Prairie Giants Crop Report

The Prairie Giants Crop Report

Field observations at your fingertips

July 17, 2023

Taylor Kurtenbach

Armyworms and Grasshoppers in Cereals

We have begun to see economic levels of armyworms and grasshoppers in cereals. This is typical in warm, dry conditions like we are experiencing this season.

Armyworms (Mythimna unipuncta) can be pests of both broadleaf and grassy plants, but prefer cereals [1][2] . Armyworms do not overwinter in Manitoba; the adult moths are blown in with the wind in spring, where they then lay their eggs. The larval stage (caterpillars) feed on the plants causing crop damage, until they pupate [1]. The caterpillars typically feed at night, so scouting for them during the day can be difficult. To scout, shake down plants in a one square foot area, then count the number of larvae present making sure to check under leaf litter. The economic threshold is 4 or more armyworms per square foot, 2 when cereal heads are being clipped, and 5 in forage crops [1].

If warranted, an insecticide application should be sprayed in the evening when armyworms are present. According to the most recent Manitoba Crop Pest Update, lots of larvae are either starting to pupate or are being controlled by parasitoids [3]. These parasitoids’ pupal clusters can be found in fields and are often confused with eggs.

Fun fact – parasoid wasps, Cotesia, lay their eggs into the armyworm caterpillar! After 2 to 3 weeks the wasp larvae emerge from the caterpillar, killing it in the process. They then spin cocoons on, or near, the caterpillar, forming the cluster of pupal cases which we see on top of the plants. Adult wasps emerge from these cocoons and start the process again [3]. Beneficial insects for the win!!

There are many species of grasshoppers, but only a few are classified as pests [4].

1. Migratory – feed on grasses and broadleaves

2. Two-striped – feed on grasses and broadleaves

3. Clearwinged – feed on grasses only

4. Packard – feed on grasses and broadleaves but prefer legumes. This species is more common in the western prairies

So far this season, I have only observed grasshoppers feeding in cereals.

Grasshopper eggs are laid in late summer to early fall. These eggs overwinter, hatching into nymphs come springtime. Both nymphs and adults feed on crops, causing damage.

To scout, choose an area, one meter squared, ahead of you. As you move towards the area, count the grasshoppers that fly away from that area. When you get to the area, disturb the plants to ensure any grasshoppers remaining fly away. Repeat this action 5 times to get an accurate estimate for the field.

The following are economic thresholds, found on the Government of Manitoba website [4].

CONTROL	FIELD NUMBER/M2	ROADSIDE NUMBER/M2
Control not usually required	0-6	0-12
May be required	7-12	13-24
Control usually required	13+	25+

If an insecticide application is required, a product like Coragen Max is preferred as it is safe for pollinators and has a 7–14 day residual depending on the rate. In some cases, an application of the headlands is often enough to control the populations. Other methods, like strip applications have also been done successfully in rangelands [4]. This is done by spraying the field in alternating strips, essentially only spraying half the amount of acres. Other preventative cultural controls include early seeding dates, and seeding guard strips of less desired plants (oats, peas) around the field.

Fortunately, our bertha armyworm and diamondback traps counts are low, and we have not been noticing any larvae in canola fields. Have questions, or need help scouting? Our agronomists are available to help!

Sources

[1] Government of Manitoba. Armyworms. [Online] Available: https://www.gov.mb.ca/agriculture/crops/insects/print,true-armyworm.html

[2] RealAgriculture Agronomy Team. 2020. Wheat School: An abundance of armyworms. [Online] Available: https://www.realagriculture.com/2020/07/wheat-school-an-abundance-of-armyworms/

[3] Manitoba Agriculture. 2023. Manitoba Crop Pest Update Issue 8. [Online] Available: https://www.gov.mb.ca/agriculture/crops/seasonal-reports/insect-report-archive/pubs/mb-crop-pest-update-2023-07-12.pdf

[4] Government of Manitoba. Grasshoppers: Identification, Monitoring and Management. [Online] Available: https://www.gov.mb.ca/agriculture/crops/insects/grasshoppers.html