Features Inoculants Seed & Chemical
Pea leaf weevil has potential to cause problems

A recent addition to the growing list of crop pests is the pea leaf weevil
(Sitona lineatus). Leaf damage from this pest was quite noticeable in pea fields
in southern Alberta in the summer of 2004. "Leaf damage from the pea weevil
starts on the lower leaves, and moves upwards in the plant," explains Mark
Olson, provincial pulse extension specialist with Alberta Agriculture, Food
and Rural Development. "The weevil makes very distinct 'U' shaped notches
in the leaves and if you look carefully, you will see the growing point of the
plants being chewed."

The pea weevil, which is native to Europe, was introduced into British Columbia
in the 1930s, and has since spread throughout the Pacific Northwest. It is also
a problem in countries such as Australia. The weevil's favourite crops are pea
and vetch. "We first started noticing the leaf damage in the early part
of June at the Southern Alberta Soil and Crop Diagnostic Field School at the
Lethbridge Research Centre," says Olson. "I also walked through several
other fields around Lethbridge, including processing pea crops, and did see
a fair amount of leaf damage."

The leaf damage was quite variable across the various fields, with the most
damage in the plots at the field school location. So far, pod damage has not
been evident at lower infestations levels.

What is of greater concern than leaf damage is the damage to the nodules on
the roots of the pea crops. In fields where leaf notching was evident, Olson
dug down into the soil and discovered larvae inside the nodules. "They
are a white shiny colour, and had actually gone inside the nodules and were
eating the nitrogen fixing structures. The larvae were totally eliminating the
ability of the plant to fix nitrogen." This potentially poses big problems
for growers who are counting on nitrogen expected from the nitrogen fixing nodules
on peas, and benefits to subsequent crops.

Because the pea leaf weevil is a new problem in Alberta, researchers have had
to rely on published literature primarily from Europe. "The literature
indicates that economic losses occur during the seedling stage when populations
approach one pea leaf weevil per plant," explains Olson. However, the economic
thresholds may be lower for emerging seedlings if the growing point is damaged.
Or, the plants may be able to withstand leaf damage if they have six to eight
expanded leaves and intact growing points.

The pea leaf weevil produces one generation per year and overwinters as an
adult in clumps of grass, alfalfa or under dirt clods or other debris. The adult
weevil is a light grayish-brown colour, with three stripes on the back and on
the wings, and moves from field to field by flying. According to the literature,
adults lay eggs in late April or early May on the soil, or on developing pea
plants. Each female can lay up to 1000 eggs and hatching occurs in one to three
weeks. The larvae feed on the nodules on the roots, or burrow into the pea pods.

"There are not any chemical control products registered in Alberta for
pea leaf weevil to-date," says Olson. "Because the weevil flies, crop
rotations aren't going to be very helpful in terms of control." The literature
indicates:

• Early seeding and harvesting can help reduce the impact of pea leaf weevil
  damage.
• Pea vines should be destroyed immediately after harvest, although this may
  not be practical.
• Avoid planting infested seed, unless it has been fumigated.
• The adult weevils can also overwinter in stored crop, so monitoring peas
  in storage is important.

Olson and other researchers need more information from growers about the extent
of infestations of pea leaf weevil in their southern Alberta fields, and damage
reports before any estimation of economic losses can be made. "We need
to hear from growers if they feel they are seeing economic losses from the pea
leaf weevil, and if it's a significant problem," says Olson.

The damage from pea leaf weevil appears to be quite variable throughout fields,
and may have been confused with damage from grasshoppers or other insects. "Once
we have some information and support, hopefully we can initiate a research project
and field survey to collect more information about the pea leaf weevil problem
in Alberta, and to be able to determine the economic impacts." -30-

The Bottom Line
Early and effective nodulation is critical to maximizing N fixation.
Any loss in N fixing capacity will dramatically change the economics of pea
production. So far, the pea leaf weevil appears to be well adapted to moderate
climates (southern Alberta, Pacific northwest). Hopefully the more frigid climate
of the prairies will keep this insect at bay for a while. John
Waterer, Winnipeg, Manitoba.