Soil insects in Queensland
Identification and management

Key points

• There are a number of soil insects that can damage establishing crops.
• The germinating grain bait technique is recommended for pest detection.
• Thresholds for control are different for summer and winter crops.
• Different soil insect pests occur under different cultivation systems.
• Weedy fallows and volunteer crops encourage soil insect buildup.
• Zero tillage encourages beneficial predatory insects and earthworms.

Identification of soil insects

Soil insects are often difficult to detect as they hide under trash or in the soil. Descriptions of common soil insects follow. Scale lines are included in the diagrams to give an indication of actual sizes.

Ants

Ants live in colonies which are mostly underground. The wingless workers gather food for the colony.

Figure 1. Ant

Beetles

Beetles are hard-bodied, small to medium-sized and drab-coloured. False wireworms (Figures 2 and 3), scarab beetles (Figure 4) and true wireworms (Figure 5) are pests. The larger species of false wireworm beetle and true wireworm beetle are black, and the scarab beetle shiny-black. The smaller false wireworm species is dirty brown in colour. True wireworm beetle adults do not damage crops, but the grubs do.

Figures 2 and 3. False wireworm beetles

Figure 4. Scarab beetle

Figure 5. Wireworm beetle

Cockroaches

Cockroaches generally grow up to 35 mm long, are wingless or winged, and have long spiny legs (Figures 6 and 7). The shield above the head is usually dark. Cockroaches are found under trash or in cracks in the soil and run very quickly when disturbed. Wingless cockroaches are dark reddish brown with contrasting pale or yellow markings. Winged cockroaches are straw-coloured and speckled with dark dots.

Crickets

The cricket's body, excluding the feelers and tail projections is up to 30 mm long (Figure 8). Crickets are brown or black in colour with long feelers. Adults are usually winged, but the younger stages are not. Crickets are best recognised by their large back legs which are used for jumping.

Earwigs

Earwigs have large pincers at the end of the body (Figure 9). They are brown or black in colour and live in cracks in the soil. The common black field earwig (about 15 mm long) is a pest of crops. Do not confuse these with the large brown earwigs (up to 30 mm long), which are valuable predators.

Figure 6. Wingless cockroach	Figure 7. Winged cockroach
Figure 8. Cricket	Figure 9. Earwig

Immature insects (grubs and caterpillars)

Immature insects are found in the soil, usually below the dry surface layer. The head is usually dark, but the body is pale or creamy-yellow. Figure 10 shows a white grub (black sunflower scarab larva). Figure 11 shows a weevil grub (note that it has no legs). Figure 12 shows a false wireworm grub.

True wireworms differ from false wireworms in that the larval head is wedge shaped, the body is flattened (rather than cylindrical) with upturned spines at its end.

Caterpillars have six legs at the front of the body and a number of stumpy projections (prolegs) on the back half of the body. They are usually multi-coloured (with stripes or patches of lighter colour). Caterpillars turn into moths or butterflies.

Figure 13 shows a caterpillar (similar to the armyworm or cutworm). Armyworms live on the soil surface, under debris or in cracks. Cutworms live under debris or may have a tunnel below the soil surface in which to shelter.

Figure 10. White grub	Figure 11. Weevil grub	Figure 12. False wireworm grub
Figure 13. Caterpillar

Other soil invertebrates

Other soil invertebrates are also common, but none damage crops.

Centipedes are long and segmented, with long feelers and tail appendages on a flattened body (Figure 14). They have legs all along the body and can move very quickly.

Millipedes have a long cylindrical segmented body with an extremely high number of legs (Figure 15). They have short feelers, no tail appendages and move more slowly than centipedes. When disturbed, they coil up into a spiral.

Isopods (woodlice or slaters) are small grey animals up to 7-10 mm long with long feelers and eight pairs of legs (Figure 16). They are usually found in groups under trash and in cracks in the soil.

Earthworms (Figure 17) are true soil animals, unlike centipedes, millipedes and isopods, which live on or near the soil surface. Earthworms are valuable in improving soil texture.

Figure 14. Centipede	Figure 16. Isopod
Figure 15. Milipede
Figure 17. Earthworm

Scouting for soil insects

Methods for sampling soil insects include spade sampling, which is time consuming and laborious, and germinating grain bait technique, which is simple, accurate and detects all soil insect pests.

Germinating grain bait technique

Immediately following planting rain:

1. soak insecticide-free crop seed in water for at least 2 hours to initiate germination;
2. bury a small handful of the seed at shallow depth and cover it lightly with 1 cm soil;
3. chose five widely spaced sites in each 100 ha; at each site, bury seed on the corners of a 5 m x 5 m square;
4. check for seedling emergence - once emerged, leave seedlings overnight;
5. dig up the entire plants the next day and put the plants on a tray to count insects.

Pre-soaked sorghum seed emerges after 3-4 days in summer and 5-6 days in autumn/spring.

Action thresholds and control

In summer crops, control is warranted when there are one or more insects (or 5 earwigs) per bait.

In winter crops, control is warranted when there are 2 or more insects (or 10 earwigs) per bait. Ant control is warranted when infestations occur on 50% of baits.

Commonly used methods of control include insecticidal grain baits for control of adults, seed treatment and in-furrow spraying for control of larvae (and adults in some cases). For more information on control of specific soil insects, see DPI&F publication "Managing insects & mites in field crops, forage crops and pastures." See Table 1 for information on insecticides registered for control of soil insects.

Table 1. Soil insect pest control

Active Ingredient	Aldicarb	Carbaryl	Chlorpyrifos	Phorate	Terbufos	Thiodicarb
Trade Name	Temik	Several	Several	Thimet	Several	Semevin
Black field cricket 1			R
Black field earwig			R			R
Black scarab beetle		R
Cutworms		R	R
Wingless cockroach			R			R
False wireworms	R		R	R		R
Wireworms	R		R	R	R
Registered Crops	cotton	cereals, sunflower, sorghum	2	cotton	maize, sorghum, sunflower	maize, sorghum

R. Registered. Always refer to the container label for current registration and safety information. These details may be subject to change. Withholding periods can vary between crops. Check container labels for details.
1. Maldison is also registered for black field cricket in sunflowers
2. Registered for use in a number of crops. Check container label for details.

Making your own insecticidal grain bait

Ingredients

• 100 mL chlorypyrifos (g/L) EC.
• 125 mL crop or vegetable oil.
• 2.5 kg of cracked wheat, sorghum or standard pellets.

Using the bait
Once mixed, the bait can be applied by a fertiliser spreader or through fertiliser tubes, dropping onto the soil surface. Using 2.5 kg/ha of cracked grain or standard pellets gives between 20-30 grains/square metre or 10-15 pellets/square metre, which is satisfactory for effective control.

If using pellets to make beetle bait, ensure that they are not too large. Large pellets applied at 2.5 kg/ha result in only 1-2 pellets/square metre which may provide inadequate control.

Ready to use baits containing chlorypyrifos are also available. Contact your local reseller for more information.

Crop rotation and stubble management effects

Crop rotation

Insect numbers decline during a clean long fallow due to lack of food. If it is less than one year since the last crop, check for soil insects. Summer cereals followed by volunteer winter crops promote rapid buildup of earwigs and crickets. Parthenium hosts black sunflower scarab beetles which can then switch to seedling sunflower crops. Grazed sorghum suits the survival and development of black sunflower scarab larvae and false wireworm larvae. Wingless cockroach populations increase under continuous zero or minimum tillage until a buildup of predatory insects occurs.

Stubble management

High stubble levels on the soil surface at planting reduce damage to seedlings by false wireworm beetles, but do not reduce seedling damage by wingless cockroaches or black scarab beetles. Black field earwig populations are higher when stubble is incorporated.

False wireworms are found under all intensities of cultivation, but are absent if stubble levels are very low. They are favoured by summer crops - populations decline under wheat and wheat stubble.

Further information

• Huf, S. and Agnew, J. 1994. Rain to Grain, Queensland Department of Primary Industries, Information Series QI94039.
• Elder, R.J., Brough, E.J. and Beavis, C.H.S. Managing Insects & Mites in Field Crops, Forage Crops and Pastures. Queensland Department of Primary Industries, Information Series QI92004.

Current national information on agricultural chemicals registered for use on soil insects is available on the Infopest CD-ROM . Write to GPO Box 46, Brisbane, Qld. 4001, email infopest@dpi.qld.gov.au or phone (07) 3239 3967 for further information.

File No. FS0050
Last reviewed January 2005.