Emerging insect pests

Phil Bowden¹, Paul Umina², and Garry McDonald²,

¹Murrumbidgee Landcare, ²cesar Australia.

Keywords: pests, minimum tillage, stubble retention, chemical control, biological control, cultural control.

Take home messages

• Minimum tillage and stubble retention have resulted in greater diversity of invertebrate species seen in crops.
• Cultural control methods such as burning, rolling or cultivating stubbles are sometimes needed to compliment chemical and biological controls.
• More research is needed on biology, life cycles and control options.

Introduction

This season has seen an abundance of invertebrates in many crops and is it any wonder?  We have had perfect conditions for plant and animal growth and the invertebrates are no different, driven by good moisture and mild temperatures they react to available resources…. so this is just the ecosystem at work.

Pest outbreaks occur from time to time, often in response to natural conditions occurring at the time, but sometimes in response to our management practices.

Many of our management practices for cropping have changed dramatically over the last 20 years.  In particular, reduced tillage and increased stubble retention have changed the cropping landscape with respect to soil moisture retention, groundcover and soil biology and this has also affected the abundance and types of invertebrate species being seen in crops.

These systems increase invertebrate biodiversity but also create more favourable conditions for many pests such as slugs, earwigs, weevils, beetles and many caterpillars. In turn they have also influenced beneficial species such as carabid and lady beetles, hoverflies and parasitic wasps.

This paper looks at a few important emerging pests and discusses what is happening in the field.

Slugs and snails

Slugs have become a more serious pest with increases in stubble retention and minimal tillage.  These management practices help to provide a suitable habitat for their survival. The main pest species of slugs attacking broad-acre crops are the black-keeled slug (Milax gagates) and the grey field slug (Deroceras reticulatum). The brown field slug (Deroceras invadens) can also be a serious pest, with species often co-occurring. Soils that retain moisture are preferred by slugs, typically heavy red loams and grey clays.

In a typical year, adult slugs are stimulated out of their summer dormancy following autumn rains and will generally be problematic in paddocks where they have previously been an issue. Slugs are best managed using both chemical and cultural methods, while also trying to preserve the role of soil dwelling beneficial species such as Carabid beetles.

The most common chemical control method for slugs is the use of baits. These must be applied at, or before sowing after good germinating rains to coincide with slug emergence and before seedlings emerge. When baiting, it is important to consider the conditions, number of baits used and how this relates to slug densities. Baiting efficacy can be reduced when there are higher slug populations, an increased availability of refuges and higher amounts of plant material. Field trials conducted in the last few years showed that:

• Ingestion of active toxicants is more effective than spray applications.
• 3% rather than 1.5% metaldehyde active ingredient on baits is economically more justifiable.
• Uniform sized baits rather than cylindrical baits spread more evenly providing greater control of slugs.

The trials have shown that the best control option for slugs is baiting combined with cultural practices; including cultivation and burning. In high rainfall areas of Victoria, with consistent slug problems, grain growers who previously were moving towards no-till systems have reportedly returned to cultivation for slug control. Cultural management options include rolling after sowing to consolidate the seedbed (particularly the drill lines) and the use of shallow discs to break the soil into a fine tilth. Other options that can be considered earlier in the season are burning stubbles to remove refuges and ensuring control of summer/autumn weeds from paddocks prior to sowing.

The shift from burning to retaining stubble and reduced tillage has also resulted in an increase in snail populations in some areas. This is particularly true in SA, but snails appear to be slowly spreading to some areas of the east and north. The highest snail numbers ever experienced in South Australia in 2009 was partially due to high stubble loads that favoured snail survival and carryover between seasons. A year-round integrated management approach including burning is recommended. A hot burn must be used for snail control as a cold burn may only reduce numbers by 50%.

Weevils

Crop weevils can cause significant damage by feeding on vegetative parts of crop plants. They are favored by minimum tillage and stubble retention. Removal of stubble through cultivation and burning destroys their habitat and reduces their numbers. Crop weevils are difficult to control with chemicals due to their behavioural habits, often only appearing at night to feed.

The Spotted vegetable weevil (Steriphus diversipes), which is often referred to as the Desiantha weevil is a sporadic pest and widely distributed across New South Wales and Victoria. Adults are mottled grey-black weevils with grey flecks on the abdomen growing to seven millimeters long. Larvae are white with orange-brown heads, grow to eight millimeters and are soil dwelling.

Weevils cause considerable damage, chewing seedlings down to the ground and scalloping leaf edges. Adult spotted vegetable weevils rarely feed on cereals, except if the affected paddock has been previously pasture. The pasture provides favorable hosts such as capeweed and grassy weeds, and so facilitates the build up of weevil populations. Highest numbers have been observed in parts of the paddock that have a thick dead layer of wireweed and sorrel trash. Although difficult to accurately estimate densities, typically finding two-three adult weevils in a meter square is not uncommon.

Crops that follow a long-term pasture phase are most likely to be at risk of spotted vegetable weevils. Inspect newly sown paddocks and germinating seedling crops, looking for poor emergence or bare patches. There are no foliar insecticides registered against this species but seed treatments can reduce the feeding damage to seedlings.

The Mandalotus weevil (Mandalotus spp.) is also a sporadic pest of canola, cereals and pulses. Adults are three-five millimeters, round and dull brown in appearance, often resembling a small clod of dirt. They are mostly active at night and during the day they can be found hiding under dirt and trash. Mandalotus weevils occur mainly on rubbly, lighter soil types in the Mallee districts, but have also been seen in central and southern NSW. Little is known about their life cycle. Usually, feeding damage is first noticed on the leaves of emerging crop plants. In severe cases the seedlings are often ‘ring barked’ just above ground level. This causes the plants to fall to the ground where the weevils continue feeding on the leaves. Crop signs may just be large bare patches.

Earwigs

There are several species of earwigs found in crops; several are pests of seedling crops including canola, cereals and legumes particularly the European earwig. However, one native species, the common brown earwig (Labidura truncata), is exclusively a beneficial insect which mainly feeds on soft-bodied insects such as caterpillars, lucerne flea and mites. The common brown earwig can be distinguished by the presence of an orange colored triangle behind the head on the elytra or ‘wing-case’.

European earwigs (Forficula auricularia) are an introduced species that appear to be spreading in southern agricultural areas. It is believed their spread is at least partly due to increased levels of stubble retention. In the past they have been found damaging canola, cereals and some legume crops. European earwigs chew developing seedlings and slow plant development. The typical appearance of damage is shredded leaf tips and/or irregular holes in leaves. European earwigs range from 12-20 millimeters long, are smooth and shiny dark brown in color with pale yellow legs. It is important to distinguish earwig species in order to make the most appropriate management decision and accurately assess the risk of attack to emerging crop seedlings.

Control options for earwigs are limited, however there is some evidence that insecticide seed treatments such as fipronil will help crop seedlings withstand attack. Cracked grain baits (wheat, sorghum, corn) containing chlorpyrifos and sunflower/vegetable oil may also be used to control pest earwigs.

Millipedes

Millipedes (Ommatoiulus moreletii) have a smooth, cylindrical body made up of 50 segments when fully developed. Adults are 30-45 millimeters long, dark grey to black in color and have two pairs of legs on each body segment. When disturbed they either curl up in a tight spiral or thrash about violently trying to escape. Portuguese millipedes can be a pest when they build up to high numbers and invade gardens and houses. They have been recorded as causing damage to several horticultural crops such as potatoes, strawberries and tomatoes. Reports suggest millipedes may also attack emerging canola seedlings in situations where minimum tillage and stubble retention are employed. However, just because millipedes are present and in high numbers within a paddock, doesn’t mean they will be a crop pest. Populations have increased in low disturbance systems over recent years likely to be as a result of increased soil moisture and organic carbon levels. Strategies to remove their habitat (stubble) are the most effective control options.

Slaters

Slaters are a common arthropod in many environments and can attack broad-acre crops, and in some instances cause serious damage. Reports of slaters causing damage to cereals, canola, lentils, chickpeas and pastures in New South Wales and Victoria are becoming more common. Feeding results in uneven rasping-type damage that often appears as ‘windows’ of transparent leaf membrane. However, the presence of slaters within a paddock (even in high numbers) does not necessarily mean a pest issue. Slaters typically feed on decaying organic matter and occasionally feed on emerging crop seedlings. The ‘trigger’ for this switching behavior is not understood.

There appears to be a strong correlation between slater occurrence and minimum tillage and stubble retention. Stubble provides a cool, moist refuge that facilitates survival and population development. Crumbly clay soil surfaces and cracking clays also seem to favor the survival of slaters. Slaters need damp conditions and will die if exposed to open and dry situations.

There are no insecticides registered against slaters in broad-acre crops, and reports indicate they are relatively unaffected by foliar sprays of both synthetic pyrethroids and organophosphates applied to control other crop establishment pests. There are chemical baits registered for use against slaters in horticulture, and reports suggest some success with chlorpyrifos baits in Western Australia.

In 2013 an unusual outbreak of slaters in northern NSW was reported.  The slaters were identified as the Flood bug (Australiodillo bifrons). The flood bug is a native species, approximately seven-eight millimeters in length and four millimeters wide. They are oval shaped and have a flattened body, with light colored legs. They have the unusual behavior of moving in ‘swarms’ which can consist of >100,000 individuals. Slaters completely chewed the tops of emerging cotyledons; in some cases only the seedling stumps remained. The affected paddock was not cultivated prior to sowing, and there was some stubble on the soil surface. Large numbers of slaters were observed moving across the paddock. Interestingly, there was no visible feeding damage in an adjacent faba bean crop, despite the presence of a large numbers of slaters.

Future research of establishment pests including slaters, earwigs and millipedes, is needed to determine the relationship between soil/crop health and the invertebrate communities with tillage systems identified as a focus area.

Some other pests we have seen this season include:

Cutworm (Bogong Moth)

There have been many reports this season of crop and pasture damage from caterpillars and the culprit species is almost certainly Agrotis infusa, the common cutworm (or the larvae of the Bogong moth). This moth is regarded as one of Australia’s most iconic insect migrants.  It is known to breed in autumn/winter in Australia’s eastern ‘grasslands’, and in spring, moths emerge and migrate to the Australian alpine region where they seek refuge from the summer heat. It is believed that the moth then returns (westwards) to ‘the grasslands’ to restart the cycle. It is however probable, that while still migrating, many moths find safe and cool refuges over summer in closer habitats across their grassland range.

The moths can lay eggs from early summer, but generally wait until conditions are more favorable in autumn. Each female can lay hundreds or thousands of eggs. They almost certainly choose to lay these following autumn rains, probably on broad-leafed plants and weeds. The survival of the young caterpillars (one-two millimeters long) can be highly variable as they are often exposed to predators (beetles and spiders), hot dry conditions, and their host plants often die out through moisture stress or herbicide applications.

So, what has happened in 2014?

This year’s widespread outbreak of cutworms has been the result of a combination of conditions triggered by an early break: (i) a larger number of moths than usual laying eggs following the unseasonably early rains, and (ii) conditions (timing of egg-lay, mild weather and food availability) that have favored, greater than average, the survival of young larvae. After all, larval survival only needs to increase from 0.1-0.5 per cent (typically) to 10 per cent to get a 200 per cent increase in full sized cutworms seeking food from crops and pastures in May and June. In most regions, the conditions this year have provided extensive green hosts much earlier in the season than normally experienced. So the prediction is that, in late spring 2014, many towns and cities of south-eastern Australia will be ‘invaded’ by countless numbers of migrating, light-seeking moths for several weeks. Something to look forward to!

Crop damage by cutworms can often present as seedlings completely cut off and consumed. When looking for cutworms, look under clods or within the top few millimeters of soil in the drill row where they often hide during the day. Searching at night is recommended. If a spray is needed, cutworms are most active just after dusk and will therefore get maximum exposure before the chemical residual declines. This is particularly important when using pyrethroids, which have little residual activity

Lucerne flea

Lucerne fleas (Sminthurus viridis) hatch from their summer diapause following periods of good soaking autumn-winter rainfall.  Egg development is stimulated when the soil remains moist for at least three days and the average daily temperatures for the following 11 days remain below 22°C. This is unlike earth mites that have much cooler temperature thresholds. The early hatch this year in some regions of southern NSW and northern Victoria is likely to result in a second generation before winter, placing later-sown crops at risk.

Lucerne fleas typically cause significant damage to emerging crops and pastures. They can also cause considerable damage to older crops in spring if numbers build up under favorable conditions throughout the season. They are generally a problem in regions with loam/clay soils because of their egg-laying requirement. Paddocks are most likely to have problems where they follow a weed-

infested crop or pasture, in which lucerne flea has not been controlled. The insects are typically found in patches which can migrate across the paddock.

Lucerne fleas have a wide host range and will attack most broad-acre crops, including canola, lucerne, pastures, some pulses and cereals. Grass species (including cereals and pasture grasses) are less preferred hosts, but they can still damage seedlings. Young nymphs feed on the soft tissue on the underside of leaves leaving transparent ‘windows’, whereas adults and older nymphs rasp and chew irregular holes in leaves and can completely defoliate plants.

If chemical control is required, do not use synthetic pyrethroids. In paddocks where damage is likely, a border spray may be sufficient to prevent movement of lucerne fleas into the crop from neighboring paddocks. As lucerne fleas are mostly distributed patchily within crops, spot spraying is often all that is required. Do not blanket spray unless the infestation warrants it.

To avoid populations building up in subsequent years, consider some of the cultural control options. Grazing with livestock in spring to reduce the height of pasture will limit food resources and increase mortality of lucerne fleas. Control broad-leaf weeds (e.g. capeweed) to remove alternative food sources that would otherwise assist in population build-up. In pastures, avoid clover varieties that are susceptible to lucerne flea damage, and avoid planting susceptible crops such as canola and lucerne into paddocks with a history of lucerne flea damage.

Aphids

Exceptional populations of aphids have been building up in cereal, canola and pulse crops all season with the perfect growing conditions providing abundant resources. There are many aphid species and most are specific to the crops attacked.

Oat aphids (Rhopalosiphum padi), sometimes called cereal aphids or wheat aphids can be found on all cereals including wheat, barley and oats. They vary in color from olive-green to black and are often characterized by a dark reddish patch on the tip of the abdomen, although under some conditions this is not apparent. Oat aphids suck sap, causing yellowing and stunting of plants. A large amount of sugary solution (honeydew) is secreted by aphids when numbers are high. This can lead to black sooty mold which can reduce plant growth. Importantly, cereal aphids can spread barley yellow dwarf virus.

Cabbage aphids (Brevicoryne brassicae) grow up to 2.5 millimeters in length, with a dull grey-green body. They are a common pest of canola in many parts of Australia. Infestations start when winged aphids fly into crops from autumn weeds. These give rise to dense colonies on the flowering spikes, which appear bluish-grey and are covered with a fine, whitish powder. Cabbage aphids suck sap and can reduce yield when numbers are high.

The green peach aphid (Myzus persicae) is one species that can attack many crops but primarily canola, lupins and other pulse crops. They are usually sparsely distributed within a crop, mainly on the undersides of leaves. Adults have an oval shape body, are approximately three millimeters long and may be pale yellow-green, orange or pink in color. Winged adults have a dark patch on the abdomen. Green peach aphids are most common in autumn and seldom cause economic losses to crops through direct feeding. However, they are an important vector of several plant diseases. Many populations have developed resistance to insecticides and are proving more difficult to control with a range of registered products. In SA this season, the green peach aphid has caused losses by direct feeding and is also suspected of transmitting the Beet Western Yellow virus in canola.

False wireworm and bronzed field beetle

False wireworms (Tenebrionidae) attack a variety of crops including cereals and canola, and are mostly found in paddocks with high amounts of stubble and trash. There are large numbers of false wireworm species, which are similar in appearance. The most common in Victoria and New South Wales are the grey false wireworm, the eastern false wireworm and the bronzed field beetle.

False wireworm larvae are relatively fast moving, have a pair of prominent spines on the last body segment and vary in color from cream-yellow to brown-grey. They generally grow between 10-50 millimeters long, and have a hardened cylindrical body. False wireworms attack germinating seeds and chew on seedling stems and roots, weakening the plant or ring-barking the plant. They feed on crop stubble, so retaining crop residues may lead to an increase in false wireworm numbers compared to stubble burning. False wireworm damage usually occurs in crops following stubble incorporation or where stubble is retained on the soil surface. Stubble provides a food source for the wireworms as well as creating a favorable habitat by increasing moisture and shelter.

Crop damage will typically occur at the two-three leaf stage, and can result in numerous bare patches. These patches are randomly distributed across paddocks. Chemical control is often not warranted as the feeding damage across paddocks is sporadic and generally not severe.

The larvae of Bronzed field beetle (Adelium brevicorne) are problematic pests of germinating canola and lupins. They are up to 11 millimeters long and shiny black in color with a slight bronze appearance. The larvae are dark brown, up to 12 millimeters in length and three millimeters wide. They have twelve body segments, the last one having two distinct upturned spines. The larvae are often confused with the grey false wireworm, which is a serious pest in parts of southern New South Wales and Victoria. They need surface plant residue, provided by stubble retention systems, for shelter and breeding. Removal or dispersal of plant residue before egg-laying in late February to early March can reduce or eliminate bronzed field beetle populations. Despite the beetles being relatively easy to find in the paddock, there has been no feeding damage observed.

Bronzed field beetles attack canola, but typically do not affect cereal and pulse crops. Most issues occur when canola crops are sown into heavy stubble. The larvae damage plants by chewing on seedlings at or above ground level, and will also feed on plant roots. However, larvae can be present in the soil with little or no damage to plant seedlings. This is because they also feed on dead organic matter and high numbers are required to cause serious crop damage. Larvae are typically found within the top one centimeter of the soil and can be found under plant material, clods of soil, rocks or wood.

To check paddocks for Bronzed field beetles place refuges such as terracotta tiles or carpet squares in areas of high stubble or where previous damage has occurred. Leave these refuges in the paddock for several days before checking and counting the number of beetles present underneath.

Beneficial species 

Some beneficial species have become more important under the current different field conditions, in addition to the common species important in keeping pest populations in check (lady beetles and lacewings)

Carabid Beetles

Carabid beetles (Coleoptera) are important for bio-control of pests in agricultural crops throughout the world. They have been found in Australian broad-acre cropping systems where they are potential bio-control agents of several species of exotic pests. Studies have shown that carabid populations increase when tillage is reduced. However, the beneficial effects of carabids as predators are generally not seen due to the use of broad-spectrum insecticides that are very effective on flightless carabid species (Horne 2007).

Hoverflies (Syrpdidae)

Hoverflies attack a range of soft-bodied insects and are a particularly voracious predator of aphids. Hoverfly larvae are often mistaken for pest caterpillars such as diamondback moth. Hoverfly adults are four-ten millimeters long and have dark-colored flattened bodies with black and yellow markings. As the name suggests they ‘hover’ over objects and look similar to bees and wasps. Adults feed on aphid honeydew and pollen, and can also be a useful pollinator. Eggs are laid on the vegetation, in close proximity to pests. Larvae are maggot-like, legless grubs with no eyes and vary in color from cream to green or brown. Many have white stripes on their dorsal side, and they can be up to 10 millimeters long. Larvae move on the vegetation searching for prey. They grip and pierce their prey with their mouth hooks and suck out the body contents.

Parasitic wasps (Aphidius spp.)

This season we have seen an abundance of aphids in crops and pastures and these wasps are one of the significant parasitic insects that can suppress pest numbers. They often take a while to build up after pests appear but can affect large numbers in a short time when conditions are favorable for them.

Adult wasps are small (typically one-three millimeters long), and usually brown-black in color. There are many different species, which are difficult to tell apart. Female wasps insert their eggs into the bodies of live aphids, where the developing larvae feed on aphid body fluids. The aphid is eventually killed and the new adult wasp cuts a hole in the skin and emerges, leaving behind an aphid 'mummy'. The activity of aphid parisitoids is often first detected by the presence of these 'mummies', which appear as swollen, bronze-brown colored aphids.

References

Baker (2008) The population dynamics of Mediterranean snails Cernuella virgatta, Cochlicella acuta and Theba pisana in pasture-cereal rotations in South Australia: a 20 year study. Australian Journal of Experimental Agriculture 48 pg 1514-22.

GRDC  2014 The Back Pocket Guide (Crop Weevils)

GRDC factsheet Slugs in Crops,

Hoffmann A, Weekes,A, Nash M, Mangano P, Umina P. (2008) The changing status of invertebrate pests and the future of pest management in the Australian grains industry. Australian Journal of Experimental Agriculture 48 pg 1481-93.

Perry K. (2012) Managing invertebrate pests in new farming systems, GRDC Updates, Adelaide 2012

PestFacts(south-eastern)2006-2014(www.cesaraustralia.com/sustainable-agriculture/pestfacts-south-eastern)

Contact Details

Phil Bowden

Murrumbidgee Landcare

weeds@mli.org.au