Plant Doctor Archive

Tiny worms eating my plants

CAN you help save my plants? Underneath at the roots of polyanthus and lots of other plants is a thin clear worm 1cm long and a thin brown-striped one about 2cm. They completely destroy the plants and there is also a very small dark fly in masses. I use Aquaticus products and Gild and have also tried Neem oil, but have no luck getting rid of these pests. Hoping you can help.

I'M not sure what the brown-striped worm is, but the smaller ones sound like the larvae of a fly, possibly sciarid fly (sometimes called fungus gnats) and the masses of small flies sound like adult sciarid flies. The female fly lays eggs in wet potting mix and the larvae feed on microscopic fungi and decaying organic matter in the mix. They thrive in warm, moist greenhouse conditions and are seldom a problem outdoors. They usually don't harm plants, but if for some reason plants are already weakened, the larvae can seize the opportunity and feed on tiny roots, making an already bad situation worse. The presence of fungus gnats is usually a sign the mix is too wet.

One of the best ways to avoid the problem is to use a free-draining potting mix and allow it to dry out between waterings. Gild should kill off the adult flies but will probably have little effect on the larvae in the soil, which at this time of year quickly turn into adults.

To control the larvae you could try drenching the mix with a solution of a contact insecticide such as Pyrethrum or use a diazinon-based product like Liquid Diazinon or No Insects Lawnguard Prills.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.