Testing the hypothesis that a cochineal insect species (Hemiptera: Dactylopiidae) may have been displaced by a congeneric biological control agent from a different cactus host

In 1913, a cochineal insect species, Dactylopius ceylonicus (Green) (Hemiptera: Dactylopiidae), was released in South Africa as a biological control agent against drooping prickly pear Opuntia monacantha. Later, in 1938, Dactylopius opuntiae (Cockerell) was used, together with several other biological control agents, to suppress populations of mission prickly pear, Opuntia ficus-indica. Both programmes achieved considerable success in alleviating these weed problems. In common with some other cochineal insect species, D. opuntiae is oligophagous which has led to allegations that it has competitively displaced D. ceylonicus on O. monacantha in South Africa. An investigation into this supposition showed that D. ceylonicus is still present at all of the seven sites where O. monacantha was monitored and that D. opuntiae was not found to occur on O. monacantha at any of the sites. Although D. opuntiae is able to use both O. monacantha and O. ficus-indica as hosts, under laboratory conditions its performance (developmental duration, survival and body mass of the females at maturity) was significantly inferior to that of D. ceylonicus on O. monacantha. These observations show that there is no evidence of the actual or potential displacement of D. ceylonicus by D. opuntiae on O. monacantha.