Spring brings friends alongside foes

Aphids will increase with the arrival of spring…and beneficials will be hot on their tails!

Autumn and winter aphid activity was relatively low in Victoria and southern NSW this season, but this trend is not expected to last – populations will likely escalate with the arrival of milder temperatures this spring.

The usual aphid suspects in spring are cabbage aphid and turnip aphid in canola, bluegreen aphid, pea aphid and cowpea aphid in legumes, and oat aphid, corn aphid, and Russian wheat aphid (RWA) in cereals.

But it’s not all bad news.

With spring comes a boom in natural enemies on the hunt for aphids (and other pests). Here are a few of these beneficial groups found throughout south-eastern Australia:

Ladybird beetles

Found in all crops, ladybird beetle larvae and adults are voracious predators of aphids, as well as leafhoppers, thrips, mites, moth eggs and small caterpillars.

Ladybird beetles undergo a significant transformation as they mature. As larvae they have grey/black elongated bodies with orange markings and may be covered in spines or white fluffy wax material. As adults they are round to oval shaped, with black spots on red, orange or yellow shells.

Ladybird beetle adult feeding on cabbage aphids (left) and ladybird beetle larva (right) (Image credit: cesar)

 

Parasitic wasps

Female wasps often lay their eggs into aphids (and caterpillars). The developing wasp larva either feed inside the host or hang on to the outside of the host while feeding. Aphid ‘mummies’ - bronze-coloured, bloated/enlarged aphids - indicate the activity of aphid parasitic wasps.

Aphid parasitic wasps are specialists in their trade, usually attacking just a single pest species. They can only live where and when their hosts occur.

 

Parasitic wasp laying an egg inside a live aphid host (left) and an aphid mummy (right) (Image credit: cesar)

 

Hoverflies

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.

Hoverflies are most noticeable in the adult fly form, when they have dark-coloured flattened bodies with black and yellow markings. But it is the larval ‘grub’ stage that is predatory. As larvae they are often mistaken for pest caterpillars such as diamondback moth but lack the typical head capsule of caterpillars.

Hoverfly adult (left) and larva (right) (Image credit: cesar)

 

Lacewings (brown and green)

Lacewings are voracious predators and will try to eat almost any insect standing in their path, including aphids, thrips, mites, caterpillars and moth eggs. Brown lacewings adults and larvae are both predatory, while only green lacewing larvae are predatory – the adults becoming nectar and pollen feeders following pupation. After sucking the internal contents of their prey, lacewing larvae impale their victim’s exoskeletons on their backs. It has been suggested that brown lacewing larvae can eat between 100 – 200 aphids during their lifetime (Horne & Page 2008).

Like ladybird beetle and hoverflies, lacewings undergo a significant transformation as they mature. Larvae lack wings and have protruding sickle-shaped mouthparts and a body that is long and varies from thin to stout-like in shape. As adults they have prominent eyes and long antennae, with large, clear wings with numerous veins giving a lacy appearance.

 

Lacewing adult (top) and lacewing larva eating an aphid (bottom) (Image credit: cesar)

 

Damsel bug and predatory shield bugs

Damsel bug and predatory shield bugs are generally found in the canopy of crop plants, feeding on a range of soft-bodied prey items including small caterpillars, moth eggs and aphids.

Damsel bugs 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

There are several species of predatory shield bugs that vary in size and shape. Adults have shiny, shield-shaped bodies, often with patterns and spikes. Juveniles are dark red and brown with the early instars being bright red.

An adult damsel bug (top) and a predatory shield bug (bottom) (Image credit: cesar)

 

For more information on beneficial species in crops visit our insect gallery.

So how many beneficials is enough to keep aphid numbers below damaging levels? Unfortunately, there is no simple answer. Currently, there are 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. If the former is the case, the beneficials are winning!