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WANTED ALIVE: Beneficials

Beneficial insects will play a key role in keeping pest populations in check this spring; but who are they?


A lacewing larvae feeding ferociously on an aphid (Source: cesar)

 

Many groups of beneficial insects can be prevalent during spring and play a key role in pest control when low to moderate numbers of crop pests such as aphids and caterpillars are present.

Beneficial insects can be encouraged by reducing insecticide applications (particularly broad-spectrums), as well as providing alternate food sources and refuge habitats on farms.  

Some beneficial insects likely to be encountered this spring include:

Ladybird beetles

There are numerous types, but three species commonly found are the white collared ladybird, the common spotted ladybird and the transverse ladybird. Adults are round to oval shaped, with black spots on red, orange or yellow shells. Larvae have grey/black elongated bodies with orange markings and may be covered in spines or white fluffy wax material. Egg to adult stage takes 3-4 weeks, while adults can live for several months.

Ladybird adults and larvae are predatory, feeding on aphids, leafhoppers, thrips, mites, moth eggs and small caterpillars.

Ladybird beetle adult feeding on aphids in canola (left); larvae are more slender and are wingless (Source: cesar).

 

Brown & green lacewing

Adult brown lacewings are mottled brown in colour and 6-10 mm long, while adult green lacewings are 15-20mm long and pale to bright green in colour. Both have prominent eyes and long antennae. Their clear membranous wings are typically folded in an upside-down v-shape and are large with numerous veins giving a lacy appearance. Larvae lack wings, have protruding sickle-shaped mouthparts and a body that is long and varies from thin to stout in shape.

Lacewings attack aphids, thrips, mites, caterpillars and moth eggs. Brown lacewings adults and larvae are both predatory, while only green lacewing larvae are predatory.

Adult brown lacewing (left) and larvae (right) (Source: cesar

 

Parasitic wasps

Adults vary in size (1-20 mm long) and colour ranging from bright orange to completely shiny black. They have two sets of wings that are clear or dark coloured. Female wasps often lay their eggs into host larvae or eggs. The developing wasp larva feeds inside the host, usually aphids or pest caterpillars. Aphid ‘mummies’ (bronze-coloured, bloated/enlarged aphids) indicate the activity of aphid parasitic wasps that are small, usually dark in colour and difficult to detect.

Beneficial wasp parasitising an aphid (left) and an aphid ‘mummy’ (right) (Source: cesar)

 

Hoverfly

Adult hoverflies are 4-7 mm long, have dark-coloured flattened bodies with black and yellow markings, and have only one set of wings (typical of flies). As the name suggests they ‘hover’ over objects and look similar to bees or wasps. Larvae are legless, green in colour, 8-10 mm long and appear grub-like. They are often mistaken for pest caterpillars such as diamondback moth but lack the typical head capsule of caterpillars.

Hoverfly larvae attack a range of soft-bodied insects, but prefer aphids. They are common in flowering crops such as canola, pasture paddocks and on some roadside flowering weeds.

Hover fly adult (left) and larvae (right) (Source: cesar)

 

Damsel bugs

Adults are 8-12 mm long and move quickly when disturbed. They have a slender light-brown body with long antennae and large protruding eyes. Juveniles are similar but smaller in size. They have a long curved ‘snout’ that is carried under the body when not feeding. 

Damsel bugs feed on a range of soft-bodied prey items including small caterpillars, moth eggs, aphids, and mites. 

Damsel bug (Source: cesar)

 

Predatory shield bug

There are several species that vary in size and shape. Adults are 10-15mm long and have shiny, shield-shaped bodies, often with patterns and spikes. Juveniles are dark red and brown with the early instars being bright red. There are multiple generations per year and adults usually live for several months.

Shield bug adults and larvae are predators of soft-body insects, particularly caterpillars, moth eggs and aphids.

Predatory shield bug (Source: cesar)

 

Our advice

So how many beneficials is enough (to keep pest numbers below damage thresholds)? Sadly, there is no simple answer. There are currently no predator:prey ratios that would guide management decisions. However, there are some guiding principles to assist:

-       Most beneficial species are highly mobile and will move from crop to crop if left unsprayed.

-       There is often a ‘lag’ time between the growth of pest populations, and increases in abundance of beneficials. This is particularly so in southern cropping systems in spring.

-       Monitor crops regularly enough so you can measure whether the relative rate of increase in beneficial insects (per sweep, per m, etc.) is faster (or slower) than that of pest populations. In the former case, the beneficials are winning!

-       Beneficials are more likely to suppress caterpillar pests in the egg and younger stages than the larger instars. This said, the larger caterpillars are still prone to parasitic wasps, predatory bugs and ground beetles.

-       Aphid parasitic wasps: it takes around 10 days from the time the wasp ‘stings’ an aphid until the aphid turns into a mummy. Recording the relative number of mummies and live aphids provides an indication of the impact wasps are having on the aphid infestation.

-       Some ladybird beetles species consume >2000 aphids in their lifetime. Consider delaying a chemical application and monitor again in 7 days time. It is amazing how quickly pest populations can crash when beneficials are left alone. 

For further information and images of beneficial invertebrates see the GRDC Beneficial Insects – The Back Pocket Guide, and cesar’s insect gallery.  

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