This is an adult pea leaf weevil feeding and they love faba beans and peas.
Actually, adult pea leaf weevils are hard to spot, they are brown, about the length of a pencil eraser, have a short snout and like to play dead.
They just fly right in, from kilometers away or may walk short distance. Other cultivated and wild legumes are also host plants for the adult pea leaf weevils (e.g. alfalfa, beans, clover, soybeans, lentils, lupins, and vetch) . Economic damage only occurs in field peas and faba beans. They may nibble on the leaves of dry beans, lentils, soybeans and lupins but such feeding does not cause economic damage. Adults overwinter under the snow in road side ditches and shelterbelts. They will also feed in any kind of bean, alfalfa or clover crop late in the growing season and then overwinter in these fields. The more rain and snow we get the more pea leaf weevils we are likely to see. With our early hot dry spring, pea leaf weevils got an earlier start than usual, and they tend to become active in waves which makes them hard to control.
“Unless something drastic happens, like a major change in precipitation, the chances are good that areas with high weevil densities last year could have high densities again this year,” said Meghan Vankosky, an entomologist with Agriculture Canada in Saskatoon
Research done around Swift Current, Sask., demonstrated that populations of pea leaf weevils completely crashed when there was a period of dry weather. It’s possible that the larvae cannot burrow through the soil crust, or they die on the soil surface when the weather is hot and dry.
In the image below it looks like someone took a hole punch to the leaves creating a scalloping effect, which means that adult pea leaf weevils have already been there and laid eggs.
It’s not the nibbles on the leaves that cause yield reduction since field pea and faba bean can compensate and outgrow this damage. It’s the fact that the adults mate and each female lays over 1,000 eggs near the soil surface in the fields from May to June. Those eggs hatch into larvae after a few weeks and burrow down into the soil.
The larvae ( about 5 mm long, white body with no legs, and a brown head) stay under ground and are only attracted to the nitrogen fixing nodules of the pea or faba bean roots.
They hollow out the nodules and eat the nitrogen producing bacteria inside . Larvae will develop through five instar stages in four to eight weeks. Plants with damaged nodules are hampered with producing their own nitrogen, and have poor growth and lower yields.
But we recommend not to spray foliar insecticide as it generally doesn’t work that well and it is too late when you see the notches since the eggs have already been laid. The insecticide spray would only kill the adults above ground, and as soon as they died, more would move in.
Insecticide seed treatments can help, but they are not 100% effective.
Thiamethoxam, a neonicotinoid seed treatment, is registered for pea leaf weevil and does get some results.
According to Meghan Vankosky, “It causes paralysis and if the weevils remain paralyzed for long enough after feeding, then they’ll starve to death. The idea is that we’re killing them before they can lay their eggs or, if we’re not killing them, we’re delaying egg laying so that the root nodules have time to establish and start supporting the plant.” It’s unclear whether a seed treated insecticide translocates to the roots. It may cause some larval mortality but not enough.
Stay tuned. MPSG is conducting seed treatment research trials in multiple western Manitoba locations.
Applying extra nitrogen isn’t the answer either as it is expensive and if you do, then even the healthy nodules won’t bother producing their nitrogen because initially they can get it from the soil. It’s tough to know how many nodules will be disrupted by the larvae feeding. Some years, if we get enough moisture and warmth, the organic matter can kick out more nitrogen that the peas can use.
It has been found that pea leaf weevils will “go for” pea varieties with a thinner wax layer on the leaves rather than those with a thicker wax layer, but research is ongoing to explore this observation.
Pea Leaf weevils have spread, from west to east, across the prairies and, as of the 2024 field survey, their current known range in Manitoba stretches as far East as Fannystelle and Rockwood,
During field surveys , the number of notches per plant and per clam shell leaf are counted. This indicates both the presence and the size of the pea leaf weevil adult population which helps to understand the potential impact on the crop. The survey also monitors the spread of this pest across the province.
If you find leaf notching in your pea or faba crops, get in touch with our Production Agronomists – Wendy or Terry, to participate in the provincial pea leaf weevil survey.
More Information on pea leaf weevil and Canadian prairie research:
Conditions right for pea leaf weevils to thrive | The Western Producer