Evaluating different biotypes of Dactylopius opuntiae and Dactylopius confusus (cochineals) for biological control of the invasive cactus Opuntia engelmannii in Kenya and South Africa
Date
2021
Authors
Machimane, Zanele
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Abstract
Opuntia engelmannii Salm-Dyck (Cactaceae) is an invasive cactus in Kenya and South Africa. Furthermore, it has become a priority for biological control in South Africa as it is NEM:BA category 1B invader, therefore it must be controlled or removed. Opuntia engelmannii varieties are found in different geographic locations in South Africa and Kenya and they vary in their morphology and genetic make-up. Therefore, the term ‘lineage’ was used in this study based on where they are currently found in Africa. Three of these lineages are found in the Eastern Cape (EC), Northern Cape (NC) and Limpopo (LM) province of South Africa. A fourth O. engelmannii lineage is found in Kenya. Biocontrol of Opuntias using cochineal insects has an excellent track record. However, success against O. engelmannii in Kenya and South Africa has not been achieved because of lack of knowledge on the exact origin of the plant. Therefore, this study aimed at finding a suitable biocontrol agent for the Kenyan and South African (EC) O. engelmannii lineages. Classical biocontrol is a preferred method compared to other control measures (e.g., chemical and mechanical control) because it is cost-effective, efficient and sustainable. This study investigated which cochineal population (biotype) of six Dactylopius opuntiae (Cockerell) (Hemiptera; Dactylopiidae) and four Dactylopius confusus (Cockerell) (Hemiptera; Dactylopiidae) collected from geographically separate lineages of O. engelmannii in the United States of America (USA) would perform best on lineages of O. engelmannii collected from Kenya and EC Province of South Africa. Each locality where the Dactylopius biotypes were collected in the USA was described as a ‘site’. Host plant acceptability and suitability of the Kenyan and EC O. engelmannii lineages to the 10 Dactylopius biotypes were investigated. This was done by measuring the proportion of settling female cochineals on both Kenyan and EC detached cladodes of O.engelmannii. Then development and fecundity of the insects was assessed by measuring fitness indices.” Results from this study revealed that five (sites 5, 13, 20, 21 and 37) out of the six D. opuntiae biotypes settled and developed into gravid females (caring eggs) on detached cladodes of Kenyan O.engelmannii lineage and only one (site 23P1) out of the four D. confusus biotypes were able to settle and develop into gravid females on the Kenyan O. engelmannii lineage. While all populations of both D. opuntiae and D. confusus settled and developed into gravid females on the EC lineage of O. engelmannii. These results described the host plant acceptability of the Kenyan and ECO. engelmannii lineage to the 10 Dactylopius biotypes. The host plant suitability was assessed by the fitness index. The fitness index (FI), calculated as the proportion of adult females multiplied by the average number of offspring per female over mean developmental time, show that the EC O. engelmannii lineage was more susceptible to both species of Dactylopius compared to the Kenyan O. engelmannii lineage, where only four (sites 5, 13, 20 and 21) of the six D. opuntiae had high fitness index (FI>1) and thus showing a greatest potential to control the Kenyan O. engelmannii lineage. No D. confusus biotypes had particularly high FI on the Kenyan O. engelmannii lineage and thus performed poorly on the Kenyan lineage of O. engelmannii. The four Dactylopius biotypes with the highest fitness indices were selected for a whole, potted-plant experiment, to assess their impact on plant health. Chlorophyll content, cladode thickness, percentage cover of cochineals and biomass were recorded. All four D. opuntiae biotypes (sites 5,13, 20 and 21) eventually killed the whole plants. However, D. opuntiae biotype from site 21 was the most effective biocontrol agent, due to its rapid reduction of plant chlorophyll content of the Kenyan O. engelmannii lineage. The biotypes that performed very well on the Kenyan lineage of O. engelmannii were biotypes coming from O. engelmannii lineages that were morphologically similar to the Kenyan O. engelmannii lineage. Geographically, these biotypes were from the same region, south of Texas. This could indicate an old association. The best performing biotypes on the Eastern Capelineage of O. engelmannii are from O. engelmannii lineages without the morphological characteristics of the Eastern Cape O. engelmannii lineage. This could have been a new association because the Eastern Cape O. engelmannii lineage appeared to be a different Opuntia species. The Eastern Cape O. engelmannii was accepted by all the D. confusus biotypes, However, D. confusus biotypes were not as damaging as the D. opuntiae biotypes on the Eastern Cape O. engelmanniil ineage.The Kenyan O. engelmannii lineage was not a suitable host for any D. confusus biotype. Failures and successes of Dactylopius biotypes to thrive on their hosts were explained by several suggested hypotheses, including plant defence mechanisms. Findings from this study have important implications for the biological control of Opuntias in Africa
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfillment of the requirements for the degree of Master of Science in the School of Animal, Plant and Environmental Sciences, 2021