Monitoring Legume Crops for Native Budworm
Native budworm (Helicoverpa punctigera) is a major pest of pulse crops. If left unchecked, these grubs can cause significant yield and quality losses in legumes and canola. Effective management begins with regular monitoring during flowering and podding. Many farmers are monitoring for grubs themselves. This is a summary of the process.
Sweeping
Sweep netting is the most common field method to detect native budworm activity. It is the best method for capturing smaller grubs. Using a standard sweep net, conduct 4-5 sets of 10 sweeps across representative areas of the paddock. Count the number of grubs collected, record their sizes, and compare against economic thresholds for each crop type. See table below. This method helps detect early infestations before damage escalates. Grubs per 10 sweeps is the base for a lot of economic threshold calculations.
Plant Shaking – Beat sheeting!
In dense or lodged crops where sweeping is less effective (such as field peas or beans), shaking plants over a clean surface—such as a bag, sheet, or canvas—can reveal larvae feeding within the canopy. This technique is useful for picking up larger instars that may not be collected by sweeping. Calculate the number of grubs per m². There is no direct conversion of grubs detected via beat sheeting compared to a sweep net, but it does help identify if any grubs are present.
Monitoring and Decision-Making
Monitoring should be carried out weekly from flowering through to pod-fill, as this is when crops are most vulnerable. Combine sweep and shake counts to get an accurate picture of pest pressure. Once larvae exceed threshold numbers (which vary by crop and grain price), a timely insecticide application may be justified. Note the size of larvae too—small caterpillars are easier to control, while larger ones may survive sprays.
This year we have witnessed huge moth flights in some regions. The native budworm are even attacking wheat in some paddocks. With low larvae thresholds and valuable crops sitting in the paddocks, Agronomists this season are budgeting on 2-3 sprays to keep grubs out of crops.
*Moisture stress not listed for these growth stages because moisture stressed crops are more susceptible to insect damage. A lower threshold may be used if extended dry periods are expected.
Summary
Using a combination of sweep netting and plant shaking provides the most reliable assessment of native budworm risk in legumes. Regular monitoring allows growers to make informed, cost-effective spray decisions and minimise unnecessary insecticide use while protecting grain yield and quality. You can see by the thresholds tabled that often the recommendations become, if you find grubs you spray.
Native budworm larvae. Photo by Andrew Weeks, Cesar Australia