As seeding continues and crops start to emerge, it is a great opportunity to scout for insects. Cutworms and wireworms are two insects that can be a concern at this time of the year and into the summer months.
There are many species of cutworm in Saskatchewan. One of the most prominent species is the red-backed cutworm. These larvae have two broad dull-red stripes along the length of their back and the head is yellowish-brown. They feed mainly on cereals, sugar beet, flax, canola and mustard but will also consume vegetables, sunflowers, sweet clover, alfalfa, various tree seedlings, and garden flowers. Small holes and notches in the foliage are symptoms of feeding by young red-backed larvae. The older larvae eat into the stems and usually break them at or just above the soil surface.
Pale western cutworms are another common species in the prairies. The larvae have a general pale yellowish-gray body color with a distinct white mid-dorsal line. The head is yellow-brown with two vertical black dashes that form an inverted V. Pale westerns can be serious in wheat crops. However, they do attack other cereals, as well as canola, mustard, flax, sugar beets, legumes and certain weeds, such as thistle. Older pale western larvae chew on the stems just below the soil level, causing the plants to topple over. Other cutworm species that cause crop damage in Saskatchewan include the dingy, army, and bristly cutworms.
The lifecycle for cutworms is the same for all species but the timing of different stages can be different. The adult stage for cutworms is a moth. Moths can lay several hundred eggs in vegetation or in soil during August and September. Depending on the species, cutworms will overwinter as eggs or as larvae. With some species the eggs will hatch in the fall and the young caterpillar-like larvae will remain in the soil and overwinter. Species that overwinter as larvae tend to be active earlier in the spring than those overwintering as eggs. Red-backed, pale western and bristly cutworms will overwinter as tiny eggs. Army and dingy cutworms overwinter as larvae and tend to be the ones we see in early spring. The larvae go through several moults until they are full size. Cutworm larvae have four sets of abdominal prolegs along the length of their body, a hardened head capsule, and will curl up into a C-shape when disturbed. The larvae stage is most damaging to crops as the caterpillars are growing rapidly and need a lot of food. Once the cutworm finishes the larva stage it pupates or starts turning into a moth. Moths will emerge during the summer months and the cycle starts over.
When scouting for cutworms, start by looking above ground as you may see bare patches, individual plants with holes or notches in foliage, or plants that have been cut off at or near the soil surface and have a wilted appearance. The next step is to dig where the damaged plants are or near the edge of the bare patch. Once you confirm cutworms are present then you need to identify the numbers present. Dig up the soil from a known area such as one square foot and remove soil to a depth of 10 centimetres. Place the soil into a basin or over a mesh screen. Loosen the soil and shake it up to activate the cutworms. A mesh screen works well to sieve the soil through so the cutworms can be collected and counted. Repeat a few times within the area where symptoms are evident and throughout the field as needed.
The economic thresholds for cutworms are three to four larvae per square metre in cereals, two to three larvae per square metre for peas, and 25-30 per cent stand reduction in oilseed. Cutworms come to or near the surface to feed during the night, and go underground during the day. Therefore, the best time to spray for cutworms is near nightfall.
Wireworms are very different from cutworms as they are more slender, have hard bodies in a jointed pattern and are yellow or orange in color. They have three pairs of legs that are grouped behind the head. The last abdominal segment of a wireworm is flattened with a keyhole-shaped notch. An adult larva can grow to the length of one to four centimetres. Adult wireworms are beetles and they lay between 200-400 eggs, usually late May and early June. The larvae will hatch from the eggs in three to ten days and spend two to five years feeding on roots and germinating seeds. When ready, the larvae go through a short pupation, which is around a month, and emerge as adult beetles in the soil.
There are 800 wireworms species discovered across North America, however nearly half of which are found in Canada. Majority of these wireworms are harmless while a few are pests of a wide variety of crops including corn, potatoes, wheat, barley and onions. Wireworms feed on germinating seeds or underground portions of plants. Wireworm damage consists of hollowed out seeds or underground plant tissue with a shredded appearance. Young seedlings are most affected as the damage can result in stunting or even plant death. The damaged areas are also an entry point for diseases which can cause further damage.
Similar to the cutworms, wireworms can create bald patches in the fields. Wireworm damage is commonly mistaken for cutworm damage. Wireworms shred tissue while cutworms tend to cut off plants or plant parts. The shredding caused by wireworms can kill plants directly or it can just cause enough damage that disease sets in and the weakened plants cannot recover. Signs of wireworm damage include: hollowed seeds, dead seedlings, shredded stems, wilting and discolored plants, thin or no stands or missing plant rows.
To find wireworms look in the soil. Wireworms move fast and will bury away from daylight. In spring, warm soil temperatures near the surface and germinating seeds will attract them and bring them closer to the surface. Another way to confirm there are wireworms present is to use bait balls. Bait balls are made from wheat flour or oatmeal and work by releasing carbon dioxide into the soil which attracts the wireworms. Note that bait balls are good measure of insect presence, not density. To make a bait ball, mix one to one and a half cups of oatmeal or wheat flour with two tablespoons of honey and up to half a cup of water until mixture sticks together in a ball. You can put the ball directly into the ground, or tie it up in a mesh bag (onion bag or cheesecloth work well). A mesh bag makes it easier to dig up from the ground. Bury the bait ball four to six inches deep and mark with a flag. Wait four to five days and then check your bait ball to see if there are wireworms. However, the bait ball is not always fool proof as the wireworms might have found other feed sources.
Unfortunately there are no insecticides registered for control of wireworms. Seed treatments are available that can provide some protection and should be considered for future planning in fields where high levels were found.
If you do find wireworms or cutworms you can help researchers learn more about the species by collecting them in containers and dropping them off at your local Regional Service office. You can also send them directly to the Crop Protection Laboratory in Regina. The insects are required to be alive so handle them gently, provide them with some food (plants from the field) and place tissue in the container to stop them from bouncing around. Your assistance is appreciated as the more we learn the more informed the decisions can be.
For more information on scouting and controlling cutworms and wireworms refer to the 2013 Crop Protection Guide, contact Alica Olson at 306-446-7650 or Agriculture Knowledge Centre at 1-866-457-2377.
