Pest Facts South East Issue Two

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12 May 2010
By Stuart McColl

PestFacts is a free service designed to keep you informed about invertebrate pest-related issues - and solutions - as they emerge during the winter growing season. The service is supported by the GRDC's National Invertebrate Pest Initiative, with a focus on pests of broad-acre grain crops in south-eastern Australia.

CONTENTS

• Redlegged Earth Mites
• Blue Oat Mites
• Yellowheaded Cockchafers
• Grey-banded Leaf Weevil
• Heliotrope Moth Larvae

REDLEGGED EARTH MITES

We have received several reports of redlegged earth mites (Halotydeus destructor) appearing in crops and pastures after hatching from summer diapause eggs. Consultant, Matthew Sparke (Dodgshun Medlin), reports finding high numbers of redlegged earth mites in several newly sown pasture paddocks around Horsham, in the Wimmera district of Victoria. The highest numbers have been found in long-term pasture paddocks that were not treated with any insecticides last spring. Redlegged earth mites have also been reported in low-moderate numbers in various parts of the Western district of Victoria and near Wagga Wagga, in the Riverina district of New South Wales.

Redlegged earth mites are up to 1 mm in length with a globular shaped black body and red legs. They are one of the most important crop establishment pests, attacking a variety of crops and pastures, including cereals, oilseeds, legumes and fodder crops. Redlegged earth mites also survive on a variety of weeds, particularly broad-leaved weeds. For this reason, management of weeds can play an important role in reducing the build-up of mite populations within crops.

At this time of the year emerging seedlings are particularly vulnerable to attack from earth mites. Examine plants for damage and search for mites on leaves and on the soil surface. Before deciding on the most appropriate control measure, ensure the correct mite species has been identified. Problems can occur when growers use targeted rates of pesticides to control a particular mite pest and have then found another unexpected mite is present that is not controlled by the sprays. For example, recommended rates of many products used against redlegged earth mites are not effective against bryobia and balaustium mites.

BLUE OAT MITES

Blue Oat Mites (Penthaleus spp.) are also active in many regions after hatching from over-summering eggs. Agronomist, Rachael Webb (Landmark), has found blue oat mites affecting many early tillering oat crops near Inverell, in the Northern Tablelands district of New South Wales. In some cases more than 10 mites/plant could be found when visually searching, and plants were showing obvious signs of feeding damage. Rachael says many crops are moisture stressed and the mites are further affecting plant development. Where chemical control has been warranted, crops have generally been sprayed with an organophosphate chemical.

Blue oat mites are often mistaken for redlegged earth mites due to their similar appearance and sympatric life-cycle. However, blue oat mites and the redlegged earth mite differ markedly in their biology and tolerance to pesticides, and require separate management strategies. Blue oat mites are the most abundant earth mites in many cropping and pastoral areas of Victoria, Tasmania and New South Wales. Blue oat mites can be distinguished from other mites by their blue-black coloured body and characteristic red mark on their back. Click here for images of blue oat mites.

If chemical control is warranted, blue oat mites should be targeted within 2-3 weeks of emergence. This approach should protect newly germinated seedlings, which are most prone to mite attack. Be aware, continual monitoring of mite populations is needed and some damage to emerging pasture and crop plants may have already occurred by the time sprays are applied. As with redlegged earth mites, weed control can suppress the build-up of blue oat mites during the growing season.

YELLOWHEADED COCKCHAFERS

Agronomist, Karla Whittaker (AGnVET Services), has reported yellowheaded cockchafers (Sericesthis spp.) in a paddock containing wheat stubble near Lockhart, in the Riverina district of New South Wales. Very high numbers were observed after digging in various parts of the paddock, which has been sown to canola this year. Yellowheaded cockchafers have also been identified for agronomist, Julian Minehan (Landmark). Julian says large patches of an emerging oat crop have been badly affected near Goulburn, in the Southern Tablelands of New South Wales.

Yellowheaded cockchafer larvae are “C” shaped, creamy-grey in colour and have a yellow head capsule. When fully grown in winter they are about 25-30 mm long. The grubs live in the soil until mid-to-late summer, where they emerge as yellow-reddish beetles about 10-15 mm in length. Unlike blackheaded pasture cockchafers, which come to the surface to feed, yellowheaded cockchafers are primarily root feeders. They mostly attack cereals, but will also cause economic damage to pastures.

Control of yellowheaded cockchafers is complicated. Insecticides are largely ineffective because of their subterranean feeding habits, although we have received reports that suggest some level of control can be achieved with insecticides when applied to light and fluffy soils, when cockchafer larvae are close to the soil surface and if significant rain occurs soon after application causing the chemical to leach into the soil. Non-chemical options are however preferred. Re-sowing bare areas using a higher seeding rate is often the most effective strategy. Cultivating the affected areas prior to sowing a crop can also help reduce pest populations as it exposes the grubs to predation by predatory invertebrates and insectivorous birds. To help facilitate biological control, existing on-farm native vegetation should be preserved, and more breeding habitats for birds and parasitic insects should be created.

To check paddocks for cockchafers, dig in the affected areas or look on the soil surface for tunnel entrances. Crops sown into long term pasture paddocks are vulnerable to attack. Be aware that if you did not have problems with cockchafers last year, it does not mean that you won’t have this year. Adult beetles achieve long distance dispersal by flying, usually at dusk on warm evenings in late spring-early summer.

GREY-BANDED LEAF WEEVIL

Agronomist, Andrew Parr (Howard Martin & Co), reports finding high numbers of grey-banded leaf weevils (Ethemaia sellata) attacking several emerging canola crops around Berrigan, in the Riverina district of New South Wales. The weevils have been found in or near paddocks where marshmallow weed was present over summer and has recently been sprayed out. In the worst affected areas, Andrew reports finding 10 weevils in half a metre of crop row, and says damage is evident across the paddock (scalloping and chewed leaf margins). The paddocks affected were all cereal crops last season.

Adult grey-banded leaf weevils are approximately 8 mm long, light brown-grey in colour with distinctive raised markings (bumps) and a white area on the bottom covering approximately 1/4 of the wing covers (elytra). Adults are thought to be flightless. Larvae are yellow to green in colour with a light green-brown head capsule. They are legless, grow to approximately 5-8 mm in length, and are often confused with vegetable weevil larvae, which are similar in size and colour.

Little is known about the feeding ecology of grey-banded leaf weevils. Adults are known to attack several winter crops including canola and lupins, and they may also be a grain contaminant at harvest. They are closely associated with malvaceous weeds (particularly marshmallow) and this has often been found to be the source of infestations in crops. Paddocks should be monitored near fence-lines and where there have been large areas of summer weeds. There are no insecticides currently registered to control grey-banded leaf weevils. However, when synthetic pyrethroids have been used at 400mL/ha against other pests, they are reported to provide adequate control. Previous reports also suggest that canola paddocks sown with insecticide-treated seed experience less feeding damage from weevils.

HELIOTROPE MOTH LARVAE

Agronomist, Tim Colvin (IK Caldwell), reports observing Heliotrope moth (Uthethesia pulchelloides) in a paddock near Mathoura, in the Riverina district of New South Wales. The caterpillars have been observed feeding on Patterson’s Curse within a fallow paddock that will be sown into cereal this year. Caterpillars have also been observed on Patterson’s Curse near Berrigan, also in the Riverina district of New South Wales.

Heliotrope moths are medium-sized, distinctly coloured and distasteful to predators. They appear almost white when flying, but at rest, the full pattern of red and black spots on the white forewings can be seen. Each hindwing is white with two black spots and an irregular black margin.

Heliotrope moth caterpillars have sparse grey hairs, and are black with orange spots and broken cream lines along the body. They contain poisonous alkaloids derived from their food-plants and reach approximately 25 mm in length. They are commonly seen in autumn and are relatively host specific, mainly feeding on Heliotrope and Echium species. They are not reported to attack pastures or broad-acre crops and are unlikely to cause damage to cereals, oilseeds or pulses.

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