Diagnosing pea weevil damage
The pea weevil (Bruchus pisorum) is in fact a beetle not a weevil and should really be called the pea beetle. It is one of the most damaging pests of field peas. It not only reduces yields but can also reduce germination rates of seed and grain quality to the point that it’s not saleable for human consumption.
What to look for
- Adult beetles are attracted to the crop at flowering but do not visibly damage the plant.
- Bright yellow cigar shaped eggs about 1.5mm long are attached individually to developing pods.
- Affected seed will contain developing beetles at harvest.
- By harvest larvae are evident if the seed is split open.
- The adult is 5mm long with white, black and grey patches.
- The white tip of the abdomen is marked by two black, oval spots.
Where did it come from?
- Adult beetles hibernate during summer, autumn and winter in sheltered positions e.g. under bark of trees, in cracks and crevices of fence posts.
- When spring temperatures reach about 20°C, the beetles become active and are attracted to crops. Even though pea weevil can travel up to 5 km, infestations usually occur from infested seed from the previous season.
- The female beetles are sexually immature when they leave hibernation and first arrive in the pea crop.
- They require a feed of pollen and further time for ovarian development to take place.
- Approximately 2 weeks after arrival in the pea crop, the females lay eggs on the developing pods.
- Female beetles lay eggs individually on the surface of pods.
- Small larvae hatch from the eggs in about 6 to 13 days, then bore through the wall of the pod and into the soft developing seed.
- After about 40 days of feeding inside the pea seed the larva prepares a 2-3 mm exit hole by chewing partially through the seed coat.It then pupates and after about 14 days is ready to emerge as an adult beetle.
- The pea weevil will not reproduce in stored grain
Pre-plantingDo not buy or plant unfumigated seed
- Monitor from flowering for pea weevil.
- If crops are grown in traditional pea growing areas, presence of pea weevil should be assumed.
- A 40 m border spray will control pea weevil that is moving into a crop. Spray with synthetic pyrethroids at registered rates.
- Insecticides are only effective on adult pea weevil so apply sprays only after adults first appear but before egg lay commences and before small pods are visible.
- In early crops beetle flights may occur over an extended period, so more than one spray application may be necessary.
- If heavy infestations are detected or if seed infested with live weevil was sown, the entire paddock needs to be sprayed.
- Monitor again 10 days after spraying and spray again if more pea weevil are detected above thresholds.
- Harvest on time to decrease yield loss and ensure no pea weevil adults have emerged and moved to hibernation sites.
- Send seed off farm straight from header, if live pea weevils are found fumigate seed.
- Fumigate stored seed in sealed silos straight after harvest. Add two phosphine generating tablets per tonne of silo capacity (internal volume of the silo) and keep sealed for 21 days. Fumigation of freshly harvested seed will also stop feeding damage by weevil larvae in storage.
- If seed is harvested at above 12% moisture it should be dried by aeration before sealing.
- Clean up spilled seed around storage sites and in paddocks. Where possible, consider grazing paddocks to decrease spilt seed as this can be a source of pea weevil infestation.
- Baled pea stubble is a source of pea weevil as it provides an idea refuge for pea weevil to hibernate, where they can remain in the seed within the bales. Fumigate bales or remove from farm as soon as baling is finished.
- Control volunteer peas to decrease the ability of pea weevil to survive and infest next seasons crops.
How can it be monitored?
- Pea weevil movement in early spring within a flowering crop is generally restricted to the crop’s edge, especially if adjacent to over-wintering sites such as trees, sheds.
- Monitor the crop edges every 3-4 days from the start of flowering. Monitor when average temperatures are above 20°C as this is when pea weevil are active. If sprays are applied, monitor crops about 10 days after spraying.
- To monitor crops use a sweep net. It should be dragged across the tops of the plants in a horizontal 160° arc with a one-metre stride between each sweep.
- Take 25 sweeps within 1-5 m of the crop edge. Repeat this at 6 or more sites.
- If weevils are found take more samples from the inside of the crop.
- Peas grown for human consumption: 1 beetle per 100 sweeps
- Peas for stock feed: 1 beetle per 25 sweeps