pdf, 57.71Mb - Entomological Society of Canada

Chapter 38
Opuntia polyacantha Haworth, Plains
Prickly-pear Cactus (Cactaceae)
M.G. MAW
There are three species of cacti on the Canadian Prairies: the prickly-pears, Opuntia
polyacantha Haw. and O. tragi/is (Nutt.) Haw., and the pincushion or ball cactus,
Mammillaria vivipara (Nutt.) Haw. Mammillaria is not considered to be a problem.
In the western provinces cacti are found on dry hillsides and flatlands throughout the
more arid parts of the brown soils zone. These areas have some grazing capacity, but
have limitations that make improvement and cultivation impractical. For example,
when the surface is disturbed the soils are likely to erode and blow easily.
Periodically, especially during droughts, prickly-pear becomes a concern. Ranchers
then worry that it will displace valuable grass, use scarce moisture, and cause injury to
stock. It is generally thought that overgrazing increases cactus growth and spread.
Certainly, severely overgrazed grasslands greatly facilitate the spread of cacti for when
soils have been denuded, cactus pads broken from clumps come into contact with bare
soil, root, and establish new clumps. On the other hand, Bement's (1968) study of heavy,
moderate, and light pasture use showed that while populations of cactus doubled in area
in 28 years, it never exceeded 2.4% of land area. Removal of cactus did not increase
forage production significantly but did make it more available for grazing. In lightly
grazed pastures, grasses tended to mask cactus while under heavy grazing, cactus was
more conspicuous. Farmers in the Avonlea area of Saskatchewan maintain that cactus
has not changed much in the past 20 years and where it has increased, it was through
careless efforts to break up the clumps with harrows (personal interviews).
The value of cactus in the North American ecosystem is not always appreciated.
During drought and under heavy grazing, cactus reduces soil erosion. Clumps of cactus
conserve moisture by holding snow, and after summer showers the soil within the
clumps remains moist for several hours longer than soil outside them. Clumps also
protect desirable plants from grazing so that seeds can be produced to repopulate the
range after drought or heavy grazing (Houston 1963).
Prickly-pear is a food for at least 44 species of birds and mammals. For example, seeds
may account for 65% of the diet of the Harris ground squirrel and up to 5.0% of the
browse of deer and antelope (Martin et al. 1961).
The earliest record of an insect attacking cactus dates back several hundred years,
and there is indication that cultivation of the cochineal, Dactylopius coccus Costa, was
conducted in Mexico and Central America for many centuries before the voyage of
Columbus (Mann 1969).
The first comprehensive study of insects of Cactaceae in the United States was done
by Hunter et al. (1912). This work described the life history and habits of various
species, summarized previous information. and provided an extensive and useful bibliography.
Surveys for insects associated with O. polyacantha and O. /ragilis in Saskatchewan
and Alberta were made from 1974 to 1979 (Maw & Molloy 1980). Although only 20
species were found. three, Chelinidea vittiger Uhler, Dacty/opius con/usus (Ckll.), and
Melitara dentata (Grote), were numerous enough to exert considerable pressure on the
cactus. D. con/usus and M. dentala can be very destructive and each year destroy about
10% of the pads (Maw & Molloy 1980)
Two species of Lepidoptera. Cactob/aslis doddi Heinrich and C. bucyrus Dyar. were
183