Columnists Features

How to fight armyworms

DONALD ZULU, KEN WILSON
MAJOR outbreaks of African armyworms are currently threatening food security in some districts of Copperbelt, Luapula and Central provinces, just as they did four years ago.
The armyworm is a serious migratory crop pest that feeds on young maize plant. It also attacks other cereal crops such as wheat, rice, sorghum, millet and most grass pastures, so affecting both crop and livestock production. But what are armyworms, where did they come from, how can we control them and what are the prospects for further outbreaks of this devastating pest?
What are armyworms?
Armyworms are the destructive caterpillar or larval stage of the moth Spodoptera exempta. They can grow to be up to three centimetres or one-and-a-quarter inches long over a period of 2-3 weeks. The young caterpillars are usually small and green and difficult to see on their green host plants, but after about 10 days growth, they switch into their familiar black coloration, and this is when most farmers will first notice them.
Unfortunately at this point, the caterpillars are growing fast and eating at an alarming rate and so farmers must act quickly if they are to protect their crops.
If left uncontrolled, the caterpillars will ravage the crop until they eventually stop feeding and drop to the ground to bury themselves in the soil. Here, they will develop into the pupal stage and remain in the soil to a depth of about five centimetres for a further 10 days.
The moth will later emerge from the ground and fly elsewhere with the prevailing winds to cause new outbreaks. The cycle could continue throughout the rainy season.
Where did the armyworms come from?
Most likely, the moths that caused these latest outbreaks originated from primary outbreak areas in southern Tanzania. Normally, when weather conditions are conducive, the adult moths from these areas will migrate hundreds of kilometres with the help of prevailing winds to other regions in southern Africa.
These flying moths are forced to land when they encounter the convective winds and storms associated with the rainy season.
The female moths then lay up to 1,000 eggs each, which will develop into caterpillars, causing new outbreaks. This process may continue throughout the rainy season, with fresh outbreaks occurring at roughly monthly intervals.
Armyworms can destroy an entire crop stand in very few days or at worst in a day. When the food supply is finished they move on to new areas and this is where the armyworm gets its name, as they march like a vast army of worms in search of food.
How can armyworms be controlled?
The conventional method of controlling armyworms is through use of recommended synthetic chemical pesticides such as cypermethrin. If used correctly, appropriate chemical pesticides are highly effective at controlling armyworms.
However, just like all chemical pesticides their use comes at a cost which could be economical, health or environmental.
Synthetic chemical pesticides are often imported products that are expensive and beyond the reach of most smallholder farmers that contribute the largest share of the “bumper harvest” at country level.
Besides the cost implication of pesticides, armyworm outbreaks often come unexpectedly without any warning, when farmers have spent most if not all their money on seed, fertiliser and labour to establish various crops at the onset of rains. Hence replanting becomes a daunting task to an average resource-constrained smallholder farmer.
If the pesticides are available, most farmers lack the necessary protective clothing and equipment, hence they will apply pesticides ill-prepared at a personal health cost.
Synthetic chemical pesticides when applied on extensive fields can become undesirable as they negatively impact on non-target organisms such as insects (including beneficial pollinators), birds, livestock and other wildlife.
Some of these issues can be addressed by taking an integrated approach to pest management involving surveillance and early warning systems together with either recommended synthetic chemical pesticides or effective non-chemical alternatives. In some African countries, this early warning system uses an extensive network of pheromone traps that attract male armyworm moths using the artificial scent of mating female armyworms.
The catches of armyworm in these traps are used in combination with local weather reports to forecast armyworm outbreaks at a local level – so called ‘community based armyworm forecasting’.
These forecasts help to alert farmers much faster to the need for control. Investment in an extensive network of pheromone traps in Zambia could pay dividends and be cost-effective.
Are there alternatives control methods?
The envi ronment a l and cost limitations of synthetic chemical pesticides call for more environmentally friendly and cheaper options for controlling these pests. There are a number of options that could be explored and these include use of naturally occurring disease of armyworm called SpexNPV and a mechanical method of control.
SpexNPV is a virus that affects armyworms in nature. In the wild, SpexNPV normally builds up late in the rainy season when the damage to crops and pasture has already been done. But by mass-producing SpexNPV, it is possible to spray infested crops early in the growing season for cheap and effective control of armyworms.
Is SpexNPV safe? Yes, because it is very specific to armyworms and it has no effect on the health of humans, livestock, wildlife, pollinators or the wider environment, as verified by the OECD and other regulatory bodies. Unfortunately, SpexNPV is not yet registered for use in Zambia or elsewhere. This is because registering new pesticides (even biological pesticides such as SpexNPV) is a very expensive process.
Therefore, until a rich donor provides the funds required for registration, SpexNPV will not be available to smallholder farmers or the Zambian government.
An alternative that is available to farmers right now is a mechanical approach. If armyworms have not yet infested your crop field but have been located nearby, the best option would be to dig a narrow ditch around the field so that incoming armyworms will fall into it.
It must have steep slope sides to be effective so that the armyworms cannot climb out of. Destroy the armyworms that accumulate in the ditch by crushing them or covering them with earth.
What are the prospects of further armyworm outbreaks?
Because armyworm moths are highly migratory, their instinct is to fly away from the outbreak sites that they were born into.
This means that it is unlikely that farmers in Copperbelt Province and elsewhere that have been affected by these latest outbreaks will be affected again this season (unless more moths migrate in from neighbouring countries).
The dominant winds at this time of year will tend to push the migrating moths eastwards and southwards.
Depending on the rainfall patterns over the coming weeks, it is most likely that any further outbreaks in Zambia will therefore tend to be in the most southern and easterly provinces, and historically these outbreaks are most likely in January, with a possibility of further outbreaks again around March.
If we are lucky, the moths emerging from the current outbreaks will migrate out of the country, but neighbouring countries such as Malawi, Mozambique and Zimbabwe will be hit next, as happened four years ago.
The future?
Outbreaks of African armyworm are highly dependent on the seasonal patterns of wind and rainfall. With global warming, the weather patterns in Africa will continue to change, which could mean more or fewer armyworm outbreaks.
Regardless of this, to effectively and sustainably assure food security in the midst of armyworm migrations, a robust, country-wide surveillance and early warning system is needed so that farmers are alerted in good time about impending outbreaks and the practical management options available, such as mechanical control.
The Zambian government has acted swiftly to ensure that chemical pesticides are made available for armyworm control but, in the longer term, more sustainable and local solutions, such as SpexNPV are required.
Donald Zulu is a researcher/ lecturer at the Copperbelt University (currently pursuing his PhD at the University of Reading, UK) and,
Ken Wilson is a professor at Lancaster University and has been working on armyworms for the past 25 years.

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