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CATERPILLARS, WORMS OR GRUBS – WHAT’S EATING MY LAWN?

Commonly known as ‘lawn grubs’, beetle larvae and moth larvae cause significant damage to lawns. Here’s how to identify the culprit in question and treat appropriately.

When it comes to insects that cause damage to lawns, they are often given the generic name ‘lawn grubs’. To be honest it’s probably not a bad generic name, in that they live in the lawn and invariably it is the larval stage – the grub – that causes the damage. But it doesn’t really help in identifying the culprit and applying the correct treatment.

Broadly speaking there are two main types lawn grub: beetle larvae, which are the white curl grubs that live in the soil and feed on the root systems, and moth larvae, the caterpillars that live both below and above ground and eat the stems of the grass. Just to confuse things further these larvae or caterpillars are commonly called ‘worms’ – cutworms, webworms and armyworms.

The various ‘beetle’ pests, including African black beetle, Argentinian scarab, Billbug and Argentine stem weevil, can be challenging pests to control and have their own unique life cycle and treatment methods. However this article focuses on the moth caterpillars, which although vary in species, can all be dealt with using the same treatment technique.

Armyworms

Much of the literature on the internet focuses on the fall armyworm (Spodoptera frugiperda) as it is a significant problem in the US, sub-Saharan Africa and parts of Asia, but it has yet to be reported in Australia. In Australia,

a number of armyworm species are found across the country, where they cause damage to cereals, pastures and turf. In the lawn situation, the most prevalent species are the common armyworm (Leucania convecta), the day-feeding armyworm (Spodoptera exempta) and the lawn armyworm (Spodoptera mauritia).

Although they have different life cycles – common armyworm, three generations a year including winter and spring generations; day-feeding armyworm, one generation a year; lawn armyworm, 2-3 generations during summer and spring – the appearance of adult moths around light sources in September and October is a warning of a potential outbreak later in the year. When numbers explode and food sources become depleted, the caterpillar can be seen marching into neighbouring fields and pastures to find food – the behaviour from which they take their name.

Armyworms are the larval stage of night flying moths. Eggs are laid in the grass, sometimes in synchronised waves that can give rise to the sudden outbreaks. They are also prolific, with a single moth laying around 300 eggs per night! Although the colour of the caterpillars may vary – green, brown or yellow – all species of armyworm can be readily identified by the three white parallel lines found behind the head, which can continue the length of the body (Figure 1, main picture above). The caterpillars will reach up to 40 mm in length before pupating in the soil.

The caterpillars feed on the leaves and do not damage the plant crown or roots. So, if a lawn suffers damage, it should readily recover with a bit of TLC. The caterpillars mainly feed during the night, resting at ground level under vegetation during the day. As such they can be quite easy to spot, with their frass – 1-2 mm green pellets – being another sign of their presence.

Cutworms

There are several species of cutworm (Agrostis spp.) in Australia: common cutworm, black cutworm (Figure 2), brown cutworm and pink cutworm. Like the armyworm, cutworms are the larvae of night flying moths – the common cutworm is the Bogong moth.

Figure 2: The black cutworm moth (photo credit/Caroline Harding, MAF/Wikipedia CC)

The adult moths lay their eggs in the soil. Upon hatching, the caterpillars remain in the ground, only coming above ground at night to feed. They only feed at ground level, cutting plants at their base, which gives rise to their name. They will eat weeds, seedlings and soft fruits such as strawberries, as well as grass. The caterpillars look similar to armyworms but lack the three white stripes (Figure 3).

Figure 3: The black cutworm (Agrostis ipsilon) lacks the white stripes of armyworm caterpillars

With the different cutworm species having different periods of maximum activity, damage to lawns can occur year round. In lawns, their activity can be more easily spotted than that of armyworms, as each caterpillar creates a circular spot devoid of grass – they come out of their burrows at night, cut the plants nearby, and retreat to their burrows. Typically they tend to be more active after rain.

Webworms

There are four species of pasture webworms (Hedonta spp.) in Australia, and despite their name actually cause more damage in cereal crops than in pastures. For homeowners it’s the sod webworm (Herpetogramma licarsisalis) that is more likely to cause damage to lawns.

The adult webworm moths are most active at dusk, when they may be disturbed when walking across the lawn or when watering (Figure 4). The females lay their eggs on the wing, flying low over the grass, dropping eggs as they go. The caterpillars hatch in 1-2 weeks and live in the thatch, coming out to feed at night. Their presence can also be readily noticed at dawn, as they leave a silky web on the grass, which is more visible when dew is present. Mainly confined to the east coast, peak population numbers occur during summer.

Figure 4: Webworm moths have slender bodies and snout-like projections

Treatment

In addition to picking up the early signs of damage, the level of adult moth activity around light sources can also provide an indication of the level of infestation. All treatments target the larval stage and a range of products, both pyrethroids and non-pyrethroids, are available. The caterpillars themselves are easy to kill, it is just a bit of a challenge to get the treatment to where the caterpillars are hiding. However, as they are all nocturnal, spraying in the late afternoon or evening when they start to come out to feed will deliver the best results. Also, as cutworms live in burrows and webworms in the thatch, watering in the treatments well after application will ensure the insecticide reaches its target.

Cluster caterpillar

Although not a pest of lawns, the cluster caterpillar (Spodoptera litura), also called the tobacco cutworm, is commonly encountered by pest managers (Figure 5).

Pest managers or their clients may notice clusters of young caterpillars that feed in groups and can defoliate plants, or spot their egg masses, which can be laid on the underside of carport and pergola roofs, or indeed inside the home (Figure 6). Treatment involves treating any caterpillars and egg masses directly.

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