| dc.description.abstract | 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. | |
