Seasonal and Habitat Variability in the Fungal Pathogens, Neozygites cf. floridana and Hirsutella thompsonii, Associated with Cassava Mites in Benin, West Africa

A survey of the pathogenic fungi associated with mites on cassava in Benin, West Africa, revealed both geographical and seasonal variation in the presence of Neozygites cf. floridana (Weiser and Muma) and Hirsutella thompsonii Fisher on Mononychellus tanajoa (Bondar) and Oligonychus gossypii (Zacher). Few dead and infected mites were found during the dry season, regardless of vegetation zone. In three of 30 surveyed sites, N. floridana was found infecting 1% of the dead M. tanajoa and 2% of the dead O. gossypii, while H. thompsonii was observed infecting 20% of the dead M. tanajoa in a single site. The frequency of sites having infected mites during the wet season was 3.5 times greater than that seen during the dry season. N. floridana infected 10% of the dead M. tanajoa and 19% of the dead O. gossypii on young leaves. Mites infected with N. floridana were found either in the coastal Southern Forest Mosaic (SFM) or in the Northern Guinea Savanna vegetation zones. N. floridana was rare in the low mite densities associated with mature leaves. H. thompsonii was found on 19% and 29% of the dead M. tanajoa on young and mature leaves respectively. All M. tanajoa infected with H. thompsonii on young leaves and mature leaves (75%) were found in the SFM. A single M. tanajoa was the only infected mite found in the Southern Guinea Savanna. Relatively few O. gossypii were infected with H. thompsonii. N. floridana and H. thompsonii were found together in three sites, but never on the same host. Phytoseiids were never found infected with either pathogen. In a regression analysis, the number of dead mites was significantly estimated from the total number of mites for both species, regardless of leaf age. The numbers of dead M. tanajoa on mature leaves were also estimated from the proportion infected with H. thompsonii. The numbers of infected mites on young leaves were estimated from their association with the SFM for M. tanajoa infected with H. thompsonii, and from total mites for O. gossypii infected with N. floridana. On mature leaves, infected mite numbers were estimated from the numbers of dead M. tanajoa infected with H. thompsonii. The merit of introducing more virulent or better adapted isolates of N. floridana to control M. tanajoa in Africa is discussed.     

Behaviour of Aphidius colemani Searching for Aphis gossypii: Functional Response and Reaction to Previously Searched Aphid Colonies

Understanding the searching strategy of Aphidius colemani Viereck (Hymenoptera: Braconidae), might gain insight into the mechanisms by which control of Aphis gossypii Glover (Homoptera: Aphididae) is obtained in glasshouses, and provide information on how to improve the biological control of aphids. The functional response of A. colemani in Petri dishes is described, as well as its behaviour on returning to a previously visited leaf disk with aphids. The number of aphids attacked during the first visit followed a sigmoid functional response. The increase of the parasitization rates was clearest at densities of up to 10 aphids/leaf disk. The duration of the first visit to a leaf disk increased with increasing aphid density. After re-entering a recently visited leaf disk, the parasitoid stayed for a much shorter time than on the first visit, and no effect of host density on the duration of the visit was observed. No evidence of patch marking was found, so the reduction of visiting times was probably caused by encounters with parasitized hosts.     

Strategies for the Control of Aphis gossypii Glover (Hom.: Aphididae) with Aphidius colemani Viereck (Hym.: Braconidae) in Protected Cucumbers

Aphis gossypii Glover (Hom.: Aphididae) is a damaging pest of protected cucumbers in the UK, and control measures are required which are compatible with other components in the overall cucumber integrated pest management programme. Two methods of establishing the parasitoids, Aphidius colemani Viereck (Hym.: Braconidae), in cucumber crops prior to invasion of the aphids were developed. The first involved weekly releases of small numbers of parasitoids beginning before A. gossypii became established on the plants. The second method used open-rearing units based on maize, wheat and rygrass plants infested with Rhopalosiphum padi L (Hom.: Aphididae), which is a common host to many natural enemies of A. gossypii but not a threat to the cucumber crop. Both methods were found to be more efficient in summer than in late spring. Parasitoid release rates were established for the two control methods at both of these times of the year. Like all biological control measures, these methods will require careful management in practice and some fine-tuning to suit individual crop production systems.