The effects of weather and soil factors on the incidence of red mite in the Gomoa District of the Central Region of Ghana

No Abstract Available Ghana J. Sci, Vol.42 2002: 29-3

Influence of weather and soil factors on the incidence and severity of damage by cassava green mite and performance of a released phytoseiid in Gomoa district of the Central Region of Ghana

The main objective of the study was to investigate how weather and soil factors in Gomoa district of the Central Region of Ghana affected the incidence and severity of damage of the cassava green mite (CGM) and also the performance of a phytoseiid – a biocontrol agent against the green spider mite. The study was carried out on 10 cassava farms along a major from Accra to Cape Coast within the area where a phytoseiid, Typhlodromalus manihoti, had earlier been released. Data collected were on: incidence and severity of cassava green mite and phytoseiid; weather and soil physical and chemical properties. The paired comparison design was employed with the sprayed and unsprayed (presence or absence of phytoseiid) as treatments and sampling sites as replicates. Subplots measured 25 x 15 meters with a minimum of 250 cassava plants. The incidence, severity and population of the cassava green mite were observed to be highest in the dry season or short dry spell in the rainy (wet) season. Rainfall and severe drought acting indirectly on food availability to cassava plant were noted to be important mortality factors of the pest. Available phosphorus and organic matter were observed to hinder development and attack of the cassava green mite. However, soil nitrogen, available phosphorus and organic matter significantly promoted the development of phytoseiids. Temperature on the other hand had detrimental effect on the general development of phytoseiids. JOURNAL OF THE GHANA SCIENCE ASSOCIATION Volume 1 Number 3, July (1999) pp. 11-3

Scaling up tests on virulence of the cassava green mite fungal pathogen Neozygites tanajoae (Entomophthorales: Neozygitaceae) under controlled conditions: first observations at the population level

Virulence of entomopathogens is often measured at the individual level using a single host individual or a group of host individuals. To what extent these virulence assessments reflect the impact of an entomopathogen on their host in the field remains largely untested, however. A methodology was developed to induce epizootics of the cassava green mite fungal pathogen Neozygites tanajoae under controlled conditions to evaluate population-level virulence of two (one Beninese and one Brazilian) isolates of the entomopathogen-which had shown similar individual-level virulence but different field impacts. In unrepeated separate experiments we inoculated mite-infested potted cassava plants with either 50 or 25 live mites (high and low inoculum) previously exposed to spores of N. tanajoae and monitored the development of fungal infections for each isolate under the same conditions. Both isolates caused mite infections and an associated decline in host mite populations relative to the control (without fungus) in all experiments, but prevalence of the fungus varied with isolate and increased with inoculum density. Peak infection levels were 90% for the Beninese isolate and 36% for the Brazilian isolate at high inoculum density, and respectively 17% and 25% at low inoculum density. We also measured dispersal from inoculated plants and found that spore dispersal increased with host infection levels, independent of host densities, whereas mite dispersal varied between isolates. These results demonstrate that epizootiology of N. tanajoae can be studied under controlled conditions and suggest that virulence tests at the population level may help to better predict performance of fungal isolates than individual-level tests