Host specificity of biotypes of Dactylopius opuntiae (Cockerell) (Hemiptera : Dactylopiidae): prospects for biological control of Opuntia stricta (Haworth) Haworth (Cactaceae) in Africa

Opuntia stricta (Haworth) Haworth and Opuntia ficus-indica (L.) Miller are the most prominent of the cactus weeds that have become invasive in several countries in Africa. Opuntia stricta has no useful attributes but O. ficus -indica is extensively exploited as fodder for livestock and fruit for huma n consumption. Biological control of Opuntia weeds has not been attempted i n North Africa for fear that introduced agents could deplete the valued res ources provided by O. ficus-indica. Recent confirmation that there are at l east two distinct biotypes of Dactylopius opuntiae (Cockerell), each specif ic to particular groups of Opuntia species, has enhanced prospects of O. st ricta being controlled biologically with no threat to O. ficus-indica. One biotype of D. opuntiae, referred to as the 'stricta' biotype, only survives on low-growing species such as O. stricta while the other, the 'ficus' bio type, is associated with tree-like cacti, including O. ficus-indica. The 's tricta' biotype has been used with considerable success for biological cont rol of O. stricta in Australia for over 60 years. To corroborate the conten tion that the 'stricta' biotype of D. opuntiae could be used specifically t o control O. stricta, with no threat to tree-like opuntias, the development and survival of the insects was monitored on several cultivars of O. ficus -indica, including the feral, spiniferous form and six spineless forms, Rob usta, Fusicaulis, American Giant, Gymno Carpo, Algerian and Morado. The stu dy showed that, with one possible exception, the O. ficus-indica varieties were unsuitable hosts for the 'stricta' biotype of D. opuntiae and demonstr ated that the insects could be used with safety for biological control of O . stricta in North Africa.