Controlling grubs in garden and lawns

We have a native grass grub here in NZ that comes from the scarab beetle. They are the reason that the birds, especially blackbirds mess up lawns in winter as they dig down to get these grubs. The local university landscaped grounds have large patches of decimated lawns each year as the birds have a field day. They need a good lecture!

In other countries, grub worms or white worms are similar and can be the grub of several beetles, most commonly the iridescent, brown and green Japanese beetle. The grubs hatch about mid summer and the beetles fly around and eat leaves, particularly of roses, some fruit trees, grapes, maples and many other leafy plants and weeds.

Over 4-6 weeks female Japanese beetles go through many cycles of eating, mating then burrowing deep into the soil to lay eggs, particularly in grassy hills, paddocks and lawns. In late summer to autumn/fall, the eggs develop into larvae (grubs) and wriggle upwards to feed on plant roots and other organic soil contents. Once the grubs have eaten enough and are mature, they then burrow down into the soil to overwinter.

When the soil is warm enough in spring, the grubs head upwards again, pupate into adults (beetles) and take off to feed on leaves.

Milky Spore Disease (Bacillus popilliae), is a recognised control for grass grubs/grub worms/white worms and can be bought online or in garden stores. Soak it into the grub infested soil at their feeding stage, and the grubs ingest it, whereupon the bacteria become alive and cause the grubs to starve and die. These bacteria stay in the soil, and in fact most soils already have some Milky Spore bacteria in them, so if you add more it helps control these grubs.

Other control methods are Beneficial Nematodes and natural predators like parasitic wasps, birds, many good beetles, and even ants will seek out the eggs. Neem oil is also recommended for grubs, but it although it's natural it will kill other insects, so don't use it on your vegetable or flower garden, only use it sparingly on lawns if you really have to.

A few grubs will not be a problem and not worth the effort and challenges to control and kill them because more will always fly in each year from outside areas. If you find one or more grubs in each handful of soil, that's getting to be an infestation which may need action. A popular control method with many people is to simply go out at night with a torch and pick off the beetles from plants and squash them or flick them into a bucket of soapy water. Check the tops of bushes and plants where they congregate and start their feasting