They’re back, … Why is the Grass Brown?

The Army Grubs plague is back again. The “tell tell” signs are the brown patches appearing around Norfolk Island on everyone’s lawns, along the roadsides and paddocks. This is an annual event on Norfolk Island and goes back many years.

It is always a concern when the army grubs appear, they destroy valuable pasture needed for the grazing cows and for the horses, sheep, and goats. Supplement food may be needed until the grubs move on and the grass recovers and grows again.

While some may be pleased to have a break from mowing lawns, the local lawn mowing businesses are also affected as the grass stops growing. Lawn mowing is a busy business on Norfolk Island until the Army Grubs damage the grass and the lawns stop growing. The good news is that Army Grub feeding rarely kills the turf, since they are just eating off the tops. However, additional stresses from heat or drought can combine with Army Grub damage to kill grass. For most lawns, a light fertilization and watering if weather turns hot and dry can help the lawn recover.

Army worms are another type of lawn pest which are commonly referred to as lawn grubs. This caterpillar of the Spodoptera mauritia moth is most prevalent throughout the warmer months of the year and repeat infestations can occur.

I’m sure we have all seen the Army Grub caterpillar, I have taken photos of the grubs crawling over each other on the edge of the grass along our driveway. This grass area has turned patchy, and the bugs were also crawling along the dirt edges. Sometimes they can be seen even crawling across the roads, there must be millions of Army Grubs.

I checked out natural, organic ways to reduce the grubs and the recommendation was to encourage birds which eat the Army Grubs. We all know the starlings and chooks enjoy feasting on the grubs. I have previously chatted with Norfolk Island locals, and they told me that there use to be many more starlings around the island, but their numbers have reduced in recent years. The possible course may be the insecticides used to kill the grubs.

Squashing caterpillars with a roller or tractor tires works well, I’m sure lots of us have tried walking on the Army Grubs to reduce their numbers. Down at the Norfolk Island Bowling Club, the lawn bowls also run over a few of them when they are on the bowls greens.

Another way to assist in reducing the grubs is to leave the grass longer by adjusting the height of your lawn mower, the reason for this is that the army grub eggs are then laid further away from the soil. Another site mentioned not to over water the lawns, as grubs love moist environments. I think most of us on Norfolk Island conserve our water and the reason for the influx of Army Grub is from the recent increase in rain fall and warm weather conditions.

Regular lawn aeration using the lawn coring or garden fork methods will greatly aid in creating a healthy strong lawn, but most importantly, aeration will encourage deeper, stronger roots which are far more resilient to lawn grub attack.

While asking questions about the control of the Army Grubs on Norfolk Island, I was told about moth zap-trap lights which use to be used across the Island. Due, possibly to the running cost, these traps were stopped and since then there has been an increase in the grubs across Norfolk Island. The traps can also kill other insects and many of these are not problems for our Norfolk Island environment.

Many parasitic wasps lay their eggs inside beetle larvae, which kills the beetle in turn. Information I read suggest it is something to keep in mind that, wasps can be a good friend to the lawn and garden owner. The wasps will also kill other garden pests, but they can also damage native insects as well. Malcom Dowling has posted images of the wasp on Facebook on Norfolk Island Horticultural Services, check out his page to see images of the good wasp which assists in reducing the Army Grubs. Malcom has also shared a link with the ichneumon wasp images.

https://www.facebook.com/Norfolk-Island-Horticultural-Services-110367120824440/

A couple of weeks ago I noticed an increase of moths around our house in the evenings. I wonder now if that was the Army Grub moths. I did some research on the life cycle of the little grub.

The life cycle of the Army Grub and its moth parent is very rapid, and as such it is possible that three different infestations can occur over the warmer months of the year. The damage caused by the Army Grub will appear as large patches of the lawn literally being eaten away, with masses of grass leaves appearing on the top of the lawn which have been cut off at the base.

When the population is high, feeding produces bare circular areas. The only life stage that is a problem is the caterpillar, when abundant. In the late evening and early morning, or anytime on cloudy days, they will be actively feeding.

I also read that many types of clover and other broadleaf weeds are left as the grass around them is consumed. Norfolk Island has kikuyu and couch grass growing in paddocks, the roadside and also these are common grasses grown as lawns at locals’ homes.

Life Cycle of Army Grubs

The life cycle begins when an adult moth will fly onto lawns at night-time and lay pockets of eggs in clusters or rows on lower leaves of grass. The female moths can lay as many as 2000 eggs during their life, so populations can build up quickly. These tiny white eggs will hatch after around 10 days and become tiny army worms.

From here on, the army worms will continue to feed on the green leaf of the lawn at a voracious rate, always feeding at night-time. They will then hide in the thatch layer or in the topsoil of the lawn during the day inside tiny cocoons. Full-grown worms are about 4cm in length with a narrow broken white stripe down the centre of the back. There also are stripes along each side of the body.

The caterpillar stage lasts about 3 weeks, but the caterpillars are usually 10 to 14 days old before damage is noticed. By this time, many of the larvae will have small, white eggs stuck to their bodies just back of the head. These eggs hatch into parasite maggots which enter the body. Those larvae that reach maturity dig into the soil to pupate. The next generation moths emerge about 15 to 18 days later. There are two and possibly a partial third generation. The second-generation larvae generally cause little damage.

I researched a little about insecticides. Some of the locals I spoke to suggest the reason for the reduction on starlings has been the introduction of sprays killing off the birds.

If you are considering controlling these Army Grubs with the use of chemical sprays, and you wish to learn more, contact the local garden supplies for guidance. It would be best to capture a few caterpillars to take to the local nursery for positive identification. This will help the staff member to recommend the right insecticide for the caterpillar type. The insecticide is mixed up and applied to the lawn just before dusk. Ensure to allow yourself enough time to finish the entire lawn treatment before night-time. The treatment is then lightly watered into the lawn so it can reach down to the base of the thatch layer. The poison is then ingested by the Army Grubs as they are feeding on the lawn. Be careful not to over water, this will simply wash the poison away, and it will not be ingested by the caterpillars.

The other thing to consider if using chemical sprays, is the impact on the Norfolk Island environment and the birds which eat the Army Grubs. Remember the grass normally recovers after the Army Grub infestation has finished.

Warning: Pesticides are poisonous.

Read and follow directions and safety precautions on labels. Handle carefully and store in original labelled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Here is a list of internet sites which were also helpful, check them out for more details. Plus, I found a couple of photos of the life cycle of the grubs. I’m not sure if these are the same as our local grub, thought they look very similar.

http://lawncareadvice.com.au/lawn-pests/146-army-worms.html

https://www.coastalturf.com.au/uncategorized/lawn-grub-control-and-prevention/

Once again, I have had a chance to research another interesting subject. Thank you to the locals who shared their knowledge of the little grub that loves our Norfolk Island grass.

Betty Matthews

March 2022