Insect pests of vegetable brassicas in Western Australia

Introduction    

This article will help you identify pests and their natural control agents in your vegetable brassica crops — cauliflower, broccoli, Brussels sprouts, cabbage, Chinese cabbage, swedes and turnips.

The aim is to provide growers with the tools to assist in preventing crop damage and unnecessary application of pesticides.

It is divided into five sections:

• Identification of pests, their characteristic damage and natural control agents — colour photographs and descriptions to identify the pests and their characteristic damage, and their natural control agents.
• Seasonality of pests — which pests are present at different times of the year.
• When to monitor pests within a crop cycle — helps with monitoring individual crops in terms of the crop stage when particular pests are most likely to be present.
• Recommended techniques for pest monitoring and pest management — principles to assist in choosing appropriate monitoring methods and control strategies for each pest.
• Pest and beneficial insects of vegetable brassica crops: common and scientific names of all pest and beneficial agents mentioned, where names are available.

Identifying pests and natural control agents

This section includes photographs and descriptions of pests and their charactertic damage, and associated beneficial insects.

By correctly identifying insects in crops, the most appropriate control strategy for a particular pest can be used. If this guide provides insufficient information to enable a confident identification to be made, seek further assistance. For example, in the Manjimup region in 2002, a pest new to industry was identified, a Staphylinid beetle that has yet to be named.

Pests are often more abundant in some regions than others. Many of the pests discussed here occur mainly in lower South-West and Great Southern areas because these crops are grown either in rotation with pasture or adjacent to areas of pasture from which the pests originate. Other insects occur as pests mainly in crops  in the metropolitan and Swan Coastal Plain regions. To clarify the locality of pests specific to each region, they are referred to as "South" or "Metro". If the pest occurs in all areas, no reference to location is made.

Insects affecting crop establishment

Figure 1 Wilting cauliflower seedling caused by root feeding whitefringed weevil grub

Figure 2 Root damage to cauliflower seedling by whitefringed weevil larva

Establishment pests include many beetle adults and larvae which occur principlally in the South, molluscs (common garden snail and slugs) and larvae of cutworm moths. The damage occurs as below-ground root pruning and/or stem girdling, resulting in the death of seedlings. Leaf feeding can also cause damage and retard plant growth, which may be severe enough to kill seedlings.

Figure 3 Adult and grub stages of weevils: from left to right – apple weevil, garden weevil, whitefringed weevil, Fuller’s rose weevil and vegetable weevil

The range of weevil pests in both adult and larval stages that can affect crop establishment is shown in the photo above. From left to right they are the apple weevil which can be important, garden weevil which is a minor pest, whitefringed weevil which can be important, Fuller's rose weevil which is a minor pest and vegetable weevil which can be important. Note that we cannot readily distinguish between the brown headed larvae of apple weevil and garden weevil, and the early stages of the white headed weevil larvae whitefringed weevil and Fuller's rose weevil.

Apple weevil (South)

Figure 4 Apple or garden weevil grub

Apple weevil grubs grow to about 10mm long, do not have legs, live in the soil and have a brown head capsule. They kill seedlings and young transplants by chewing through the stem below ground.

Before seeding or transplanting assess their abundance by examining 50 to 100 square spade soil samples on a grid pattern across the area in which the crop will be grown. There is no established threshold, but it is probably worth taking some action if 5 to 10% of samples have larvae. These larvae are similar in appearance to garden weevil, which has not been reported to be a pest to date.

Apple weevil adults are dark brown (see photo of adults and larvae above), about 8mm long and nocturnal. All adults are female and do not fly. They survive well in pastures in the South-West.

During the day, they burrow into the upper soil layer. They damage seedlings and young transplants by feeding on leaves, the growing tip or ringbarking the main stem. Dig around the base of a damaged plant to assess the abundance of burrowing adults. If crop damage cannot be associated with any pest seen during the day, check for weevils feeding on plants at night.

Whitefringed weevil (South)

Whitefringed weevil larvae (see photo of adults and larvae above) do not have legs, live in the soil and have a white head capsule. They grow to about 15mm long.

They kill seedlings and young transplants by chewing through the stem below ground. Assess their abundance in the same manner as for apple weevil larvae, using the same threshold for action.

Adults are grey with a pale or white stripe down each side and are about 15mm long (see photo of adults and larvae above). All adults are female and do not fly. They may feed on the leaves of brassica crops, but are not a major pest. Adults are active day and night.

Vegetable weevil

Figure 5 Vegetable weevil grub

Vegetable weevil larvae do not have legs, range from cream to green and grow to about 15mm long (Figure 5). They have a dark brown/black head capsule and a brown plate across the top of the thorax, just behind the head. These larvae, which feed at night, can damage leaves. Dig around the base of damaged plants during the day to find larvae just below the soil surface.

Adults are grey with two diagonal bands near the tip of the abdomen (see photo of adults and larvae above). The weevils are about 10mm long, feed at night and damage leaves.

Check for these weevils by looking around the base of the plant by day or at night when the adults may be seen feeding on the plants.

Small lucerne weevil (South)

Figure 6 Small lucerne weevil adults

Small lucerne weevil adults are grey and similar in shape to whitefringed weevil adults, but are only about 8mm long. All adults are female and do not fly. They damage seedlings and young transplants by leaf and stem feeding.

Monitor for these pests during the early stages of crop establishment where plant damage is noticed. Adults are active day and night.

Spotted vegetable weevil (South)

Spotted vegetable weevil larvae have a brown head capsule and a spiracle or air pore under each abdominal lateral spine

Larvae of spotted vegetable weevil are legless, have a brown head capsule and can be distinguished from larvae of apple weevil and garden weevil by the presence of a breathing pore under each lateral abdominal spine. This pore may be seen using a ten times hand lens or microscope.

These soil borne larvae can be very abundant and may be mistaken for other more important soil pests. The pest status of vegetable weevil larvae is thought to be minor.

Figure 7 Spotted vegetable weevil adult

Spotted vegetable weevil adults are speckled grey/black and about 5mm long. They damage brassica crops by feeding on leaves and stems of young plants. When damage is noticed, look for these pests at the base of the plant during the early stages of crop establishment. Adults are active day and night.

Vegetable beetle (South)

Figure 8 Vegetable beetle grubs

Vegetable beetle larvae or false wireworm have three pairs of legs on the thorax. They are long, thin, shiny, hard-skinned and live in the soil, although may be found just under surface organic matter. They may damage seedlings or young transplants by feeding on the stem, but are not usually a pest.

Figure 9 Vegetable beetle adults (left) and African black beetle adult (right) feed on the stem near ground level and leaves of a young cauliflower

Adults are hard-shelled, slightly flat, ground-dwelling beetles about 10mm long, which can fly. During the day they may be feeding or resting under clods of soil or rocks, or under organic matter. They can kill or retard plant growth by feeding on leaves and stems of transplanted or recently emerged seedlings.

African black beetle (South)

African black beetle adults (see photo above) are shiny black, hard-shelled beetles about 10–15mm long. They live in soil but also crawl on the soil surface. They can fly and often undertake mass dispersal flights, sometimes in spring but more commonly in late March and April. They can kill seedlings and young transplanted plants by feeding on the stem at or just below ground level.

Snails and slugs

Figure 10 Common garden snails

Figure 11 Slug

Common garden snails and slugsfeed on leaves of young plants, retarding growth. But they are more important because they feed on the harvested portion of the crop (head or curd) rendering it unfit for sale. Monitor before planting by an even placement of mounds of crushed grain, cereal bran or snail/slug bait across the crop area.

Cutworm

Figure 12 Black cutworm grub

Cutworm grubs are soft-bodied moth larvae, with variable colour from pink to grey to black. They have three pairs of legs on the thorax.

They kill young plants by feeding on the stem at or just below ground level. Check for these grubs by digging around the base of damaged plants during the day or check for grubs feeding at night.

Insects affecting crop growth and quality

A range of insects feed on leaves of brassica plants of any age, which may retard plant growth.

Figure 13 Leaf damage caused by cabbage white butterfly grubs

The extent of leaf damage by larvae of the pest diamondback moth to cauliflowers can be seen in this photo where the section of the crop in the front left was not treated with insecticide.

Figure 14 Cauliflower curd damage caused by diamondback moth grubs

Some leaf feeding pests feed directly on the harvested part of the crop such as this damage to a cauliflower curd by diamondback moth larvae, resulting in direct crop loss through rejection.

Diamondback moth

Figure 15 Diamondback moth (DBM) adult

Diamondback moth adults are small, grey/brown moths about 10mm long. When the wings are folded over the back of resting moths, they form a diamond pattern.

Figure 16 Pheromone trap for adult diamondback moths 

Pheromone traps to monitor diamondback moth adults help define the time of arrival of spring moths. Later in the season, the number of moths caught may indicate the level of pest pressure and assist with monitoring crops for larvae and deciding whether sprays are required.

One trap per crop near the crop edge and at crop height is sufficient to gauge moth numbers. Adjust trap height to be the same as the crop as it grows. Check traps weekly. Traps should be considered secondary to regular crop scouting for larvae.

Diamondback moth eggs are laid on any part of the above-ground brassica plant, but usually on the upper side of cauliflower leaves. They are relatively flat, about 0.5mm long and can be laid singly, but usually in groups of two to six.

Figure 17 Newly-laid diamondback moth eggs

Figure 18 Older diamondback moth eggs

When first laid eggs are white to yellow but turn dark yellow/grey as the grub inside develops, showing the head capsule of the grub. After eggs hatch the transparent shell remains. Use a x10 hand lens to help identify eggs.

Figure 19 Diamondback moth first instar grub

Figure 20 Leaf mining caused by newly emerged diamondback moth grubs

After hatching, diamondback moth first instar grubs commence feeding by leaf mining. This gives a ‘scribble pattern’ of lines to the leaf. Very young grubs can be distinguished from cabbage white butterfly grubs (see photo below) by the presence of a dark band just behind the head. The dark band is a series of hairs not present on young cabbage white butterfly grubs. Use crop monitoring charts to assess the need to spray to protect crops from attack.

Figure 21 Diamondback moth mid-size grubs

Diamondback moth larvae grow to 15 to 20mm long. They are grey-green and soft-bodied. They exhibit rapid forward or backward movement or may hang from a leaf by a silk thread when disturbed.

They feed on leaves of brassica crops which vcan be important if infestations are heavy during crop establishment. But they are also important because they can damage the harvested part of the crop - leaves, if these remain with the curd, and the curd itself. This considerably reduces quality and increases rejection of product.

Figure 22 Diamondback moth pupa (courtesy Mike Furlong)

When mature, diamondback larvae pupate inside a woven silk cocoon on any above-ground part of brassica plants. Pupae are around 10 to 12mm long.

Cabbage white butterfly

Cabbage white butterfly adults on wild radish

Adult cabbage white butterflies are large day-flying butterflies about 25mm long, with a wingspan of about 50mm. They are cream-coloured. Females have two black spots on the forewing and males one. Both have black tips on the forwing and one black spot on the hind wing.

Figure 24 Cabbage white butterfly eggs

Eggs look like sharp-pointed rockets about 1mm high. They are laid on leaves and are white when first laid but later darken to yellow.

Figure 25 Young cabbage white butterfly grub

Early stage larvae of cabbage white butterfly larvae do not have dark hairs behind their heads as is the case for diamondback moth early stage larvae. Cabbage white butterfly larvae are green with yellow stripes alongg their back and sides and covered with hairs. They grow to 25 to 30mm long.

Figure 27 Faecal deposits by cabbage white butterfly grubs on a cauliflower curd

Larvae of cabbage white butterfly are mainly leaf feeders, but their faecal deposits may foul the harvested parts of brassica vegetables.

Figure 26 Older cabbage white butterfly grub

Pupae form at the completion of grub development on the above-ground part of a brassica plant. The grub weaves a silken thread attached to the leaf to support and anchor the pupa while it develops into a butterfly.

Onion maggot (South)

Figure 28 Onion maggot adult

Onion maggot adults are grey flies, about 2 to 3mm long. They are difficult to detect in brassica crops because other very similar flies such as ferment flies are often present. The maggots of these other flies may feed on decaying organic matter in or near the crop. Ferment fruit fly or drosophila fly, which is honey-brown with red eyes, is easily distinguished from onion maggot flies.

Figure 29 Onion maggot

The larval stage of onion maggot feeds on cauliflower curds producing unsightly brown grooves, rendering curds unfit for sale. The feeding of the maggot provides an entry point for disease and soft rot may develop on the curd.

Figure 30 Onion maggot pupa

Onion maggot may pupate on cauliflower curds.

Curd-feeding fly (South)

Figure 31 Cauliflower curd feeding fly maggot

Another species of curd-feeding fly has small brown to red worm-like larvae or maggots which cause similar damage to onion maggot. The adults of this fly are even smaller than onion maggot adults but are also grey and not readily detected. This species is very uncommon and has not been identified.

Cabbage aphid and green peach aphid

Figure 32 Cabbage aphid

Figure 33 Green peach aphid

Cabbage aphid and green peach aphid are pests of brassica crops because numbers build up and retard plant growth during establishment of crops, and their presence reduces the quality of the harvested crop.

In the cool moist conditions of spring, aphids are likely to be held in check by natural enemies.

In autumn, they are more likely to build up to damaging numbers before natural enemies control them. Turnip aphid, which appears similar to cabbage aphid, can also infest vegetable brassica crops.

Cabbage centre grub (Metro)

Figure 34 Cabbage centre grub moth

Cabbage centre grub moths are about 12mm long and rapid fliers. They are light brown with dark brown markings. Cabbage centre grub is a minor pest of WA vegetable brassicas and has been restricted to the Perth and northern regions of the state. It is more often seen during hot weather.

Figure 35 Cabbage centre grub

The cabbage centre grub larva is cream-coloured with red to brown longitudinal stripes. Young larvae have a black head capsule and a small black mark behind the head. Older grubs have a red to brown head capsule and reach up to 12mm long.

They sometimes tie the leaves together with webbing or create large blisters inside the leaf. They may also burrow into the growing point or the main vein of a leaf. Larvae pupate on the plant within their feeding tunnel.

Heliothis moth

Figure 36 Heliothis moth

Heliothis moths are stout, buff-coloured, strong flying, moths, about 20mm long.

Figure 37 Heliothis eggs

Eggs of heliothis are dome-shaped and white when first laid, and turn pink then almost black before hatching. If parasitised by a small wasp, they turn completely black.

Figure 37 Heliothis grub

Heliothis larvae vary in colour from green, through yellow to brown and black. They have a leathery skin and a prominent light stripe along each side. These larvae damage brassica plants by feeding on the growing tip, heads and leaves. They are not a common pest of vegetable brassica crops.

Looper grubs

Figure 38 Tobacco looper grub

Looper larvae are bright green and grow to about 35mm long. They have legs on the thorax and abdomen and move in a characteristic looping action. They feed on leaves but their faeces may foul the harvested parts of brassica crops. They are not a common pest of vegetable brassica cops.

European earwig (South)

Figure 39 European earwig

European earwig has a black body and distinctive yellow legs, pincers and shoulders. Adults are 12 to 20mm long. Males (left) have curved pincers while females have long straight pincers. They feed on leaves and cauliflower curds. Their faeces may foul harvested portions of crops. They are not a common pest and can be distinguished from the native earwigs, which are reddish brown. European earwigs are often present in large numbers but preatory earwigs are usuall solitary.

Staphylinid beetle (South)

Figure 40 Staphylinid beetle adult on a cauliflower leaf

Figure 41 Staphylinid beetle adult close up (under a microscope)

Staphylinid beetle is a relatively new pest of cauliflower in the South-West. Adults are small, 1 to 2mm long, black elongate beetles which can fly. Little is known of their biology but it is likely the larval stage feeds on decaying organic matter in moist situations near brassica crops. Adults would fly into nearby crops and may reach heavy infestations over a short period of time.

Figure 42 Cauliflower curd damaged by Staphylinid beetle adults

Adults browse on the curd, rendering it brown and unmarketable. By the time damage is noticed, control is impossible. Should this insect become a consistent pest, monitoring methods will need to be developed to detect the pest early enough to prevent damage.

Figure 43 Destroy crop residues

Destroying crop residues as soon as possible after harvest is completed can help reduce the risk of pests building up in these unsprayed areas which can infest nearby crops.

Beneficial natural control agents

For diamondback moth

Figure 44 Diadegma wasp stinging diamondback moth larva

Figure 45 Diadegma wasp cocoon

Diadegma, a wasp parasite of diamondback moth larvae, is the most abundant of the natural control agents for this pest. Once the wasp parasite larva completes its development inside the diamondback larva, it emerges to spin a cocoon within the silken case produced by the host diamondback moth larva.

Figure 46 Apanteles wasp

Figure 47 Apanteles parasite cocoon

Apanteles, another species of wasp parasite that attacks diamondback moth larvae, is also abundant. The wasp larvae pupate in small white cocoons.

Figure 48 Parasitic fungus of Diamondback moth

A fungus, Zoophthora radicans can kill larvae and pupae. Infected larvae disintegrate on the plant. Mass larval deaths from this fungus occur in moist conditions and where larval density is high.

For cabbage white butterfly

Figure 49 Cabbage white butterfly pupa showing grubs of a wasp parasite

Figure 50 Cocoon of wasp parasite of cabbage white butterfly pupae

Grubs of a wasp parasite of cabbage white butterfly emerge from a cabbage white butterfly pupa (Figure 50). These wasp grubs then spin white cocoons (Figure 51). The adult wasp chews a hole in the cocoon and emerges.

For aphids

Figure 51 Aphid wasp parasite

Wasp parasites help keep aphid numbers in check. The wasp completes the larval stage of its life cycle within the aphid body producing an aphid ‘mummy’ from which the adult wasp cuts an emergence hole.

Figure 52 Predatory ladybird adult

Predatory ladybird adults are usually brightly coloured orange and yellow with black stripes. Adults are around 7mm long.

Figure 53 Predatory ladybird egg cluster

Predatory ladybird eggs are bright yellow and are laid in groups.

Figure 54 Predatory ladybird grub

Predatory ladybird larvae are common predators of aphids as well as eggs and young stages of other insect pests. They are 8–10mm long and grey with yellow bands.

Figure 55 Hover fly

Adult hover flies are about 10mm long with a black and yellow striped abdomen. Adults are not predatory.

Figure 56 Hover fly maggot

Hover fly maggots are predators of aphids.

Figure 57 Fungal disease of aphids

A fungal disease can kill aphids. The disease is normally only seen in the more humid conditions of spring.

Seasonality of pests

Pest monitoring and treatment calendar

This table indicates the times of year the major pests of vegetable brassicas are likely to be present.

This is a guide only:

• Not all these pests will occur in your crop.
• The status of each pest varies across districts.
• Monitor throughout the year to avoid unnecessary or poorly timed spraying.

When to monitor pests within a crop cycle

Note:

• Monitoring and applying management options at the correct time will help to prevent damage to crops.
• Continue to monitor regularly throughout the crop cycle.

Pest monitoring and management

Before planting

After crop emergence/transplanting

Pest and beneficial insects: common and scientific names

Pests

Beneficial agents

Acknowledgement

This information was originally compiled by Stewart Learmonth, Françoise Berlandier and Rachel Lancaster.