Cassava Green Mite
Taxonomy and distribution
The cassava green mite (CGM), Mononychellus tanajoa (Bondar) (Figure 1) is an exotic pest introduced from the South America into Africa in the early 1970s. This tiny mite belongs order Acari and the family Tetranychidae.

 


Figure 1. Cassava green mite eggs and active stages

From Uganda where CGM was first introduced, it spread quickly, carried away by wind and movement of infested planting materials, to all countries of the cassava belt. CGM attacks cassava only. The red mite, Olygonychus gossypii (Zacker) is widely distributed mite on cassava in Africa, but much less economically important than CGM.
 
Biology, damage, ecology, and phenology.
Apart from the egg which is inactive, there are 4 active stages including larva, protonymph, deutonymph and adult male and female (Figure 3). CGM development occurs in about 10 days and a females can live up to 30 days and lay more than 60 eggs during her life span. Peak CGM densities occur during the first half of the dry season, with a smaller peak occurring within about a month of the start of the long rainy season.

Figure 2. Yellow spotting caused by cassava green
mite feeding.
The active stages feed on the bottom surface of leaves by sucking fluids from cells. This causes yellow spotting of leaves (chlorosis) (Figure 2), which can increase from a few spots to complete loss of chlorophyll (green pigments in leaves). Most CGM individuals are generally found on the upper third or the cassava plant. Leaves damaged by CGM may also show mottled symptoms which can confuse it with symptoms of cassava mosaic virus disease (CMD).

Severely damaged leaves dry out and fall off, which can cause a characteristic candle stick appearance (Figure 3). Because of reduced plant growth, accumulation of starch in the storage roots is slowed, sometimes even reversed, and root yield losses in the absence of any control measures can reach 50%. Where leaves are eaten as vegetables by farmers, a corresponding loss ensues. Reduced growth and stunting of the tips is also responsible for contorted and thin stems, thus affecting the planting material to be used for the next season.

   
Figure 3. Candlestick appearance
The size of CGM populations, and hence yield losses, is generally influenced by several factors including: (1) age of the host plant – young plants are more exposed and susceptible to CGM attacks than older plants; (2) season - damage severity is greater during dry than wet season, and heavy rainfall can reduce CGM populations, (3) temperature– populations increase with increasing temperature leading at times to very rapid increase in populations and damage, and (4) poor agronomic practices - plants grown in poor soils are more susceptible to mite attacks.

 
Control
Cassava varieties respond differently to CGM attacks. Some are more susceptible/tolerant to CGM attacks than others. Scientists can screen local and/or introduced germplasms to identify tolerant/resistant varieties to CGM, determine their source of resistance and incorporate that into productive and acceptable varieties. The selected varieties can then be released through national extension system. However, this method can take long time before such varieties can be developed.

Although several insecticides and miticides (such as Dimethoate and Dicofol) can control CGM, they are often too expensive for farmers and may be dangerous if not well applied and in countries where leaves are consumed as vegetables. Several cultural methods, such as adjusting planting time for the crop to escape severe damage at young age, mixing varieties to avoid genetic uniformity, and removing infested tips have been tried but without much success, primarily because these practices were not well suited for traditional farming systems.

Because CGM was introduced into Africa from South America, where it is widespread and generally not a serious cassava pest, biological control was chosen as the most appropriate method of control. Several predatory mites of the family Phytoseiidae that generally keep the mites under control in the South America, were introduced into Africa from Colombia and Brazil. All Colombian species failed to establish. However, three predators from Brazil, Neoseiulus idaeus, Typhlodromalus manihoti and Typhlodromalus aripo (Figure 4) became established in several countries. Of the three predators, T. aripo is the most widely distributed.

Figure 4. Introduced phytoseiid predators of cassava green.


In the year 2000, T. aripo establishment has been confirmed in 19 countries including Mozambique where it is established in Nampula, Zambezia and Inhambane provinces. T. manihoti is presently established in limited distribution in Ghana, Benin and Nigeria. It is not known if N. idaeus has persisted in the countries where it was first established (Kenya and Benin). Where T. aripo has been present for three or more years it can reduce CGM by an average of 50% and increase root yield by an average of 30%. T. manihoti can reduce CGM populations by half, but effects on cassava yield have not been rigorously determined. Evidence also suggests that CGM control is greater where both T. aripo and T. manihoti are present together.

In addition to T. aripo and T. manihoti several species of indigenous predatory mites such as Euseius fustus, Typhlodromalus saltus and Ipheseius degnerans are found on cassava and can feed on CGM and red mite. While these predators may be capable of reducing peak infestations of CGM they cannot keep it under control. Pathogenic fungi have also been reported infecting CGM and red mite, but these pathogens are unpredictable and generally cause very low levels of CGM infections in Africa. Recently, however, IITA has introduced isolates of Neozygites floridana (Figure 5) from Brazil, where this fungus causes heavy but irregular epizootics of CGM and can contribute to rapid crashes in CGM populations. Presently, where the fungus has been released in southern Benin, infection levels were increased 10 folds but remain too low (about 12%) to cause substantial reductions in CGM populations.

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