Figure 3: Army Worm infestation in Paddy Nursery at Tendrelthang, Thimphu

Figure 3: Army Worm infestation in Paddy Nursery at Tendrelthang, Thimphu

This is to inform all concerned, especially farmers and extension officers, that armyworm outbreak has been reported from the following locations in the month of April and May. With weather conditions becoming more favourable for the pest outbreaks, all extension officials are instructed to carry out regular monitoring of the pests, and assist farmers to implement control measures in time.

District Gewogs Crop Date of observation
Punakha Barp, Chhubu, Dzomi, Goenshari, Guma, Kbaisa, Lingbukha, Shengana, Talo, Toepisa, Toedwang Paddy nursery, maize 5th April-6th May 2015
Wangdi Thetsho and Nahi
Thimphu (Tendrelthang paddy nursery) Paddy nursery 13th May 2015

Armyworm:

Army Worm

Figure 1: Army Worm in paddy Nursery at Tendrelthang, Thimphu

This pest is known as a migrant species, especially in the years of mass outbreaks; its larvae move in rows that are 15-20 m wide, searching food at night and in the afternoon; the pest in swarms is even able to cross small rivers. The adult moths can migrate over long distance upto 1500 Km; the direction of migration is determined by monsoon winds and air streams.

Army worm is a moisture loving and thermophilic species. Optimum temperature for growth and emergence is 23-30°C for larvae, 25°C for pupae, and 20-25°C for moths. Optimum relative air humidity is 80-95%.

Adult moth flight of the overwintered generation occurs from end of May until the third week of June; moth flight of 1st generation occurs in 1st-2nd weeks of August; moth flight of 2nd generation in September-October.

Army Worm Larva

Figure 2: Army Worm larva

Army worm species belonging to insect family, Noctuidae, are polyphages, preferring cereals; they populate plains (wheatgrass fallows, shorthear meadows) by rivers, lakes, or bogs, preferring false wheat, shorthear, chicken millet, and reed. Larvae of all instars feed mainly on leaves. The older larvae of 4-6 instars cause the main harm, roughly gnawing out and eating around leaf plate, and damaging inflorescences, ears, growth point, and grain in ears. Starting with weeds, the larvae then pass to cultural plants. Larvae of 1st generation cause the greatest harm.

Management:

  • Flooding seedbed is the best defence against armyworms when the population is in the larvae stage. Flooding drowns the swarming larvae.
  • Field Sanitation: Cut grass weeds from bordering fields (particularly gramineae) regularly to reduce breeding sites and shelter for armyworm.
  • To prevent the caterpillars from moving to another field, apply a 40 foot border spray around the non-infested field.
  • Plough a deep ditch and filled it with water. This method is helpful when caterpillars are found to be moving towards your field from the adjacent fields.
  • Army Worm

    Army Worm infestation in Paddy Nursery

    Another method is to dig a deep ditch with vertical sides to trap the caterpillars and prevent them from crawling out. Caterpillars are lured to congregate in the holes. Collect and properly dispose the trapped caterpillars.

  • If there is high infestation of army worm chemical spray is necessary. Spray cypermethrin@1 ml/1 L water. Since the armyworm usually feeds at night, the best time to spray is late in the day. 
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