The term "armyworm" can refer to several species, often describing the large-scale invasive behaviour of the species' larval stage. Fall Armyworm (FAW) is a very destructive and dreaded pest insect in many crops including maize. It has wide host range and attacks more than 80 different crops including weed species. It is not fastidious to many crops, rather the larva are polyphagy in nature. It may cause significant yield reduction on major cultivated cereals such as maize, rice, sorghum, and also to legumes, vegetable crops and cotton, if not managed properly. Its adult is a moth which is nocturnal in nature. The larvae are also known as worm, they swarm in mass scale and are the major culprit since they involve in major damage. They are highly voracious and becoming resistant to many chemical pesticides. Hence, they have become very difficult pests to manage below the economic injury level.
How they spread? The original habitat of the FAW was sub-tropical and tropical regions of America and it was detected for the first time in early 2016. It was then quickly spread across Sub-Saharan Africa and finally invaded India and Yemen in 2018. By December 2018, it had been reported in Bangladesh, Sri Lanka and Thailand. As at June 2019, it was reported in Myanmar, China, Indonesia, Laos, Malaysia and Vietnam, Egypt and the Republic of Korea, while Japan reported presence of FAW in July 2019. Because of trade and the moths' strong flying ability, it has the potential to spread in vast areas. Travelling all the way through America to Africa and Asia, it finally invaded Nepal. Its first occurrence was noted in Nepal (Gaidakot, Nawalparasi district) on 9th May 2019. Since then, it has been devastating crops in a number of districts across Nepal, however, east part of Nepal has been hit harder.
How can they be recognized? The adult is about 1.6 cm long and mixed brown ash coloured moth. The male moth can be recognized with its white spots on its forewings whereas the female species lacks such spots. Upon copulation the female lays eggs on the underside of the leaves in masses of about 100-200. They are very cleaver as they cover their eggs with the cottony substance like mass. One female may lay more than 1000 eggs in her lifetime. The eggs hatch into larvae within 2-5 days and move to the plants with the help of silken thread. In the beginning the larvae are light green in colour with black head, however the colour gradually changes into brown. The larva can be recognized easily with the Y shaped white marks on the head. There are three yellow lines on the dorsal side of the larvae. Cannibalisms occur in larvae and are active during night time and hibernate inside the soil in daytime. Fifth and sixth instars larvae are more voracious feeders hence it is very important to target these instars. The larval instars complete within six moultings and reach into the pupae state in two weeks' time. It is very interesting to note that pupae remain inside soil onto fasting during whole stage. Adult comes out from the cracks and crevices during agricultural operation within a week. The single life cycle completes within about a month. Understanding of pest dynamics is very important in planning the suitable management practice, and this is what exactly advocated in integrated pest management.
Management preparations The word "control" was dominant until 1960s, however it was soon replaced by "management". It was mainly due to the wider scale realization of pest resistance, resurgence, outbreak and flair-back. Hence, our options to regulate this notorious pest should be through management orientation. In this prevention endeavour, monitoring, survey-surveillance and then eco-friendly management through integrated approach are the obvious choices. Early sown maize following synchronized date of sowing in one community may help in escaping pest infestation compared to late sown staggering maize. Mixed cropping enhances the activity of natural enemies. Use of increased dose of nitrogenous fertilizer may increase the chance of attack by the pest. Hence, balanced use of fertilizer is always suggested. Strip plantation of Desmodium grass is very useful in repealing the insects by its volatiles. In contrast, Napier grass can be used as an attractant to the armyworm, which then can be blanketed with strong pesticides. These crops serve for lure and kill approach. Monitoring of armyworm in ten plants in "W" or "M" shaped pattern is suggested to determine the early occurrence. Larvae can be picked up by hand and dipped into hot water in a small scale. It is interesting to note that male members of this insect can be attracted into the suitable insect sex pheromone traps using armyworm lures. For this, ten armyworm lures per hectare of land helps knocking down large population of male insects. Besides these, 10,000 parasitized wasp eggs of Trichogramma chilonis or Telenomus sp, three times per week suppresses the larval emergence. The larvae can be managed using bio-pesticides such as insect pathogenic virus (NPV), fungi (Metarhizium anisopliae and Beauveria bassiana) and bacteria (Bacillus thuringiensis). After 10 days of emergence of the seedlings neem-based Azadiractine is advised to spray mixed with water. If non-chemical measures are ineffective or un-successful then safer chemicals such as Spinetoram or Emamectine benzoate or Spinosad or Chlorantraniliprol or Immdiachloprid can be used as per the recommended dose and time either in the morning or evening. While applying chemicals use of protective clothing and safety measures are always suggested. Ministry of Agriculture and Livestock Development has formed national coordination and working group teams to fight against this pest including the concerned representatives from research, extension, states and non-governmental organizations. Standard working protocols are prepared and management plans are under way. Coordinated efforts are necessary nationally, regionally and internationally against this pest, since it has spread internationally.
The author is Secretary at the Ministry of Agriculture and Livestock Development