This background to Fall Armyworm management is from a consultant managing outbreaks of Fall Armyworm in Uganda who provided us with a valuable insight into managing this very damaging insect as it moves across the country.
Fall armyworm (Spodoptera frugiperda) is a new pest threatening maize crops in Uganda and southern and central African regions. It is the first complete season the pest has been found in Uganda following suspected introduction either through imported goods or via the strong Atlantic wind currents. In fact, the Fall Armyworm in its moth stage is a very adept flier capable of covering many thousands of kilometres in a lifetime. It was first sighted in Uganda in mid-2016. Uncontrolled, this insect will potentially force many farmers and families into poverty and severe food insecurity.
In Rwanda, the army is out in force spraying maize crops to protect farmers from the insect. Uganda has set aside thousands of litres of insecticide and funds for accompanying educational programs to encourage farmers to protect their crops and food. On a continental scale, the United Nations Food and Agriculture Organisation is organising a similar program of insecticide and education for farmers in the regions under attack.
The fall armyworm is native to southern US, and central and south Americas causing millions of dollars in crop damage every year. The name Fall Armyworm comes from the way the caterpillars ‘march’ into crops, leaving a trail of destruction as they pass through and because in the US the caterpillar is most active in Fall.
The Fall Armyworm differs to the African Armyworm (Spodoptera exempta) in host plant range and timing of attack. As the name suggests the fall armyworm is present during the autumn months in the US. The adult female Fall Armyworm is a strong flier and the moths move in large swarms at night. A mature female Armyworm can lay in excess of 1000 eggs during its lifetime. The host range of plants spans nearly 100 species in 27 different families with preferred hosts of grass based plants such as maize, sorghum, millet, rice and sugar cane.
Damage from Armyworm can appear to come in waves often a month apart as the populations rapidly increase and decrease in density. Armyworms will eat the tender portions of the leaf first – so farmers will check for the ragged edges of the leaves. However, damage may be present on all parts of the plant. For example, larger fall armyworms will act as cutworms (very damaging caterpillars in Uganda) and completely cut off the young maize plant at the base. Other damage will appear as clear leaf windows (picture below).
The Armyworm has a tendency to build up resistance to insecticides given the high egg numbers and rapid breeding rates.
There are several factors which make Fall Armyworm more damaging to maize than other Spodoptera pests:
- Adult females place eggs directly on maize compared to other species that firstly oviposit on wild grasses before older larvae move onto crops.
- The mandibles of caterpillars have comparatively stronger, serrated cutting edges which increase the ability to chew on higher silica content leaves.
- Older larvae are cannibalistic and can dominate both inter and intra specific competition.
- The damage seems appears almost overnight and without warning as the final growth stage caterpillars eat more than all the other stages combined (Figure 2)
- This reduces the ability to detect and enact control techniques before widespread damage.
Pest Management Strategies
Control options for the fall armyworm are limited although as with all pest management strategies an integrated approach is generally most effective. Combining host plant resistance (CIMMYT currently breeding resistant maize), chemical control, pheromone traps, biological control, intercropping with legumes and habitat management has been able to reduce infestation levels in CIMMYT trials. In 2016 the Ugandan Government reported infestations from May through to July. Farmers should begin monitoring crops immediately for infestation.
Insecticides and GM (Bt toxin producing) plants are the most effective methods although Fall Armyworm is very adept at becoming resistant to the Cry1F toxin produced in Bt plants. The MAAIF is recommending that farmers use a combination of Lambda-cyhalothrin (106g/l) and Thiamethoxam (141g/l) and Rocket at a rate of 20-30mls in 15 – 20 litres. Care must be taken to prevent the development of resistance.
There are a number of natural predators and diseases that kill Fall Armyworm. Certain practices such as clearing a buffer zone around the crop in combination with weed management may slow the march of Armyworm as it approaches. However, the moths are able to fly long distances at night and on storms. Recent research has shown that infestations have been reduced by 20-30% by intercropping maize with beans. Further, the range of crops will spread the risk of food shortage following infestation damage.
While some research suggests that reducing the number of attractive weeds within the crop will reduce armyworm numbers by reducing breeding and feeding areas, others suggest that having other weeds in the crop may spread the damage. The latter (leaving weeds in crop) may be effective in developed systems whereby larval control can be rapid and potentially effective. However, increasing the area of breeding and feeding for the caterpillar with the possibility of no action or a lack of effective action may ultimately make the situation worse for the maize itself or neighbouring crops and other farmers. Thus a clean field and clean borders are crucial in controlling the pest. Clean interrows will enable the farmers to get a better look at the pest on the plants and narrow down exactly where to apply chemical. If chemical is limited and expensive, the fewer plants to spray the better. Timing of the spray is also very important. In the US the recommendation is to apply insecticide early or late in the day when the armyworms are most active. In African Armyworm management, the destruction of their native habitat on native grasses will lead to an influx of armyworms in the crop. However, as maize is a preferred plant for the fall armyworm this technique may be of limited use. Even still, the impact of control techniques on the prevalence of African armyworm and other insects must be considered carefully.
The most important factor behind a management strategy is monitoring and area specific control. Pest identification sheets or sticky sheets allow the establishment of a ‘treatment threshold’ whereby once a certain number of caterpillars are found per square metre (in the US 3 per sq ft is a common value or an equivalent 32.28 per sqm) insecticide should be applied. Too frequent and ineffective applications of insecticide will ultimately lead to the development of resistance. For example, certain insecticides will not effectively manage large larvae and may result in the development of resistant populations able to metabolise the chemical. As with many pests, a smaller size and prevalence of the pest is much easier to manage effectively. Further, these sheets are relatively inexpensive and easy to use. Light traps provide another alternative to sticky sheets to assess a potential outbreak by monitoring the moth stage. They are constructed by using a tripod of sticks to hold up a lantern of kerosene hanging over water. The moths are attracted to the light, die from heat exposure and fall into the water.
Another method of control for the marching front of armyworms can be a 45cm deep by 60cm wide ditch. The caterpillars crawl and fall into the ditch and then are unable to crawl out. They can then be killed with logs, fire or any other suitable technique. This is used commonly in African armyworm management.
All in all, effective control requires the combination of all avenues of control in an integrated pest management approach. Further, healthy plants free from disease and weed competition will recover more quickly and will be less likely to die from a minor attack.