Cassava Green Mite
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
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.
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
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.
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.
