Summer is boom time for army worms, considered one of the major international agricultural pests on crops and pastures. They’re not really worms but are the caterpillars of the moth Spodoptera Mauritia, a prolific breeder than can produce 200 to 300 eggs in a single laying, then do the same thing the following night, and again the night after that. That’s about 1000 eggs, from just one moth. And there’ll be more than just one moth building a nursery in the soil of your lawn.
After about a week, the larvae (caterpillars) emerge with the sole objective being to eat as much green grass as is possible within the next 20 to 35 days before they move onto their pupae stage, then emerge as moths about two weeks later for the next life cycle to begin. This means there can up to three generations of army worm infestations during summer and early autumn.
Be prepared for the invasion
Because they only work at night and bury themselves under the grass during the day, most often the damage is done before you realise they’re there. One way to be prepared is to leave an outside light on during the warm nights and check every couple of hours to see how many moths are fluttering around it. If there’s a multitude of them, it’s time to develop a defense strategy because the eggs become caterpillars in about seven days.
You can implement your strategy at night or during the daytime. To catch them in the act, go out with a torch and check if there are any caterpillars eating your grass or crawling over then. Or during the day mix water with some dishwashing detergent in a bucket and pour over a small patch of grass. This will cause sleeping caterpillars to rise from their slumber and come to the surface within about 10 minutes.
The very young caterpillars are only a couple of centimetres long and are light green with whitish longitudinal strips. As they mature they grow to about 4.5cm and become dark green, or brown, or black, with two stripes down their back.
How you can stop them
Army worms don’t generally kill a lawn, but their voracious feeding will badly damage it and, if untreated, patches of your lawn may die, so it’s important to restore growth as soon as possible.
You can destroy the grubs with insecticide, then add nutrients (make sure it’s high in nitrogen, which encourages green growth) and several deep waterings a week, rather than more frequent light waterings.