Host plant associations of two cochineal insect species, Dactylopius ceylonicus and D. opuntiae (Dactylopiidae: Hemiptera), on the invasive cactus species Opuntia monacantha, O. ficus-indica and a possible hybrid cactus, in South Africa
Master Thesis
2014
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University of Cape Town
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Several cochineal species have been used with great success for biological control of invasive cactus species, both in South Africa and elsewhere. Although most cochineal species are oligophagous, they are all exclusively associated with opuntioid cacti and therefore pose minimal risk of non-target effects in the Old World where there are no native Cactaceae. However, the ability of cochineal to use more than one host plant species has validated reports of inadvertent displacement of Dactylopius ceylonicus (the original agent released in 1913) by D. opuntiae (which was originally released on Opuntia ficus-indica in 1938) on O. monacantha in South Africa. There are also reports that the two cactus hosts, O. monacantha and O. ficus-indica, have hybridized in South Africa, but there are uncertainties as to which of the two cochineal species is associated with the alleged hybrid. The primary aim of this study was to determine the present status of D. ceylonicus and D. opuntiae on O. monacantha and on the possible hybrid. Cochineal samples were collected off cactus plants from sites in the Eastern Cape and Western Cape and identified using bioassays and morphological features. The performance of each cochineal species on each of the three cactus hosts (O. monacantha, O. ficus-indica and the hybrid) was determined through measures of female developmental rates, survival and mass at reproductive maturity. Results showed that D. ceylonicus remains present on O. monacantha at each of seven study sites, while D. opuntiae did not occur on O. monacantha at any of the sites. However, D. opuntiae is capable of establishing on O. monacantha, O. ficus-indica and the possible hybrid cactus under laboratory conditions. Dactylopius ceylonicus is also compatible with both O. monacantha and the possible hybrid, although it did not establish on O. ficus-indica. These findings showed that D. ceylonicus has not been displaced by D. opuntiae on O. monacantha in South Africa and it is expected that O. monacantha will continue to be kept under excellent biological control into the future. Results also indicated that possible hybridisation between the two Opuntia species has not produced a new taxon that is immune to either of the two control agents. However, further research is required to improve our understanding of this new plant taxon and confirm it will be suppressed by one or both of the cochineal species.
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De Souza, N. 2014. Host plant associations of two cochineal insect species, Dactylopius ceylonicus and D. opuntiae (Dactylopiidae: Hemiptera), on the invasive cactus species Opuntia monacantha, O. ficus-indica and a possible hybrid cactus, in South Africa. University of Cape Town.