Field biology and mortality factors affecting Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), a biological control agent of Canada thistle (Cirsium arvense (L.) Scop.)

Abstract

Mean overwintering survival of beetles was 20.5 ± 17.2% and 21.3 ± 3.6% for two consecutive years. Leaf litter was the preferred hibernaculum followed by straw. Successful overwintering was not compatible with water saturated soil. The mean supercooling point of C. rubiginosa in summer was -5.2 ± 1.5°C and was significantly different (P<0.05) from mean supercooling points of -9.3 ± 2.1°C in fall and -8.8 ± 2.3°C in winter. Females studied individually produced an average of 61.1 ± 29.9 oöthecae/female under field temperatures for the period of May 2 through August 13. Development from egg to adult under field temperatures required an average of 27.4 ± 1.7 days when reared on Canada thistle and 28.5 ± 2.1 days on an alternate host, musk thistle. Mortality due to incomplete development totaled 52.5% for immatures reared on Canada thistle and 79.0% for immatures reared on musk thistle. Mean survival from egg to adult of beetles maintained in field cages ranged from 18.3 ± 26.7%. Mean survival from early (1st - 3rd) instar to adult averaged 22% for beetles confined in field cages and 16% for beetles not confined in cages. Ingestion of leaves treated with the herbicides glyphosate and 2,4-D had neither a positive nor negative effect on amount of feeding by C. rubiginosa adults and did not appear to affect the longevity of adult beetles or the fecundity of females.