PRICKLY-PEAR ON THE CANADIAN PRAIRIES

M. G. MAW and M. M. MOLLOY, Agriculture Canada Research Station, Regina, Saskatchewan, S4P 3A2.

There are three species of cacti on becomes a concern on the prairies. the Canadian prairies: the prickly- Ranchers then worry that the pears, polyacantha Haw. and may displace valuable grass, use (Nutt.) Haw. and the scarce moisture and cause injury to pincushion or ball cactus, Mamillaria stock. During these periods various vivipara (Nutt.) Haw. This paper con¬ control methods are examined. siders only the two prickly-pear Because spectacular control of cactus species because they are often con¬ has been achieved in Australia sidered to be unwanted or pest through the introduction of the , species. Cacti are found on dry Cactoblastis cactorum (Berg), hillsides and flatlands throughout the biological control is often considered more arid parts of the Brown Soils in other countries, including Canada. Zone. Here soils are of poor quality, In biological control programmes, comprising a shallow layer over shale. preliminary studies are made to These areas have some grazing assess the problem and to determine capacity but have limitations that what predators are already present make improvement and cultivation and how effective they are in controll¬ impractical. For example, when the ing pest populations. The following is surface is disturbed, the soils are apt a summary of surveys made to erode and be blown about easily. from 1974 to 1979 on cactus in Some people are of the opinion that Saskatchewan and Alberta and of a overgrazing increases cactus growth. study of two from Argentina as However, this may be an illusion. possible agents for cactus control. Bement’s study of heavy, moderate and light pasture use found that while Survey of Associated with populations of cactus doubled in area Prickly-Pears in 28 years, it never exceeded 2.4% of the land area.1 Removal of cactus did The following insects were not significantly increase forage collected by aspirator or picked by production but did make it more hand from prickly-pear. Roots and available to the cattle. In lightly grazed fruit were not examined. pastures, grass tended to mask the cactus while under heavy grazing the ORTHOPTERA: cactus was more conspicuous. Acrididae Farmers near Avonlea, Dissosteira venusta Stal — adult, Saskatchewan, maintain that cactus Buffalo Pound, Sask., 29 May 1974. has not extended its total range of Peranabrus scabriollis (Thom.) — growth much in the past 20 years. adult, Val Marie, Sask., 16 May 1975. Periodically, especially during HOMOPTERA: droughts, prickly-pear cactus Coccidae

208 Blue Jay Fig. 1. dentata (Grote), a common cactus insect M. M. Molloy

Dactylopius confusus (CK11.) — all Listrus provincialis Blais — adult, stages, Buffalo Pound, Sask. — 29 Irvine, Alta. — 26 June 1979. May 1974. Avonlea, Sask. — 1 June Meloidae 1974; 23 June 1977. Aylesbury, Sask. Epicauta atrata (Fab.) — adult, — June 1974; Manyberries, Alta. — Readlyn, Sask. — 2 July 1973. June, 1974. A widespread cactus Epicauta ferruginea Say — adult, mealybug. Most northerly recorded Readlyn, Sask. — 2 July 1973, Buffalo point, Aylesbury, Sask. Can be very Pound — 7 July 1977. destructive. Anthicidae : Anthicus hastatus Casey — adult, Val * Marie Sask. — 17 May 1975. vittiger Uhler — immatures, Irving, Alta. — 26 June, Nitidulidae 1979. A gregareous feeder on the Carpophilus pal Iipen n is Say — adult, pads. Readlyn, Sask. — 2 June 1973, Chlorochroa uhleri Stal — adult, Val Avonlea, Sask. — 23 June 1977, Marie, Sask. — 10 May 1973. A Irvine, Alta. — 29 June 1977. In the general plant feeder. flowers, very numerous. Tenebrionidae COLEOPTERA: Hyperaspis sp. — adult — Carabidae Manyberries, Alta. — 19 June 1974. Microlestes linearis LeC. — adult, Val Melanastus sp. — adult, Manyberries, Marie, Sask. — 17 May 1975. Alta. — 19 June 1974. Melyridae Elcodes obsoleta Say — adult, Val Collops vittatus Say — adult, Buffalo Marie, Sask. — 17 May 1975. Pound, Sask. — 7 July 1977.

December, 1980. 38(4) 209 Curculionidae species became weak and died out Gerstaekeris basalis LeC. — adult, after a few generations in the Val Marie, Sask. — 17 May 1975, laboratory. Eastend, Sask. — 10 June 1975. Breeds in cactus pads. Bagous sp. — adult, Val Marie, Sask. Discussion and Conclusions — 17 May 1975. No further introductions of cactus DIPTERA: insects are planned. C. doddi and C. Milichiidae bucyrus apparently are not suited to Milichia sp. — adult, Buffalo Pound, our Canadian cacti. Other species Sask. — 7 July 1975. Genus could, however, be tested. It appears associated with decayed plant that any harm or inconvenience material. caused by our prairie cactus is balanced by benefits derived from it. : The native biotic agents already exert Phycitidae considerable pressure on the cactus (Grote) — larvae, and our native grasses are well able to Buffalo Pound, Sask. — 29 May 1974, compete satisfactorily when the land Manyberries, Alta. — 19 June 1974, is not overgrazed. Avonlea, Sask. — 23 June 1977. Larvae mine the pads. Can be very The value of cactus in the destructful. About ten percent of pads ecosystem has not always been appreciated. During drought or under in Saskatchewan and Alberta heavy grazing, cactus reduces soil attacked. erosion. Clumps of cactus conserve Hesperiidae moisture by holding snow, and after Pyrgus communis Grote — adult, summer showers, the soil within the Irvine, Alta. — 26 June 1979. clumps remains moist for several hours longer than soil surrounding them. The clumps slow runoff and protect seeds of desirable so Biological Control Studies necessary for repopulating the range after severe drought or heavy Agriculture Canada’s Regina grazing.2 Research Station obtained two species of moth, Cactoblastis doddi Prickly-pear provides food for at Heinrich and Cactoblastis bucyrus least 44 species of birds and Dyar, for biological control studies. mammals. The seeds may account for Their origins were the high altitude 65% of the diet of the Harris ground valleys of Jujury province in northern squirrel and up to 5.0% of the browse Argentina. In the laboratory both of deer and antelope.3 During the species fed on Canadian prickly-pear summer, cactus may constitute 30% but it was necessary to injure the pads of the food of the black-tailed jack before the larvae would enter and rabbit4 and is the main food of the feed. Once inside, the larvae hollowed pocket gopher5. the pads but would not leave to go to fresh ones when there was no more The introduction and release of a food. Failure to leave the depleted biotic agent to control prickly-pear in pads is contrary to their habits in Canada would be vigorously opposed Argentina where they move from pad by both the United States and to pad as conditions dictate. Either Mexican authorities because they the Canadian cactus was unsuitable regard cacti as an emergency fodder or the insects were diseased for both for cattle, and as human food.

210 Blue Jay CEMENT, R. E. 1968. Plains prickly-pear; plants. A guide to wildlife food habits relation to grazing intensity and blue Dover pubs. Inc. New York. 500 pp. grama yield on Central . 4SPARKS, D. R. 1968. Diet of black-tailed J. Range Manage. 19:83-86. jack rabbits on sandhill range in HOUSTON, W. R. 1963. Plains prickly- Colorado. J. Range Manage. 21:203- pear, weather, and grazing in the 208. Northern great plains. Ecology 5VAUGHAN, T. A. 1967. Food habits of the 44:569-574. northern pocket gopher on 3MARTIN, A. C., H. S. ZIM and A. L. shortgrass prairie. Am. Midld. Natur. NELSON. 1961. American wildlife and 77:176-189.

December, 1980. 38(4)