J. S. Yaninek

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 dens...

Survival of Neozygites cf. floridana (Zygomycetes: Entomophthorales) in mummified cassava green mites and the viability of its primary conidia

The survival of Neozygites cf. floridana (Weiser and Muma) as dry hyphal bodies in mummified cassava green mites, Mononychellus tanajoa (Bondar), at 5.0% RH in the dark was affected by storage temperature. Survival of the fungus in mummies kept at 24±1.0°C could be demonstrated for 6–7 months. When stored at 4°C, the fungus sporulated from 90% of the mummies liberating an average of 186.9 primary conidia per mummy even after a storage period of 16 months, when the experiment was terminated. The temperature, humidity and light condition significantly affected the viability of primary conidia. The percent viability across all factors dropped from 98.4% after 0 h (beginning of the experiment) to 23.4% after a 1 h exposure to the conditions tested. Lower temperatures maintained higher viabilities with 86.3% of the conidia surviving after 18 h at 18°C, whereas almost all conidia died after 12 h at 33°C. Conidia survived less than 1 h when exposed to SDs (saturation deficit) of 2.0 mm Hg or higher at any tested temperature.

Continent-wide releases of neotropical phytoseiids against the exotic cassava green mite in Africa

Since the initiation of the classical biological control campaign by the International Institute of Tropical Agriculture to control the exotic mite, Mononychellus tanajoa, 11 species (18 distinct populations) of neotropical phytoseiids have been selected and shipped to Africa for experimental releases. Initially, natural enemies were selected because of their abundance and frequency on cassava. Between 1984 and 1988, more than 5.2 million phytoseiids belonging to 7 species of Colombian origin were imported to Africa and released in 348 sites in 10 countries. None of these species and populations ever became established in the wide range of agronomic and ecological conditions tested, apparently because of inadequate alternative food sources when M. tanajoa densities were low and extended periods of low relative humidity. Foreign exploration was adjusted in 1988 to focus on Neotropical regions that were agrometeorologically homologous to areas in Africa where the potential for severe M. tanajoa damage exists. Natural enemies associated temporally and spatially with M. tanajoa and capable of surviving periods of low M. tanajoa densities on alternative food sources in the new exploration sites were given selection priority. Several natural enemy candidates were immediately identified in Northeast Brazil and shipped to Africa. Approximately 1.9 million phytoseiids of the species Neoseiulus idaeus and Typhlodromalus limonicus of Brazilian origin were released in 133 sites in 6 countries in 1989 and 1990. N. idaeus has since become established in Benin, while T. limonicus may be established in Benin, Burundi, and Ghana.

Alternative plant habitats for common phytoseiid predators of the cassava green mite (Acari: Phytoseiidae, Tetranychidae) in northeast Brazil

A survey was conducted to identify possible alternative plant habitats of the most common phytoseiid predators associated with the cassava green mite, Mononychellus tanajoa (Bondar), in their native environment in northeast Brazil. Thirty-two phytoseiid species were collected, including Amblyseius aripo (DeLeon), Amblyseius idaeus (Denmark and Muma) and Amblyseius limonicus Garman and McGregor s.l., the three predators previously shown as the most common on cassava. In increasing order, A. idaeus, Phytoseius guianensis DeLeon and A. aripo were the most common phytoseiids collected on the plant habitats examined. A. limonicus s.l. was one of the least common phytoseiids on plants other than cassava. Altermative plant habitats seem important in harboring A. aripo and A. idaeus, but not A. limonicus s.l..

Effect of temperature, humidity and photoperiod on mortality of Mononychellus tanajoa (Acari: Tetranychidae) infected by Neozygites cf. floridana (Zygomycetes: Entomophthorales

The effect of temperature, humidity and photoperiod on the development of Neozygites cf. floridana (Weiser and Muma) in the cassava green mite, Mononychellus tanajoa (Bondar) was studied in the laboratory. Dead infected mites began to appear 2.5 days after inoculation. At 33 and 28°C peak mortalities were higher and occurred earlier (after 2.5 days), than at 23 and 18°C. Mean LT50 (time for half the infected mites to die) decreased with increasing temperature as follows: 3.9, 3.0, 2.9 and 2.5 days at 18, 23, 28 and 33°C, respectively. When placed under conditions of high relative humidity for a period of 24 h, the percentage of dead infected mites from which the fungus sporulated was highest at 28°C (51.4%) and lowest at 33°C (6.5%). The development of the fungus inside the mite was not significantly affected by ambient humidity or photoperiod. No significant interactions between tested factors were found.

Continental dispersal of the cassava green mite, an exotic pest in Africa, and implications for biological control.

The continental dispersal of an exotic spider mite species is described for the first time. The cassava green mite,Mononychellus tanajoa (Bondar) (Acari: Tetranychidae), has been found to be dispersed across the cassava belt of Africa in less than 10 years after first being discovered in 1971. This mite disperses within plants by walking, and within and between fields by drifting aerially. Widespread transportation of mite-infested plant material, however, is proposed to explain the rapid spread ofM. tanajoa in Africa. Observations of mite-infested plant material being transported in the field, and laboratory evidence of mite populations surviving up to 60 days on cassava stems removed from the field and isolated from external contaminates, support this hypothesis. The spread ofM. tanajoa in Africa as a model for future introductions on cassava suggests a pattern of movement at species-specific rates. Exotic natural enemies ofM. tanajoa, especially phytoseiid predators, are expected to spread at a rate slower than their host; consequently, large-scale and long-range releases will be needed to accelerate their spread.

Host specificity of the cassava green mite pathogen Neozygites floridana

Tests were conducted on the hostspecificity of a Brazilian isolate of thefungus Neozygites floridana, a potentialbiological control agent for the cassava greenmite, Mononychellus tanajoa, in Africa.Five insect and two mite species, mostly fromthe cassava agroecosystem, were evaluated forsusceptibility to N. floridana, namelyEuseius concordis, E. citrifolius, Phenacoccus herreni, Stethorus sp., Aleurothrixus aepim, Apoanagyrusdiversicornis, and Bombyx mori.Individuals of each species were exposed tocapilliconidia (the infective stage of thefungus). None of the tested individuals wasfound with hyphal bodies (the vegetative stageof the fungus), whereas 73 to 94% of thecassava green mites in the controls becameinfected. Non-germinated capilliconidia were,however, found attached to several individualsin most species. N. floridana appears tobe safe for exportation. Further evaluation ofits performance against M. tanajoa inAfrica is therefore desirable.

Effect of some food sources associated with cassava in Africa on the development, fecundity and longevity of Euseius fustis (Pritchard and Baker) (Acari: Phytoseiidae)

Various foods associated with cassava were tested for their effect on the development, fecundity and longevity of Euseius fustis, the most common phytoseiid species found on cassava in Africa. Euseius fustis developed successfully to adulthood on the spider mite prey species Mononychellus tanajoa (Bondar) and Oligonychus gossypii (Zacher) and on pollen from maize, castor bean and cassava. Euseius fustis also completed development on water-diluted phloem exudate from cassava, diluted honeydew from the cassava mealybug and on various pollen and prey combinations. When reared on Tetranychus urticae Koch prey or free water only, E. fustis did not develop past the deutonymphal stage. All larvae held on clean leaf discs on water-soaked cotton died without moulting, suggesting that E. fustis must feed in order to moult to the nymphal stages. Diets of maize plus castor bean pollen and maize pollen plus M. tanajoa resulted in the highest rate of development, the highest fecundity and the greatest longevity. Castor bean pollen alone and maize pollen alone produced a higher fecundity and greater longevity than M. tanajoa tested alone. A colony of E. fustis reared continuously for seven generations on castor bean pollen produced nine times more adult females than a colony of E. fustis reared continuously on M. tanajoa. No negative effects on the development and fecundity of E. fustis were observed after seven generations were reared on pollen.

Non-prey food for subsistence of Amblyseius idaeus (Acari: Phytoseiidae) on cassava in Africa

The nutritional effects of pollen from cassava, oil palm, castor bean, Leucaena leucocephala and a phloem exudate from cassava were tested in the laboratory for their effect on survivorship and oviposition of Amblyseius idaeus (Denmark & Muma). Survival and longevity of even-aged females were enhanced on the aforementioned non-prey diets when compared with the control of no food. Castor bean pollen provided sufficient nutrition to induce an irregular and low ovipositional rate of A. idaeus over a five week interval. Oviposition ceased after 48 h in the presence of the other non-prey foods or free-water only. These studies suggest that A. idaeus is capable of switching or supplementing its diet with non-prey foods during periods when spider mite populations are low.

Establishment of the neotropical predator Amblyseius idaeus (Acari: Phytoseiidae) in Benin, West Africa

Populations of the phytoseiid predator Amblyseius(=Neoseiulus) idaeus (Denmark & Muma) from northeastern Brazil, have been successfully introduced into Benin, West Africa, as part of a classical biological control campaign to control the exotic cassava green mite Mononychellus tanajoa (Bondar). Monthly follow‐up surveys revealed the presence of A. idaeus in most release sites. Some populations have persisted for at least 18 months, including two cycles of potentially limiting wet and dry season conditions. In some sites A. idaeus has been the numerically dominant phytoseiid predator on cassava Manihot esculenta, where it is associated with the tetranychids M. tanajoa and Oligonychus gossypii Zacher. During periods of low M. tanajoa densities A. idaeus disappeared from cassava, but were found on weeds with O. gossypii until prey densities on cassava increased.