Garry McDonald, La Trobe University
Pea weevil (Bruchus pisorum) is a major pest of field peas. It is now established in all major field pea growing areas of south eastern Australia. Infested seed reduces the weight and quality of the grain and may cause rejection by millers and exporters. Importantly, control can only be achieved by preventing egg laying.
Infestation begins in the paddock when beetles lay eggs on the developing pea pods during or immediately after flowering. It is only when the next generation of beetles emerge from the harvested seed that the damage is seen. The problem of control of pea weevil must be tackled in the paddock. It is quite a different problem to that caused by other storage pests.
Figure 1. Characteristic black and white markings on the stocky body of the adult pea weevil
Adults are stocky, brownish flecked beetles with black, white and grey patches (Figure 1). They are about 5 mm long. The tip of the abdomen (body) extends beyond the wing covers. The portion of the abdomen that is visible is white and marked with two black spots. Contrary to the name, they are not true weevils and lack the typical weevil snout. Larvae are not obvious, as they eat the seed within the pods. Eggs are a shiny bright yellow/orange, 1.5 mm long and are attached singly to developing pea pods. Several eggs may be laid on the pods, and usually, groups of pods are affected. The larvae, which enter and remain in the pea, are cream coloured and have four instars (moults).
Adults emerge from hibernation in spring and fly to pea crops when day temperatures reach about 16-18° C, generally in early September to October. The period of emergence from hibernation sites varies from about 25 to 40 days in south eastern Australia; the longer the duration of emergence, the greater the risk that beetles will reinvade a sprayed crop. When their hibernation sites are away from a pea crop, they will fly in search of pea crops, attracted by the scent of the flowers. They feed on pea pollen for a few days before mating. Irrespective of the distances adults fly to reach the crop, their movement within the crop is largely restricted to the crop edge.
The females lay their eggs on developing pods of any size; eggs are first laid on young pods at about the time the flower begins to wither. Eggs hatch in about 14 days and the larvae bore directly from the egg, through the pod wall and into a seed. Only one larva can survive in each seed.
After about 40 days of feeding in the centre of the pea seed, the larva prepares a 2 to 3 mm exit hole by chewing partly through the seed coat. The larva then pupates, and after about 14 days changes into a beetle. By this time the seed has usually been harvested and some beetles emerge from the stored seed to find suitable hibernation sites. Others may remain within the seed to emerge next spring, or whenever the seed is disturbed.
Pea weevil larvae can reduce yield directly by consuming the seed or by increasing the number of peas split during threshing. Insect contamination may cause price reductions through dockage. The pea weevil population is mostly at third instar at the time of early harvest, and at this stage, only 26% of the maximum possible damage (seed weight loss) has occurred. The remainder of weight loss occurs around or after harvest because pea weevil larvae continue feeding for another two months.
Hence about 74% of seed weight loss can be prevented by an early harvest and fumigation of the crop immediately following harvest. This may mean fumigating 'pest unseen' because feeding larvae can be hard to see even when peas are cracked open. The sooner it is done the better to minimise weight loss. The longer the delay between harvest and fumigation, the better the chance to see a developed beetle when peas are cracked open, but by then the insect has caused the maximum damage.
Most beetles begin emerging through the exit hole in early January, leaving behind a hollowed-out pea (Figure 2). Some may not emerge from the seed until the following spring or until the seed is tumbled or moved. Damage is only obvious after the beetle has emerged. For example:
- If harvesting has been delayed, the beetles may fly out of the crop and alight on the advancing harvester;
- The beetles may emerge from seed stored in the barn;
- The beetles may emerge from seed in the combine while sowing;
- No beetles are seen, but the exit holes in the seed tell the story;
- Pea processors test a sample and find damaged peas, or beetles within peas, during a splitting test and reject the delivery.
Figure 2. Peas damaged by pea weevil
Sampling and detection
Sampling peas at and immediately after flowering is vital for the detection and control of pea weevil. In contrast, growers may also find spraying unwarranted if they sample effectively and find pea weevils absent or in low numbers. The ideal time to spray crops is prior to pod formation before oviposition commences. Control can only be achieved by preventing egg laying. That is, only the beetles are susceptible to insecticide treatment. If beetle numbers exceed the economic threshold, they should be controlled at early flowering, when the beetles first fly into the crop and before they lay eggs. Do not be tempted to delay spraying until native budworms appear as it is usually too late for a pea weevil spray. Plan the first crop inspection for the day the first flowers open. Pay particular attention to the crop edges.
Crops can be checked by using a sweep net. These nets are now available at some reseller outlets. A butterfly net, a fish net covered in terylene mesh or a home-made net (ring 400 mm to 600 mm) are suitable alternatives. Even a house bucket is acceptable if a net is not available, although this is not a very reliable technique. A sweep net provides the most effective means of sampling for pea weevil, although even this technique catches no more than 45% of the beetles present in the crop canopy. Take 25 sets of sweeps at 10 sites within the crop. As the beetles do not fly far into the crop, sampling need only be conducted on the edge. There are some important guidelines to maximise sweep net efficiency: sample on warm, humid (or wet foliage) days with no wind, and avoid areas of unusually high or low vine density.
An average of 3 beetles per 25 sweeps, taken as an average over 10 sites, is a reliable economic threshold. 10 beetles per 25 sweeps is considered a severe infestation. If few or no beetles are present, it is necessary to repeat check during the flowering and early-podding stages because beetles may still fly into the crop.
Using a boom or mister, only spray the first 40 metres of crop border where most beetles are located prior to pod formation. A border spray in a large crop results in a considerable saving, relative to a full paddock spray, in both chemical costs and time. Insecticides are therefore used to kill the adult beetles as they enter crops prior to egg laying. There is often only a short period during which effective action can be taken. If pea weevil is detected in seed it should be fumigated in sealed silos. Crops are unlikely to suffer a significant reinvasion after a spray applied at late flowering.
Crop hygiene and management practices
To avoid unintentionally introducing pea weevil, enquires should be made about the presence of pea weevil from the intended seed supplier and its occurrence in the supplier's area.The seed should be inspected regularly after delivery. Collect samples (1-2 kg) in plastic bags, shake vigorously and securely tie. Check for the emergence of beetles after a few hours. Sampling and checking in this way should be repeated monthly up to the time of sowing. Even if beetles are detected in only some bags of peas, all of the stored seed should be fumigated.
Timing of harvest: Shattering can be kept to a minimum by harvesting the crop as soon as it is ripe, using a correctly set machine. Harvesting of peas should not be left until after harvesting cereals.
Self-sown peas, or peas sown for grazing: Sheep should be used to clean up crop residues as soon as possible after harvest. Sheep eating infested peas and leaving the remainder exposed to the sunlight will kill pea weevil larvae. Heavy grazing also reduces the likelihood of volunteer peas surviving as hosts for weevil in subsequent years.
Pea hay: Peas grown for hay are usually cut when they have partly finished flowering and have some pods containing small seeds. Pea weevils can complete their development in such seeds and so it is preferable to cut pea hay no later than the beginning of flowering. Attention to these details can help reduce pea weevil numbers on farms where the problem is already established.
Spraying the crop of field peas when beetles first fly in will ensure control. If this is not done or is mistimed, fumigation of the harvested seed will be necessary if the seed appears to be infested. Silo fumigation of bulk peas with aluminium phosphide tablets is the most effective method. Pea seed in bags can also be fumigated with aluminium phosphide. Lay down gasproof plastic sheeting or a tarpaulin and stack the bags on it. The sheeting should be large enough to be able to wrap the stack and contain the fumigant. Add one tablet of aluminium phosphide to each bag, then cover the stack for from five to 12 days. When the covers are removed air the bags for another five days.