Native budworm

Adult moth

The adult form of the native budworm is a moth that has rapid, low-level flight in a zig-zag path and ends with a dive into a crop or shrubs.

The moths have light brown, patterned forewings, and mostly pale hind wings with a black patch at the tail end.

Eggs

White spherical eggs (0.5 mm) are laid singly, mostly near the top of the plant. The eggs darken as they mature and tiny caterpillars hatch after about 7 days.

Caterpillars

The young caterpillars feed on leaf or pod material for about 2 weeks before they are large enough (5 mm long) to be noticed in the crop. It takes a further 4 weeks until they are fully grown (40 mm long); about 7 weeks from the time of egg laying.

These development times are based on average spring temperatures when caterpillars are active in central cropping areas of WA. Later in the season, or in more northerly areas, developmental rates for caterpillars will be faster.

The caterpillars vary greatly in colour from green through orange to dark brown and are often seen with their heads inside pods. They usually have dark stripes along the body and are sparsely covered with fine bristles.

New moth flights and egg laying result in caterpillars of varying sizes in a crop. Caterpillars eat increasing quantities of seed and plant material as they grow, with the last 2 growth stages (fifth and sixth instar) responsible for eating over 90% of their total grain consumption.

Pupa

When fully mature, the caterpillars crawl to the ground, burrow into the soil and pupate. The length of the pupal stage depends on several environmental factors and varies from 2 weeks to several months.

Mortality of eggs and caterpillars

Only a small proportion of eggs and small caterpillars survive to the damaging large caterpillar stage. Eggs may be dislodged, and small caterpillars may become stuck or drown due to wet weather.

Predators and parasitoids also affect population numbers and can suppress them below economic damage thresholds. If numbers of native budworm are high, populations can crash due to viruses and disease.

Native budworm caterpillars most frequently attack the fruiting parts of plants but also feed on the terminal growth, flowers, and leaves.

All pulse and canola crops grown in Western Australia, including field pea, faba bean, lentil, chickpea, lupin, and canola, are susceptible to the pest, especially when pods are present.

Serradella, lucerne, clover and annual medic seed crops, and a large range of horticultural crops, may be affected.

Losses attributed to native budworm come from direct weight loss through seeds being wholly or partly eaten.

Grain quality, especially faba bean and albus lupin pods, may also be downgraded through unacceptable levels of chewed grain or fungal infections introduced via caterpillar entry into pods.

The percentage of broken, chewed, and defective seed found in grain samples affects the final price of pulse crops marketed for human consumption. This applies particularly to the large-seeded crops, such as faba bean, kabuli chickpea, and field pea.

Adult moths

Male moths are easily captured in pheromone (female sex scent) traps. These traps are maintained by department staff and volunteer farmers and provide an early warning of moth arrival and abundance.

Caterpillars

Sampling of crops to determine the abundance of caterpillars is essential. The quickest and easiest method to sample most crops is to sweep with an insect net, taking 2 m long sweeping arcs, using the standard net size (380 mm diameter).

Multiples of 10 sweeps should be taken in several parts of the crop. If more than 10 consecutive sweeps are made, there are likely to be too many dead flowers and leaves in the sample to easily locate the small caterpillars.

Sweep netting is most efficient in short and thin crops and less efficient in tall dense crops. It is important to keep the lower leading edge of the sweep net slightly forward of the net opening so dislodged grubs are picked up and carried into the net.

Field pea crops

To monitor for native budworm caterpillars in:

• semi-leafless field pea crops, use grub counts based on 20 sweeps of a sweep net
• for trailing type field pea crops, use 10 sweeps of a sweep net.

The intertwined tendrils of semi-leafless peas are more difficult to sample. Sweeping efficiency in semi-leafless peas is about half the efficiency of that in conventional trailing types like Helena.

Lupin crops

Narrow-leafed lupin crops will not be damaged until they are close to maturity and the pods are losing their green colouration. Until then, it will be difficult to use a sweep net as the plants are stiff and the pods are spiky.

An alternative to sweeping to count caterpillar numbers, is to cut plants from several places in the crop and at each sample point, to shake plants into a bin.

The easiest method for assessing damage levels for the entire crop is to take small areas from the crop, either a tenth of a square metre, or an equivalent number of plants to those found in a square metre.

When to spray

The decision to spray for:

• field pea, chickpea, lentil, and faba bean crops needs to be considered from the time of first podding
• lupin and canola crops can be left until pods are beginning to mature.

If caterpillar numbers are below the threshold levels provided in the Thresholds section below, the decision to spray should be delayed and periodic sampling continued. One well-timed spray to control native budworm caterpillars should be sufficient in most situations.

Sweep netting of the crop should be carried out after spraying to confirm that the required level of control has been obtained. Effectively applied synthetic pyrethroids will prevent reinfestation for up to 6 weeks after spraying. Subsequent caterpillar hatchings will usually be too late to cause any damage.

Economics of spraying

The number of caterpillars present in a crop is the major factor determining whether economic damage will occur. Results from many trials conducted by the department have been used to generate the Native budworm spraying threshold to give a personalised and more precise measure of potential loss from native budworm damage.

Crop loss (kilograms per hectare) for each caterpillar netted in 10 sweeps (or found per square metre) is shown in the table. For one caterpillar netted in 10 sweeps is equivalent to about 20,000 caterpillars per hectare for most pulse crops.

Thresholds

See the Native budworm spraying threshold factsheet to determine the economics of spraying your crop.

Native budworm spraying threshold factsheet

Crops vary in their attractiveness to moths as sites for egg-laying. Moths generally prefer crops in the following order: lentil, field pea, faba bean, chickpea, lupin, canola.

Crop density and crop growth stage (flowering and podding) will affect the number of eggs laid by native budworm moths.

The feeding behaviour of caterpillars also changes according to the type of crop the caterpillars are feeding on.

Field pea, chickpea, lentil and faba bean crops

• Are very susceptible to all sizes of caterpillar during the formation and development of pods.
• Tiny caterpillars can enter emerging pods and damage developing seed or devour the entire contents of the pod.
• Some field pea growers have had unexpected losses from native budworm damage in semi-leafless field peas, like Kaspa, as their intertwined tendrils make them more difficult to sample.

Lupin crops

• Narrow-leafed lupin crops will not be damaged until they are close to maturity and the pods are losing their green colouration.
• Pod walls are not penetrated until the caterpillars are over 15 mm in length.

Canola crops

• Canola is like narrow-leafed lupin - pods only become attractive to caterpillars as the crop nears maturity and begins to hay-off.
• Caterpillars of all sizes will enter pods at this stage, with larger caterpillars doing the most damage.
• Native budworm can also cause damage after a canola crop has been swathed. This will occur if the crop is swathed when many pods are still green with soft seed or if swath drying is prolonged due to cool damp conditions.