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Nocturnal nuisances: identifying night-active pests

The cause of crop damage is not always easily identifiable as some critters feed at night. Here are some tips to find the culprit.


When a mystery pest has been feasting on a crop, but the culprit is nowhere to be found, visually inspecting the type of damage usually provides a clue or two as to who is a likely suspect.

Invertebrate feeding mechanisms come in several shapes and forms, and damage is largely indicative of their mouthparts. For example, symptoms of sucking pests (e.g. aphids and mites) include leaf-cupping, silvering, and shrivelling, while symptoms of chewing pests (e.g. caterpillars and weevils) include scalloped leaves, cut stems and leaf holes. Some pests damage plants more by rasping than chewing. Rasping damage from slugs and snails occurs as they scrape off the soft tissue of leaves during feeding, with a rasp-like feeding device known as a radula, often resulting in shredding or irregular-shaped holes in leaves. In the case of lucerne flea, the leaf ‘windows’ result from the upper leaf surface being rasped.

But damage type may only take you so far and the reality is that very few pests can be solely identified to a species or even genus level by their damage. For example, chewing damage in vegetative cereals may point to a caterpillar but reveal relatively little about the individual species.

Many pests attack crops at night. To locate them during the day, inspect the ground and plant crown thoroughly, breaking up soils clods and stubble if present.

These extra strategies may also be useful in identifying critters that take shelter during the day.


Inspections at night

Night inspections with a torch can be essential to get a reasonable indication of the pest and their densities. This is useful for pests such as millipedes, slaters and slugs. Remember to disturb and agitate stubble and plant residues which provide shelter to crop pests. In vegetative crops, night sampling can be sped up using a sweep net.


Pitfall traps

Pitfall traps are useful for identifying pests that are active on the soil surface such as beetles, weevils, caterpillars, earwigs, slaters, and millipedes. To establish a pitfall trap, bury a small disposable plastic cup in the soil so that the rim is level with the soil surface. Pour some water (1/4 cup) and a little detergent to immobilise the organisms. Setup multiple traps across the paddock, especially where the damaged area meets the healthy crop. Inspect the traps after a few days.


Refuge or shelter traps

Refuges are an effective tool in luring pests such as slugs, cutworm, earwigs and millipedes, and are easy to setup. These traps can be tiles, wet carpet, hessian squares or even old chemical drums that are placed on the soil surface in multiple locations across the paddock. Refuge traps are not as effective when the soil is moist and are only effective against pests that have high moisture requirements.

We always say that best practice management begins with correct identification. Employing one or more of these monitoring methods is your best bet to achieving this when the culprit is not very obvious.

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