these studies assess how changing food quality influences spruce budworm, choristoneura fumiferana (clem.), in terms of the performance characteristics of survival, development, growth, and fecundity. the components of food quality that were investigated were nitrogen (n), water, indigestible fibre (if), and leaf toughness. i believe that budworm feeding, at a given defoliation intensity triggers a relative increase of foliar if in comparison to the other nutritional constituents that are essential to the growth and development of spruce budworm it was my hypothesis that there is a threshold if:nutrient above which performance and fitness declines. four experiments were conducted to test this hypothesis. in experiment 1, foliar samples of balsam fir (bf), abies balsamea (l.), white spruce (sw), picea glauca (moench) voss, and black spruce (sb), picea mariana (mill.) b.s.p., were chemically analyzed for levels of n and if to determine the seasonal variation in these components and to quantify the physiological bounds of if:n in the natural budworm diet. as well, water content was assessed. foliar n levels were initially high and decreased rapidly in the early growing season and then gradually for the remainder of the season. little difference was observed among species and crown positions. if followed a trend of low levels in the swollen buds, a rapid increase to about early july and then a general tapering off. there was slight variability among crown positions. at the start of budworm feeding early in the growing season, if:n was about 20:1 for all species. by early july, when feeding stops, the if:n ratio was about 50:1 for sw and 30:1 for bf and sb. foliar water content followed a trend of high levels at growth initiation that increased soon to seasonal peaks. water content then decreased from about mid-june to late august. little difference was observed between crown positions for all species. in experiment 2 spruce budworm were reared on early- and late-season foliage of bf, sw, and sb to test the hypothesis that budworm can perform better on foliage with low if:n (early-season foliage) than on foliage with high if:n (late-season foliage). spruce budworm showed superior development, growth and fecundity on early-season compared to late-season foliage. development times were shorter and pupal weights and number of eggs laid were greater on early-season than on late-season foliage. the early- and late-season, current-year needles of bf, sw and sb were collected and measured with a penetrometer for leaf toughness in order to quantify the seasonal variation of this food quality component. early-season needles were significantly softer than late-season needles for all species, although toughness measurements were highly variable. among early-season measurements, bf foliage was significantly softer than the two spruces, which themselves did not differ. among late-season measurements, species differed significantly from each other with sb being the toughest and bf being the softest. it is interesting that the two spruces differed significantly considering that they had similar if levels. it appears that a component other than if may be contributing to leaf toughness for sb. in experiment 3, to test the hypothesis that below some level of dietary n budworm performance would be low, casein was added to the standard mcmorran diet at 12, 18, 25 and 100 percent casein in the standard diet. this lower level would then be included into the design of experiment 4. as well, components of the standard diet were analyzed for sources and levels of n and if. spruce budworm reared on the 12 percent casein diet showed the poorest performance; survival, weights at sixth instar and pupa, and number of eggs laid were lowest while development times in sixth instar were longest. no clear association between pupal weight and fecundity occurred; fecundity was greatest on diets with 25 percent casein although female pupal weights were significantly lighter than those on 100 percent casein. it may be that there are inherent problems with the weight/fecundity relationship or that the standard diet is too n-rich and not optimally nutrient balanced for egg production. these findings have implications for optimizing the standard mcmorran diet. to test the hypothesis that there is a threshold if:n above which spruce budworm performance declines, experiment 4 was conducted with 12, 18, 25, and 100 percent casein in the standard diet and varying cellulose:casein n. cellulose:casein n ratios of 3:1 (standard mcmorran diet), 15:1, and 50:1 were included to imitate the physiological bounds of if:n in host foliage diets. budworm performed well on all diets with few significant differences. the reason for these unexpected results, in comparison to experiment 3, was probably because budworm were initially reared on the standard diet before being transferred, whereas in experiment 3, the insects were reared from second instar to pupa on treatment diets. the standard diet probably gave the larvae sufficient protein reserves to support their growth when reared on the deemed protein-deficient diets. as well, the inclusion of powdered cellulose as the primary source of fibre had little effect on performance, probably because it passed through the larval gut easily without interfering with digestion. an exception was the positive effect that increasing cellulose to 15:1 in the 100 percent diet had on egg production, compared to the standard diet which was 3:1, cellulose:casein n. females on the former diet had 14 percent heavier pupal weights and laid 19 percent more eggs than those on the latter. it appears that the standard diet could be improved through the addition of fibre. from these studies, it is still inconclusive that there is an interaction between n and indigestible fibres which leads to diminished budworm performance.