Scarab beetle larvae, also known as white curl grub (or cockchafer in southern states), are a serious lawn pest. The signs of infestation are easily confused with other pests, diseases and disorders in turf and present as a general yellowing, then browning, followed by the death of lawn.
In subtropical areas, lawn injury is commonly seen from November through to January. The most common causal agent is African black beetle (Heteronychus arator), although a number of native and non-native scarabs look similar and produce comparable damage. These include pruinose scarab (Sericesthis geminata) and Argentine scarab (Cyclocephala signaticollis). If in doubt, have the pest formally identified.
Third instar African black beetle larvae grow to 20-25 mm in length before pupating in the soil. They have an orange-brown head capsule. Oval-shaped, shiny black adults, 12-15 mm long, emerge during February, feeding on stems just below ground level. They are less active through winter and mate in spring after the female has reached sexual maturity.
Only one generation is produced each year. Deceptively, different larval stages are sometimes found in the soil. This is mainly due to eggs being laid at different times.
White curl grub, scarab beetle larvae, lawn beetle larvae or cockchafer are the correct common names for the juvenile stage of lawn beetle. However, white curl grub is sometimes incorrectly referred to as 'lawn grub' and 'witchety grub'. 'Lawn grub' is a colloquial term for surface-dwelling caterpillars such as sod webworm, army worm and cutworm, which become moths. The true witchety grub is the wood-feeding larva of two families of giant Australian moth.
White curl grubs have a characteristic 'C' shape and three pairs of legs. They live underground, protected by soil. Animals such as magpies, crows, wood duck and other carnivorous birds, bandicoots and even foxes enjoy this food source. Damage from animal feeding can be the first indicator that the insects are present.
Some white curl grubs are parasitised by the yellow (hairy) flower wasp (Campsomeris tasmaniensis) in southern Queensland. This 30 mm-long hairy wasp with yellow and black banding on its abdomen can also act as an indicator of the presence of beetle larvae.
The late second instar and third instar phases of the beetle’s lifecycle are the most damaging to turf. These larger larvae are voracious feeders on roots and underground stems. The adults also feed on turf, but cause much less damage.
What often differentiates white curl grub damage from other types of lawn dieback, such as that caused by drought or water repellent soils, is that the lawn starts to slip or roll up like a carpet. If this symptom is detected it is time to bring out a large corer or shovel and dig for beetle larva.
A problem infestation is generally regarded to be 25 or more white curl grubs per square metre. If fewer larvae are present, healthy turf is likely to outgrow the minor damage it will sustain. Under heat and drought stress, the problem may be exacerbated by poor rates of regrowth and smaller numbers of larvae can cause significant damage.
African black beetles establish in a wide range of grasses including green couch, blue couch, soft leaf buffalo grass and kikuyu. The insect has a broad range of dietary preferences and larvae will attack, among other things, strawberries, pineapples, potatoes and grape vines.
Control measures are most effective when insect activity is monitored. One way of doing this is to moisten a hessian bag or piece of carpet and place it on the lawn overnight. In the morning the adults can be collected and disposed of. Check for adult beetles from late spring to early summer when egg laying commences.
It is thought that garden lighting may be helpful in attracting and detecting adult beetles. However, this may have the unwanted side effect of increasing egg laying activity in adjacent lawn areas. Turning off unnecessary garden lighting may reduce pest numbers.
Some householders encourage carnivorous birds into their garden to control the pest. However, if the white curl grub problem is severe, bird feeding can cause extensive damage in its own right. Free range poultry will also keep pest numbers in check.
A bucket of soapy water made with a biodegradable detergent can be poured onto affected areas, encouraging the larvae and beetles to move to the surface where they might be picked off by birds.
Effective control of later larval stages is only achievable with insect killing nematodes, known as entomopathogenic nematodes (ENs). They are active only against specific soil-dwelling insects, safe to handle and safe for plants. These nematodes were commercialised in 1999 after extensive research by the CSIRO Division of Entomology in Canberra.
ENs for African black beetle are raised in a laboratory and shipped in a dormant state. When received, the ENs must first be hydrated in water, and then lightly stirred to avoid settling. The suspension can then be watered onto a pre-moistened lawn. This needs to be done in the late afternoon because ENs are sensitive to the sun’s ultra-violet rays.
Upon release, the nematodes sense their target, move to it, and enter their prey through openings in its body. They then release bacteria that feed on the inside of the larva. The bacterium nurtures the nematode population, which builds up to the point where the larvae dies, rupturing to release a new generation of ENs into the soil.
Read garden chemical product labels carefully prior to purchase. Make sure the product is registered for use on home lawns for lawn beetle. There are three stages of the lawn beetles’ lifecycle for which a chemical may be registered. Use the chemical on the correct part of the lifecycle, strictly following the directions on the label.
Chemical control measures are most effective on newly hatched larvae. The presence of adult beetles is a cue to check the soil for early stages of the lifecycle, which are vulnerable to imidacloprid (Confidor) and thiamethoxam (Meridan) applications.
The organophosphate, chlorpyrifos (various lawn beetle and lawn grub formulations), is registered for the control of lawn beetle larvae and adults. In practice, the chemical is only effective on larvae if it infiltrates the soil and reaches the insect. It does not work well on larvae with high body fat. In addition, chlorpyrifos is highly toxic to the user and needs to be handled with caution.
Prior to treatment, water the lawn well to bring the larvae closer to the surface. Penetration of chemical will also be enhanced by mowing, then raking out thatch, before treatment.
The adult beetle is easier to control. Other chemicals registered for the control of adults have the active ingredients beta-cyfluthrin, bifenthrin, cyfluthrin (Baythroid) and diazinon (Pennside). Synthetic pyrethroids (such as bifenthrin and cyfluthrin forms) are safer to handle than organophosphates such as diazinon and chlorpyrifos. Pennside has been micro-encapsuled, reducing its toxicity to users.
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- Posted by Zeal Property Maintenance P/L from iPad.