Vergara - 1976 - Physiological and morphological adaptability of ri

EXPERIMENTAL APPROACH TO msEcr-trrirt-rars RELATIONSHIPS 361
the abilities of the respective natural enemies to influence populations of the
host.
The results of these laboratory studies were then compared with. and used to
interpret. the natural distributions and performances of these parasitoids in
California alfalfa fields (Messenger. 1970).
To evaluate the influences of fluctuating temperatures on parasitoid and host
biologies, a series of constant temperature studies was also camed out. using the
physiological life table technique as the experimental approach (Force and
Messenger. 1964a. 1964b). Studies on the possible outcome of competition
among these parasitoids were also made under constant conditions by exposing
hosts to simultaneous or sequential attack by different combinations of the
three parasitoids, taken two at a time (Foree and Messenger. 1965).
An overall summary of the results of all these studies has not yet been published.
Brieflyn the host aphid has a ‘wider temperature tolerance range than any
one parasitoid. or all three together. However, with all three together, the parasitoid
tolerance range is widened sufficiently so that the host can only find
"enenrv-free” situations in the most extreme climatic conditions (midwinter.
midsummer). Within the range of activity of the parasitoids. T. complanarizs- is
active over the widest range of conditions, A. asychis is next. and P. exsoleticm
is least. Each parasitoid alone can check rising populations of the host. but only
over relatively narrow ranges of climate. In this respect, T. complanatus can
control its host over the widest range of temperatures, A. asychis next, and P.
exsoferizm the narrowest. However, in the coolest conditions within which these
parasitoids remain active. P. exsoletiznz remains the most effective.
These results correlate well with the field performance of these natural enemies.
Pram: exsoletmn occupies the northerly. cooler. and higher habitats in California.
where it is the dominant parasitoid affecting the alfalfa aphid. It is less
numerous and in places even absent in the hotter. lower. and more southerly
regions (Messenger, 1970). Triorvs COIHPIGITGIIIS. on the other hand. is the
principal enemy in the southerly’. warmer areas. The aphelinid. A. asychis, is
restricted to coastal and intermediary localities because of its inability to tolerate
or function in summer heat or winter cold.