Seasonal factors cause outbreak of armyworms

According to the National Plant Protection Center (NPPC), Ministry of Agriculture and Forests, armyworm outbreaks in Bhutan occur sporadically, especially in crops such as paddy and maize. The seasonal factors also play a role in their outbreak.

Deputy Chief Plant Protection Officer, Kiran Mahat (PhD), said that with the onset of spring, rains followed by warm and dry weather conditions, can greatly favor armyworm development and outbreak.

“The wet conditions promote rapid growth of their food (primarily plants in the grass family-Gramineae). Due to the abundance of food supply, armyworms can multiply rapidly. On the other hand, the prolonged dry weather conditions can adversely affect the survival of their natural control agents, such as insect predators and/or natural disease causing agents. When these natural control agents cannot survive and flourish, the armyworm population cannot be kept at check, and hence outbreaks can happen,” he said.

There are many species of armyworms. However, the endemic species Mythimna separata is present in almost all the dzongkhags in the country, and with favorable conditions sporadic outbreaks are commonly observed.

He said that armyworms have a wide host range, in which they feed on different species of plants and crops. However, most armyworm species prefer to feed primarily on grass family- Gramineae.

“In Bhutan, armyworms commonly feed on rice and maize crops. However, people can easily confuse any lepidopteran larva (caterpillars) for armyworm. For instance, the outbreaks reported in potatoes recently are not caused by armyworm, but is due to another species of insect called the semi-loppers,” Kiran Mahat (PhD) said.

Armyworms can be managed easily with regular monitoring of crop health and early detection of insect activity. The pests, when detected in the early stages of their development, just hatched from the moth’s eggs, are relatively easy to control than those that have developed into bigger caterpillars.

Therefore, any control intervention initiated when the caterpillars have grown into large caterpillars, will remain futile. Hence, early detection is very important. However, as most farmers fail to regularly monitor their field, and they observe the pest at a stage when the majority of the crop have been damaged, and when it is too late to undertake any control measures.  

He said that due to uncertain weather patterns, armyworm outbreaks will remain a common issue, hence farmers will need to be prepared for outbreaks and be familiar with the insect pests and management options.

“If unsure, they will need to actively seek expert advice from their respective agriculture extension officials. More awareness and training programs on armyworm management is required,” he said.

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