The ecology of Cactoblastis cactorum (Berg) (Lepidoptera : phycitidae) in relation to its effectiveness as a biological control agent of prickly pear and jointed cactus in South Africa
- Authors: Robertson, Hamish Gibson
- Date: 1985
- Subjects: Insect-plant relationships Lepidoptera -- South Africa Cactus -- South Africa Weeds -- Biological control -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5671 , http://hdl.handle.net/10962/d1005356
- Description: The successful biological control of the shrub-like prickly pear Opuntia stricta Haworth in Australia by cactoblastis cactorum (Berg) was not repeated when C. cactorum, derived from the Australian population, was released in South Africa in the 1930's against the tree prickly pear Opuntia ficus-indica (L.) Miller. Resistance of the woody portions of o. ficus-indica to attack by C. cactorum was regarded as the main reason for the poor performance of C. cactorum in South Africa. C. cactorum also oviposits and feeds on Opuntia aurantiaca Lindley, which is currently South Africa's most important weed and which is also considered to be partly resistant to attack by C. cactorum. This study had three main objectives: (i) to compare the ecology and effectiveness of C. cactorum as a biological control agent on O. ficusindica and O. aurantiaca; (ii) to reassess why C. cactorum has not been as effective a biological control agent in South Africa as it has been in Australia; and (iii) to evaluate whether inundative release or the importation of new biotypes of C. cactorum from South America (where it is indigenous) might be feasible methods of improving its effectiveness as a biological control agent of O. aurantiaca in South Africa. All field work was undertaken at a site near Grahamstown in South Africa. The ecology and effectiveness of C. cactorum on O. ficus-indica and O. aurantiaca was assessed in terms of its oviposition behaviour, survival and feeding on these host plants. The proportion of C. cactorum eggs laid on O. ficus-indica and O. aurantiaca was similar and was influenced by the size, conspicuousness and condition of the host plant as well as by the proximity of the host plant to moth emergence sites. Factors affecting oviposition site selection on the plant are also considered. Life tables, compiled for a summer and a winter generation, showed that the survival of C. cactorum was greater on O. ficus-indica than on O. aurantiaca, mainly because higher egg predation by ants occurred on the latter host plant species. During the period of study, the population size of C. cactorum was reduced by a number of mortality factors, of which egg predation and the effects of low temperatures on fecundity were the most important. Although there was evidence of a partial, positive response by predatory ants to C. cactorum egg densities on plants, the extent of egg predation was also affected by other factors, particularly seasonal effects. C. cactorum destroyed a greater percentage of cladodes on O. ficu-indica than on O. aurantiaca, but even on O. ficus-indica it was unable to contain the growth of plants within the study area. C. cactorum larvae rarely killed the woody rooted cladodes of O. ficus-indica and O. aurantiaca and consequently whole plants were not often destroyed. The detrimental effects of host plant resistance, natural enemies and climate on the effectiveness of C. cactorum as a biological control agent all appear to be greater in South Africa than in most of the regions occupied by C. cactorum in Australia. A field experiment conducted at the study site showed that inundative release methods for improving the effectiveness of C. cactorum on O. aurantiaca are not feasible. The importation of biotypes of C. cactorum from South America that might be better suited for destroying O. aurantiaca infestations in South Africa, is also not a viable option. Results of a survey of a 218 ha area that is regarded as being heavily infested with O. aurantiaca, illustrate how this cactus species has been overrated as a weed problem. It is argued that the present strategy for O. aurantiaca control in South Africa is not based on sound economic or ecological criteria.
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The host-searching behaviour of coccophagus atratus compere (Aphalinidae: hymenoptera)
- Authors: Clark, Maxwell Maitland
- Date: 1985
- Subjects: Coccophagus , Hymenoptera , Insects -- Host plants , Insect-plant relationships
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5630 , http://hdl.handle.net/10962/d1004923 , Coccophagus , Hymenoptera , Insects -- Host plants , Insect-plant relationships
- Description: The host-searching behaviour of the parasitoid Coccophagus atratus Compere was investigated. C. atratus parasitoids have unusual host relationships. Female offspring develop in scale insects but male offspring develop hyperparasitically on their conspecific females, or on other parasitoid species. C. atratus females, therefore, must locate, identify and oviposit into two different types of hosts. A primary aim of this thesis, was to identify when and how the behaviour of a female, searching for hosts suitable for female offspring, differed from that of a female searching for hosts suitable for male offspring. This was done by investigating and comparing the behaviour of virgin and mated females. Virgin females can lay only male eggs while mated females can lay both male and female eggs. The role of plant odours and host odours in attracting C. atratus females to the host habitat and to their scale insect hosts was examined with the aid of an olfactometer. Field observations, to test the validity of results obtained in laboratory experiments, indicated that C. atratus females do not search initially for for their hosts' food plants, but search directly for hosts. Only when hosts were physically located did the behaviour of virgin and mated females differ. Recognition cues used by the females to distinguish between the two types of hosts were identified. Finally, the implications of results obtained were discussed in relation to ecological and evolutionary aspects of heteronomous parasitoid biology.
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The influence of ants on the insect fauna of broad-leaved, savanna trees
- Authors: Grant, Susan
- Date: 1985
- Subjects: Ants -- Behavior , Insects -- Host plants
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5811 , http://hdl.handle.net/10962/d1006971 , Ants -- Behavior , Insects -- Host plants
- Description: The influence of foraging ants on the insect fauna within the canopy of the tree species Terminalia sericea, Burkea africana and Ochna pulchra was studied in an area of typical South African savanna, over a two year period. The number of individual insects and their species composition was compared on unbanded, ant-infested plants and on banded plants where ants had been excluded. Differences in the level of herbivory recorded on banded and unbanded trees were related to the guild composition of insects within the canopy, and the results are discussed in terms of plant protection as a consequence of ant - insect interactions. Twenty-six ant species were recorded on the study trees at Nylsvley, with individuals belonging to the genus Crematogaster being numerically abundant and dominant within the canopy of each species of tree. These dominant ant species influenced the insect fauna by their strong dependence on honeydew, encouraging a build up in numbers of Homoptera on the branches and leaves of foraged trees, and supporting homopterous populations within the confines of their nest compartments . The exclusion of ants from trees led to fewer "mobile" homopterans (Aphididae, Membracidae, Psyllidae and Cicadellidae) and "sessile" homopterans (mainly Coccidae but also Pseudococcidae). Pyrethrum spraying showed that the guild composition of non-homopterous insects was similar on banded and unbanded trees. Differences in the level of herbivory on banded and unbanded trees suggested that, although slight, foraged trees were protected from some damage by the presence of ant species within the canopy. A trend did exist towards a greater number of insect individuals and species on unbanded trees, and it is postulated that during the period 1982 1984 when drought conditions prevailed over Nylsvley, ants do not reduce insect numbers through predation or disturbance but simply deter phyllophagous feeding. A separate experiment showed that Crematogaster constructor would feed on the eggs and early instar larvae of the saturnid moth, Cirina forda, but low numbers of lepidopterous larvae on the trees may have forced ants to seek honeydew. The negative impact of large homopterous populations on foraged trees was only seen in an isolated field observation where Polyrachis schistacea was found to associate with the lac insect Tachardina sp . . In conclusion it can be said that where homopterans are not the dominant phyllophages, plants do benefit from foraging populations of ants in that damage to the leaves is reduced.
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