16/06/2026
We've had a few enquiries lately as to whether the Bt we sell on our website for use on cabbage white caterpillars, will work on the Fall army worm - thankfully, we haven't seen the dreaded army worm here yet, but below is some info we have gathered.
Bacillus thuringiensis (Bt) is a bacterium found naturally in soils, and produces spores containing proteins which are toxic to caterpillar larvae when eaten. The gut must have a pH of 9.0 to 10.5 (a high pH) in order to activate the toxin, which is how it specifically targets caterpillar and doesn't harm mammals, birds, fish or beneficial insects. As Bt has no direct contact action, it must be eaten by the caterpillar, so applying it fortnightly during periods of high infestation allows the larvae to ingest it while they are tiny, and stop feeding before they (& the damage they do) becomes large.
While the Fall armyworm (Spodoptera frugiperda) is a relatively new pest here in NZ (& thankfully responds to Bt at the moment), Fall armyworm is one of the main pests of corn, rice, sorghum, and cotton in many countries in the Americas, Africa, Asia and in Australia, and in several countries has developed resistance to some Bt protein strains, and perhaps unsurprisingly, also in the genetically engineered (GE) corn engineered to produce the insecticidal proteins from Bacillus thuringiensis.
So in short, the Fall armyworm is a relatively new arrival to these shores, Bt appears to show effectiveness here and now, but it's likely that in years to come we will begin to see the Fall army worm develop resistance to some of the Bt protein strains.
Key tips for effective use:
🐛 Regular application (fortnightly) during high periods of infestation is key to minimise the damage caused (compared to when the caterpillars are large and can decimate crops overnight),
🐛 Avoid applying Bt when it's not necessary to try and limit developed resistance.
🐛 Apply Bt in the evening (when no rain is forecast), as Bt is broken down by UV light.
🐛 Store Bt in a cool, dry, dark place.
🐛 Alternate the Bt strain you apply (if possible) to reduce the chance of resistance developing
