PestFacts 21 September 2023

Parasitic wasps and entomopathogenic fungi on aphids

Large colonies of aphids have been reported on canola at Tumby Bay, but luckily most of them have been attacked by parasitic wasps and entomopathogenic fungi (fungi that kill insects).

Parasitic wasps

Several species of parasitic wasps lay their eggs into aphids. The developing wasp larvae either feed inside the host or hang on to the outside of the host while feeding. Aphid 'mummies' – bronze-coloured, bloated or enlarged aphids – indicate the activity of parasitic wasps and when a new wasp is about to emerge. Even with few mummies visible, parasitism can be quite high, simply because the mummy is only the very last stage of wasp development.

Entomopathogenic fungi

An entomopathogenic fungus is a fungus that attaches to the body of an insect and under the right conditions of temperature and (usually high) humidity they grow within the insect, typically killing it eventually. Dense aphid colonies often produce favourable conditions for entomopathogenic fungi, which can be very helpful in limiting aphid numbers.

Source of reports: Chris Pearce (Nutrien Tumby Bay)

Monitoring aphids in canola and threshold guidelines

Monitoring for canola aphids should begin in crop edges as these are typically infested first. Inspect at least 20 plants at 5 sampling points over the paddock  .

• Cabbage aphid (Brevicoryne brassicae) colonies have a characteristic blue-grey appearance and are normally covered in a thick, whitish powder.
• Turnip aphid (Lipaphis pseudobrasssicae) colonies have a lighter covering of wax and appear green in colour.

Threshold guidelines for cabbage aphid and turnip aphid:

• consider control where more than 20% of plants are infested, or
• more than10% of plants with greater than 25 mm of stem infested.

When determining economic thresholds for aphids it is critical to consider several other factors before deciding:

• growth stage
• current growing conditions
• moisture availability
• populations of natural enemies or beneficials.

Drought stressed plants often have higher numbers of aphids.

Other natural enemies that prey on aphids include ladybird beetles, hoverfly larvae, lacewings, predatory bugs, and earwigs.

Cabbage white butterfly activity

Cabbage white butterflies (Pieris rapae) have been reported flying around bean paddocks at Tumby Bay. Caterpillars feed on cruciferous plants (such as cabbages and canola). They will not feed on beans.

Identifying cabbage whites

Cabbage white butterfly adults are creamy white in colour with black spots on the centre of each forewing (1 spot for males, 2 for females). The tips of the forewings are also a light charcoal colour which is easily visible when their wings are open.

Mating occurs during spring and eggs are laid on the underside of the leaves of brassica crops. The eggs are yellow in colour, bottle-shaped and are laid singly or in small clusters. The larvae grow to 30 mm in length, are covered in short white hairs, and are a velvety green colour with a faint yellow stripe running along their back and the sides of their body.

Source of reports: Chris Pearce (Nutrien Tumby Bay)

Native budworm update

Over the last month, native budworm (Helicoverpa punctigeria) has been reported at Kimba on pre-flowering field peas and early moth flights and young caterpillars reported at Kadina. Current budworm trapping has not detected flights over the past week on the Upper Yorke Peninsula and has seen low numbers (76 moths) on Lower Eyre Peninsula.

Identifying native budworm

Native budworm caterpillars can vary substantially in colour that includes shades of brown, green and orange. They usually have darkish stripes along the body and bumpy skin with sparse, stiff, stout hairs. They grow up to 40 mm in length, with a sharp downward angling in the last segment of the body.

Similar caterpillars include the corn earworm (H. armigera) and lesser budworm (Heliothis punctifera). Native budworm can be recognised by the black hairs around the head and along the body, compared to the white hairs around the head on both corn earworm and lesser budworm.

Armyworm activity

Armyworm caterpillars have been reported at Mannanarie with patches in cereal paddocks grazed to the ground, and at Hallett in wheat where numbers were above threshold.

Identifying armyworm

There are three main species of armyworm that are broadacre crop pests  and are difficult to tell apart in the field but their feeding habits and therefore management is similar. Armyworms can be identified from other caterpillars by the 3   longitudinal stripes from the collar (behind the head) running down the body. They can grow up to 40 mm in length.

Fall armyworm (FAW, Spodoptera frugiperda) has not been detected in South Australia. They have the distinctive 3 stripes but can be told apart from other native armyworms by an inverted Y-shape on the head and a distinctive pattern of 4 large raised dark spots on the second to last body segment.

Source of reports: Steve Richmond (Nutrien), Damien Sommerville (Sommerville Partners).

Russian wheat aphid in cereals

High numbers of Russian wheat aphid (RWA, Diuraphis noxia) have been reported in a drought-stressed cereal crop alongside white heads at Loxton. The plants are at growth stage 40 (GS40) and aphids are present inside rolled leaves.

When considering how to manage RWA the Russian wheat aphid action threshold calculator can help. For this situation the relevant factors include:

• the crop is likely to be low yielding (estimated at 1.2 tonnes per hectare)
• a spray would have to be a specific pass and therefore control application costs are likely to be higher (estimated at $20 per hectare)
• aphids inside the rolled leaves are difficult to reach, so pesticide application may not reach all the aphids, if spraying a high volume (at least 100l/ha) would be required
• aphid infestation might progress if there are rolled leaves in the crop and drought persists. RWA calculator expects an increase until GS50.

The RWA calculator (based on current wheat price level) would suggest that a spray in these conditions is justifiable only if there are greater than 14% of tillers with aphids that could increase to 20% at GS50.

In recent years drought stress in cereals has favoured RWA. It was also reported that aphids were still present in other paddocks that had received a pesticide, likely due to rolled leaves reducing application and not enough spray volume.

To learn more on how to use the RWA action threshold calculator for your situation see PestFacts 4 September 2023.

Identifying RWA

Often the first signs of RWA are symptoms from feeding, such as striped leaves. White heads can also be a symptom, but this can also be caused by frost damage. The best indicator of RWA is the trapping of heads by leaves.

RWA can grow up to 2 mm in length, are pale green in colour, elongate and spindle-shaped. The siphunculi ('exhaust pipes') are very short and initially appear absent to the naked eye. They have two caudal processes (short tails) on the end of the abdomen, one above the other, giving the appearance of a double tail.