By Jessica Davey, Extension Summer Student, Moose Jaw
The wheat stem sawfly attacks a variety of grasses, including cereal crops. This insect prefers spring wheat and durum and can be economically important in periods with successive dry years. Rye, triticale and barley can also be affected.
This insect has distinct features for identification and goes through a unique life cycle. An adult sawfly has a shiny black body with three yellow bands around the abdomen, yellow legs and the wings have a smoky tint. The adults are about eight to 13 millimetres long and are relatively inactive, often seen resting with their head downward on wheat stems. Females can be easily differentiated from males by their ovipositor, located at the posterior end of the abdomen. Larvae are found in stems of infested fields from May to June. Pupae are typically seen in this time. In June and July, the pupae have matured into adults and then eggs are laid in the prime growing season.
The adults do not feed and only live for up to ten days while adult females can lay up to 50 eggs, with around one egg laid per stem. The life span of an adult sawfly is short, but in that brief time they can contribute to damage by laying eggs and multiplying the pest population at an extremely high rate. Only one larva per plant will survive if multiple females lay eggs on one stem as the first egg to hatch is cannibalistic. Hatching occurs in about five to eight days and the larvae are a light creamy colour, wrinkled and have an “S” shape.
The larvae cause damage to cereal crops and other grasses as they feed upward on the inside of the stems, after they emerge from the eggs. Most importantly, larvae also cut a groove around the whole inside of the stem less than one inch above the ground. This causes damage to the tissue, making it difficult for nutrients to pass through the stem to the upper portions of the plant affecting the stem strength, the seed head and overall yield and quality. Cutting damage weakens the stem causing it to break and fall over. Weather conditions can also increase lodging. Crop damage can result in a difficult harvest with equipment damage and increased fuel costs and lower yields.
Monitoring and scouting for sawflies can be helpful in identifying potential risk or impact to a cereal crop. If 10 to 15 per cent of the crop was affected by the pest in the previous year, then management options are warranted. Scouting should be done in late June and the first week of July when adults typically emerge. Sweep nets can be used to sample for adult sawflies. Scouting for sawflies can also be done at the end of July.
Although crop injury by wheat stem sawfly is usually more prevalent within the first 20 metres of the field edges, damage is not always confined to the margins. To monitor and scout for sawflies there are several methods such as counting the number of an affected stems in a one-metre row of a crop in five to ten different spots, using a sweep net in late June to July to find adults, splitting 50 to 100 stems at each of the five to ten locations to determine the number of stems containing larvae.
Natural enemies help regulate wheat stem sawfly populations; in fact, there are nine species of wasps that can attack them. The most important of these are the parasitic Braconid wasps, Bracon cephi and B. lissogaster, that attack the larvae of the wheat stem sawfly causing major reductions of sawfly populations.
Cultural management options include delayed seeding, applying nitrogen at proper rates, as well as using shallow tillage in the fall, reducing planting of successive wheat crops and other host crops (rotating crops) and swathing and harvesting the crop earlier than normal.
To reduce damage from the wheat stem sawfly, the most effective solution is incorporating resistant cultivars and/or crops. If you are planning to plant wheat in crop rotation, it is important to use solid stemmed varieties that are filled with pith that is harder for sawflies to attack. An excellent choice for a resistant variety is AC Lillian.
More information on this insect can be found on the ministry's Wheat Stem Sawfly page.