Gardening – Armyworm arrival

We’ve just had our first ever infestation of fall armyworm, a new pest blown over on wind from Australia, and first detected in New Zealand in early 2022. Adult moths can travel hundreds of kilometres on wind, so it was inevitable that it would appear sooner rather than later in our little village.

This devastating pest can feed on over 350 plant species (including some natives) but is usually found first on brassicas and sweetcorn. In our case, Angela noticed what appeared to be thin little windows in the leaves, about a centimetre long, which are caused by the first larval stage chewing out the underside of the leaves.

It’s very important to try to control this pest in the very early stages of the infestation. I was aghast at the sheer number of caterpillars that we found on each leaf. As the name indicates, the pest is most active in autumn (fall), but in the warm climate of Northland and Rodney, infestations can start in October. The ‘army’ part refers to the habit of mass exodus from a decimated crop to the next available food source, like an army column of caterpillars!

The eggs are laid on the undersurface of leaves, house eaves or fences in masses of up to 200 eggs, covered with a distinctive fuzzy felt-like layer. Wipe these out when you see them. Early-stage larvae are light green to pale brown, with a characteristic dark head capsule. The larvae grow to three to four centimetres long, becoming darker as they mature, with pale white stripes along the length of the body, a pale inverted “Y” shape between the eyes and dark spots with dark spines on each body segment.

The pupal stage is a shiny dark brown chrysalis, about the same size as a monarch butterfly chrysalis, but thinner, and more segmented, found in the upper level of soil or under mulch or leaf litter. Squash these if you find them, even though they may be pupae of other species, as all the army worm group of moths are a pest for home gardens.

The adult moths can be trapped using blue light moth traps or homemade traps (there are numerous recipes available online) The moths are relatively large, up to four centimetres across the wings, with a white hindwing that has a dark brown margin. The males also have triangular white spots at the tip and near the centre of each forewing.

I sprayed with Neem at 7ml/litre plus Success at 10ml/litre (both are considered organic) which seems to have done the trick. The Neem can burn sensitive plants such as tomatoes, so I back the rate down to 5 or 6ml/litre for these and, in any case, I spray in the evening when the temperature is lower and the bees have gone to bed.

Another option is spraying with Bacillus thuringiensis (Bt), a biological insecticide based on a bacteria that the caterpillars must eat to die, so good coverage of both sides of the leaves is important. Insecticidal soap also provides some control, and this product can also be applied with other sprays to increase their efficacy.

If you’ve lost the battle, pull up the plants and kill the caterpillars by plunging the above ground parts of the plants in a drum of soapy hot water. Don’t put them in the compost without dealing with them, as the caterpillars may just decamp to a nearby crop. Above all though, keep a close eye on your gardens.