All News

Insects to Scout for in 2021

By: John Gavloski, Entomologist, Manitoba Agriculture and Resource Development.

A few crop feeding insects got to levels that were of concern in 2020. Among these were grasshoppers, cutworms and armyworms. Cutworms and grasshoppers can successfully overwinter in Manitoba, provided conditions remain favourable, and are insects to make sure to scout for in 2021. Armyworms are not likely to successfully overwinter in Manitoba, but can migrate in. Some quite high levels were present last year. Cereal leaf beetle is a cereal feeding insect that has not been at economic levels in Manitoba, and is being managed with biological control. In this article we will look at these four insects and provide some tips on how you can monitor levels in your crops, and possibly help out with a biological control program.

Grasshopper Levels Remain High

There has been an increase in grasshopper populations over the past few years, and higher populations were again present in 2020. Grasshopper populations have more successful development in dry years and generally increase over a series of dry years. The generally dry summers over the last few years have likely contributed to their increase. If this trend of drier summers continues in 2021, farmers and agronomists should keep an eye on grasshopper levels around and in their crops.

Each year, Manitoba Agriculture and Resource Development, with assistance from organizations like Manitoba Crop Alliance, does a grasshopper survey in August to forecast levels for the next year. Along with grasshopper levels, the dominant species are recorded. Twostriped grasshopper was generally the most common species of grasshopper at most of the count sites where species was determined. Migratory grasshopper was noted as one of the dominant species in at least one count site in each region, and clearwinged grasshopper was noted as one of the most common species at 1 site in the Central region and 2 sites in the Interlake. Both migratory and twostriped grasshoppers feed on a variety of types of plants (both crops and non-crop). Clearwinged grasshopper is primarily a grass feeder, and seldom feeds on broad-leaved plants. Carolina grasshopper, a non-pest species of grasshopper was noted to be one of the dominant species at 1 site in each of the Southwest and Central regions, and 3 sites in the Interlake. Populations of bee flies, field crickets, and Epicauta species of blister beetles, all of which feed on grasshopper eggs, were also quite noticeable in some locations of Manitoba in 2020.

One of the things that can reduce grasshopper levels is heavy rainfall, but timing of this is important. The egg stage, which is the overwintering stage of our pest species of grasshoppers, is very resilient to excess moisture. Heavy rains in early spring would not be a major mortality factor for grasshopper eggs. The same heavy rains in early or mid-June, as the nymphs are emerging from the eggs, could be quite detrimental. Start scouting for grasshoppers in late-may or early-June, and focus this early scouting on areas that would have had lush green vegetation late last summer, such as field edges, pastures, late crops, etc.

Twostriped Grasshopper Stages JG Carman MB July172019

Figure 1. Twostriped grasshopper (right) and nymphs.

Cutworms: Still Main Early-Season Concern in Many Crops

Cutworm levels were high in many field crops in 2020. Overall the cutworm levels may not have been as high or damaging as 2019, but still a concern in many fields. Hopefully the population cycles for our main species of cutworms are now declining, after a few troublesome years. Cutworms remain an insect you want to make sure you are scouting for in 2021.

The two dominant species were once again the redbacked and dingy cutworm. Both of these species overwinter in Manitoba. Redbacked cutworm overwinters as an egg, while dingy cutworm overwinters as a partially grown larva. Scout for these cutworms as soon as the crop emerges. Cutworm populations can be quite patchy in a field. If high levels are found, determine if they are patchy or more widespread over a field. Cutworms are nocturnal feeders, and will be hidden during the day. You have to look in the soil or under debris to find them during the day. Insecticide applications are best done late in the day or at night.

The seed treatment Lumivia CPL is registered for cutworms in cereals and some pulse crops. The active ingredient in this seed treatment is chlorantraniliprole, the same active ingredient in the foliar insecticide Coragen. Predators such as ground beetles, and parasitoids such as bee flies Tachinid flies, and several species of parasitic wasps can help manage cutworms. Wet soil conditions promote fungal diseases among cutworms, and also forces the larvae to feed at the soil surface where they are subject to the attack of parasites and predators. High populations of these natural enemies can help regulate cutworm levels. To promote healthy levels of natural enemies, a recommended approach is to scout and get an idea what levels of cutworms and cutworm feeding are like early in the season, and control them if levels are high. Applying insecticides when cutworm levels are naturally low can be uneconomical, not only because of the insecticide costs, but also by disrupting cycles of natural enemies.

Picture4

Figure 2. Redbacked cutworm (left) and dingy cutworm (right).

Armyworms – The Troop was Large in 2020

Armyworms do not overwinter in Manitoba. The adult moths move in from the south, so populations can be very different in successive years. In 2020, armyworms were controlled in some fields of small grains in all agricultural regions of Manitoba. Populations of larvae were quite high and widespread from the end of June through July. Fall rye fields had very high levels of armyworms in some areas.

Natural enemies also seemed to be at work on the armyworm population. In some fields that had armyworms, pupal clusters from a parasitic wasp called Cotesia were very visible at the top of the plants. In some cases, people may mistake these for insect eggs. Cotesia larvae live inside the armyworms, and once mature dozens of Cotesia larvae can emerge within minutes from a parasitized armyworm.

Manitoba Agriculture and Resource Development will be organizing a program to monitor the time of arrival and abundance of armyworm moths in 2021. Approximately 30 pheromone-baited traps will be set up to monitor moth counts.

Picture5

Figure 3. Left: armyworm; Middle: Cotesia pupal cluster; Right: Cotesia emerging from armyworm.

Cereal Leaf Beetle – A Biological Control Success Story

The cereal leaf beetle was first found in Manitoba in 2009, in the Swan River Valley. Larva were dissected and found to not contain the parasitoid that in many other areas of Canada help keep levels of this cereal feeding insect low. Since 2009, regular releases of a tiny parasitic wasp called Tetrastichus julis, which targets cereal leaf beetle, have been done in Manitoba, and the level of parasitism monitored. Cereal leaf beetle can now be found in most cereal growing areas of Manitoba, but so can this parasitoid. Damage from cereal leaf beetle remains below economic levels.

In 2020, cereal leaf beetle larvae from fields near Pilot Mound, Tolstoi, Sarto and Dugald were collect and sent to Agriculture and Agri-Food Canada in Lethbridge where they were tested for percent parasitism by larvae of Tetrastichus julis. Collection dates ranged from June 22 to July 7th. Percent parasitism ranged from 10 to 58 percent of larvae parasitized by T. julis.

Percent parasitism by Tetrastichus julis of cereal leaf beetle (clb) larvae from Manitoba in 2020.

Nearest Town

Crop

Collection Date

Number of clb larvae collected

Number of clb parasitized

Percent Parasitism

Pilot Mound

Oats

June 23

31

18

58.1

Tolstoi

Oats

July 7

30

3

10.0

Sarto

barley

June 22

33

12

36.4

Dugald

Wheat

June 26

69

37

53.6

No new releases of T. julis were done in Manitoba in 2020 due to supplies not being available. Areas in Eastern Manitoba that had lower levels of percent parasitism in 2019 remain priority areas for future releases of T. julis once sufficient supplies of the parasitoid are available.

Farmers and agronomists can help with this biological control program. If you notice cereal leaf beetle larvae on any of your cereal crops, contact John Gavloski, with Manitoba Agriculture and Resource Development. You will be directed as to how to collect a sample, of about 30 larvae, to be tested for parasitism, or we will come to your field to collect a sample.

Picture6

Figure 4. Left: Cereal leaf beetle; Right: Cereal leaf beetle feeding and larva.