affected by nitrogen in the leaves upon which the females developed. It appears that larval food acquisition of the female has less effect on fecundity than does adult food acquisition. Parasitoids were allowed to feed and oviposit onL. trifoliireared on leaves with differing nitrogen levels. Fecundity of adultC. oscinidis(averaged 69 progeny per female over a 10-day period) was significantly highest at the 4.88% leaf nitrogen level. Similar to the shortest developmental time, fecundity was enhanced at this level and adversely affected at the other levels.C. oscinidissex ratio averaged 1:1.23 (male:female); no significant differences in sex ratio were found among nitrogen treatments. Total progeny production of adultC. oscinidis,allowed to feed and oviposit onL. trifoliilarva reared on leaves with differing nitrogen levels, may increase by as much as 40%. These changes in fecundity are highly significant and could greatly affect pest/parasitoid populations in both agroecosystems and mass rearing programs.