This week’s Insect of the Week is the soybean aphid. This pest overwinters in the US and is blown into Canada, where winged females migrate to soybeans and produce several generations over the summer.
In 2016, 2.5 million acres of soybeans were planted in Alberta, Saskatchewan, and Manitoba. This amount is expected to rise in coming years. Thus, it is important to know how to scout for and manage soybean aphids.
The Wheat Stem Maggot, Meromyza americana (Diptera) isa minorpest that causes eye-catching damage in wheat, rye, barley, oat, millet, timothy, brome, crested wheatgrass and bluegrass. This is a timely insect of the week, because the larval damage (dead white heads in an otherwise green field) started to appear in the last few weeks and is now highly visible in many fields.
Your eyes are naturally drawn to these white heads and can cause you to overestimate the actual amount of damage to your fields. However, the damage is usually limited to 1-5% of the crop. To identify if the wheat stem maggot is the culprit, gently pull on the white head to see if it easily separates from the flag leaf sheath and shows evidence of feeding damage at the base of the culm.
By the time you see damage, the greenish-white larva has exited and is off to begin a second generation that will overwinter in volunteer cereals. There are no registered chemicals or resistant varieties so your best management practices are rotate to non-cereals/non-grassses, destroy infested stubble, and control volunteer hosts and grassy weeds. Delayed seeding, where/when possible may also limit damage.
Ross Weiss, David Giffen, Owen Olfert and prairiepest_admin
Weather synopsis – This week’s temperatures were similar to last week, both the seven-day (Fig. 1) and 30-day average temperatures (Fig. 2) were similar to long term averages (Fig. 3). Compared to 30-day average temperatures, Alberta was above normal, whereas Saskatchewan and Manitoba were slightly below normal (Fig. 2).
Figure 1. Accumulated precipitation the past 7 days (from July 17-24, 2017) across the Canadian prairies.
Figure 2. Accumulated precipitation the past 30 days (from June 24-July 24, 2017) across the Canadian prairies.
Figure 3. Long Term Normal (LTN) average temperatures over 30 days
(from June 24-July 24) across the Canadian prairies.
Seven-day rainfall accumulations were low across the prairies ( Fig. 4). Total 30-day rainfall accumulations indicate that conditions are normal to dryer than normal for most of the prairies (Fig. 5). Growing season (April 1 – July 23, 2017) percent of average precipitation continues to be average for most of Alberta and below average for most of Saskatchewan and Manitoba.
Figure 4. Accumulated precipitation the past seven days (July 17-24, 2017) across the Canadian prairies.
Figure 5. Accumulated precipitation the past 30 days (June 24-July 24, 2017) across the Canadian prairies.
Figure 6. Percent of average precipitation for the across the Canadian prairies for
the growing season (April 1-July 24, 2017).
The lowest temperatures across the prairies over the past seven days (July 18-24, 2017) are mapped below.
In contrast, the highest temperatures recorded over the past seven days (July 13-19, 2017) are presented below.
The growing degree day map (GDD) (Base 10ºC, March 1 – July 23, 2017) is below:
The growing degree day map (GDD) (Base 5ºC, March 1 – July 23, 2017) is below:
The maps above are all produced by Agriculture and Agri-Food Canada. Growers may wish to bookmark the AAFC Drought Watch Maps for the growing season.
Cabbage seedpod weevil (Ceutorhynchus obstrictus) – There is one generation of CSPW per year and the overwintering stage is the adult which is an ash-grey weevil measuring 3-4mm long (Refer to lower left photo). Adults typically overwinter in soil beneath leaf litter within shelter belts and roadside ditches.
Monitoring: ● Begin sampling when the crop first enters the bud stage and continue through the flowering. ● Sweep-net samples should be taken at ten locations within the field with ten 180° sweeps per location. ● Count the number of weevils at each location. Samples should be taken in the field perimeter as well as throughout the field. ● Adults will invade fields from the margins and if infestations are high in the borders, application of an insecticide to the field margins may be effective in reducing the population to levels below which economic injury will occur. ● An insecticide application is recommended when three to four weevils per sweep are collected and has been shown to be the most effective when canola is in the 10 to 20% bloom stage (2-4 days after flowering starts). ● Consider making insecticide applications late in the day to reduce the impact on pollinators. Whenever possible, provide advanced warning of intended insecticide applications to commercial beekeepers operating in the vicinity to help protect foraging pollinators. ● High numbers of adults in the fall may indicate the potential for economic infestations the following spring.
Damage: Adult feeding damage to buds is more evident in dry years when canola is unable to compensate for bud loss. Adults mate following a pollen meal then the female will deposit a single egg through the wall of a developing pod or adjacent to a developing seed within the pod (refer to lower right photo). Eggs are oval and an opaque white, each measuring ~1mm long. Typically a single egg is laid per pod although, when CSPW densities are high, two or more eggs may be laid per pod. There are four larval instar stages of the CSPW and each stage is white and grub-like in appearance ranging up to 5-6mm in length (refer to lower left photo). The first instar larva feeds on the cuticle on the outside of the pod while the second instar larva bores into the pod, feeding on the developing seeds. A single larva consumes about 5 canola seeds. The mature larva chews a small, circular exit hole from which it drops to the soil surface and pupation takes place in the soil within an earthen cell. Approximately 10 days later, the new adult emerges to feed on maturing canola pods. Later in the season these new adults migrate to overwintering sites beyond the field.
Grasshopper Simulation Model Output – Based on model output, grasshopper development is slightly ahead of long term averages. The greatest development was predicted to be across southern regions in all three provinces, particularly southern Alberta. Grasshoppers should be predominantly in the fourth and fifth instar stages with adults present across most of the region.
Grasshopper Scouting Steps:
● Measure off a distance of 50 m on the level road surface and mark both starting and finishing points using markers or specific posts on the field margin.
● Start at one end in either the field or the roadside and walk toward the other end of the 50 m, making some disturbance with your feet to encourage any grasshoppers to jump.
● Grasshoppers that jump/fly through the field of view within a one meter width in front of the observer are counted.
● A meter stick can be carried as a visual tool to give perspective for a one meter width. However, after a few stops one can often visualize the necessary width and a meter stick may not be required. Also, a hand-held counter can be useful in counting while the observer counts off the required distance.
● At the end point the total number of grasshoppers is divided by 50 to give an average per meter. For 100 m, repeat this procedure. ● Compare counts to the following damage levels associated with pest species of grasshoppers:
0-2 per m² – None to very light damage
2-4 per m² – Very light damage
4-8 per m² – Light damage
8-12 per m² – Action threshold in cereals and canola
12-24 per m² – Severe damage
>24 per m² – Very severe damage
* For lentils at flowering and pod stages, >2 per m² will cause yield loss.
* For flax at boll stages, >2 per m² will cause yield loss.
Diamondback moth (Plutellidae: Plutella xylostella) – Throughout April and May, both forward and backward wind trajectory data was compiled weekly to identify potential DBM arrival events from southerly source areas including Mexico and southwest USA or the Pacific Northwest. This week, two biofix dates were selected as “starting points” used to apply the DBM model based on Harcourt (1954).By selecting and presenting mapped model outputs for both a biofix date of May 1 (Fig. 1) AND May 21 (Fig. 3), the predicted number of generations of DBM can be estimated across the Canadian prairies as of July 24, 2017. The following maps indicate that potentially two generations have been completed across most of the prairies for both biofix dates (Fig. 1 and 3). Using Biofix of May 1 – Based on the biofix date of May 1, 2017, the model predicts two generations of DBM (e.g., areas highlighted in yellow) whereas southern Alberta populations are potentially in the third generation (Fig. 1). The second map (Fig. 2), showing predicted results for Long Term Normal (LTN) data, indicates that populations in southern Alberta and the Peace River region are ahead of normal development (based on May 1 introductions).
Figure 1. Predicted number of generations of Diamondback moth based on a biofix date of May 1, 2017.
Figure 2. Predicted number of generations of Diamondback moth based on a biofix date of
May 1, 2017, but using Long Term Normal (LTN) data.
Using Biofix of May 21 – In the following scenario using biofix date of May 21, 2017 (Fig. 3), the number of generations of DBM are marginally behind the early May introduction presented above in Figure 1.
Figure 3. Predicted number of generations of Diamondback moth based on a biofix date of May 21, 2017.
REMINDER – Once diamondback moth is present in the area, it is important to monitor individual canola fields for larvae. Remove the plants in an area measuring 0.1 m² (about 12″ square), beat them on to a clean surface and count the number of larvae (Fig. 4) dislodged from the plant. Repeat this procedure at least in five locations in the field to get an accurate count. The economic threshold for diamondback moth in canola at the advanced pod stage is 20 to 30 larvae/ 0.1 m² (approximately 2-3 larvae per plant). Economic thresholds for canola or mustard in the early flowering stage are not available. However, insecticide applications are likely required at larval densities of 10 to 15 larvae/ 0.1 m² (approximately 1-2 larvae per plant).
Figure 4. Diamondback larva measuring ~8mm long. Note brown head capsule and forked appearance of prolegs on posterior.
Figure 5. Diamondback moth pupa within silken cocoon.
Across the prairies, provincial staff coordinate diamondback pheromone trapping during the growing season: ● Low numbers of moths have been reported across Saskatchewan for the 2017 pheromone monitoring. ● Manitoba Agriculture and Rural Initiatives posted low DBM counts which can be reviewed here. ● Alberta Agriculture and Forestry has a live 2016 map reporting Diamondback moth pheromone trap interceptions. A copy of the map (retrieved July 20, 2017) is below for reference.
In-field monitoring for egg masses and newly emerged larvae (photo below) should initially focus on the undersides of leaves plus watch the margins of leaves for feeding. Bertha armyworm larvae will also feed on newly developing pods so the whole plant should be examined. Watch for the following life stages:
Scouting tips: ● Some bertha armyworm larvae remain green or pale brown throughout their larval life. ● Large larvae may drop off the plants and curl up when disturbed, a defensive behavior typical of cutworms and armyworms. ● Young larvae chew irregular holes in leaves, but normally cause little damage. The fifth and sixth instar stages cause the most damage by defoliation and seed pod consumption. Crop losses due to pod feeding will be most severe if there are few leaves. ● Larvae eat the outer green layer of the stems and pods exposing the white tissue. ● At maturity, in late summer or early fall, larvae burrow into the ground and form pupae. Monitoring: – Larval sampling should commence once the adult moths are noted. – Sample at least three locations, a minimum of 50 m apart. – At each location, mark an area of 1 m2 and beat the plants growing within that area to dislodge the larvae. – Count them and compare the average against the values in the economic threshold table below:
Provincial entomologists provide insect pest updates throughout the growing season so we have attempted to link to their most recent information:
● Manitoba’s Insect and Disease Update for 2017 is prepared by John Gavloski and Pratisara Bajracharya and read Issue #10 (posted July 26, 2017) noting diamondback moth exceeding economic thresholds in some fields within the southwest and Gladstone areas, cereal aphids and soybean aphids plus the appearance of green cloverworm in some eastern Manitoba soybean fields.
● Saskatchewan’s Crop Production News – 2017 – Issue #4includes information related to soybean pestsprepared by Joel Peru. That report includes an update on scouting and management tips for painted lady butterflies (also described in Week 6) and Aphanomyces root rot.
Additionally, Danielle Stephens with Sask Ag reported that bertha moth counts in pheromone traps appears to have peaked over most areas of Saskatchewan during the week of July 17th. Cooperators operating BAW traps are asked to take down their traps the week of August 2nd and to submit their counts but also thanked for their ongoing support which makes the 2017 map possible.
● Watch for Alberta Agriculture and Forestry’s Call of the Land and access the most recent Insect Update (July 27, 2017) provided by Scott Meers. That reports notes surveying is underway targeting a ‘new’ midge in canola, continued scouting needed for diamondback moth with larval populations exceeding thresholds in a few canola fields at early pod stages in southern Alberta, appearance of low numbers of wheat leaf miner which is typically an occasional pest but seems to be accompanied by its parasitoid by K. Fry this year.
West Nile Virus Risk – The regions most advanced in degree-day accumulations for Culex tarsalis, the vector for West Nile Virus, are shown in the map below. As of July 23, 2017, areas highlighted in yellow on the map below have accumulated sufficient heat for the initial C. tarsalis to begin to fly. Areas highlighted in orange will have C. tarsalis flying so wear your DEET to stay protected!
The Canadian Wildlife Health Cooperative compiles and posts information related to their disease surveillance for West Nile Virus in birds. As of July 20, 2017, 833 birds were examined and eight have tested positive for West Nile virus; four from Ontario, one from Saskatchewan, and one from Quebec.
Active Wildfires – Natural Resources Canada posts live interactive maps like the one below. Access their webpage for more information and to stay current on the various active wildfires burning across Canada.
Canola growers at the far south of Alberta will want to note this preliminary update kindly provided by Dr. H. Carcamo (AAFC-Lethbridge) on July 20, 2017 related to flea beetles in canola…..
“We just collected sticky cards from a canola field just east of Stirling that is part of a collaborative study led by Dr. Alejandro Costamagna. I noticed an unusual dominance by striped flea beetles in this field. 15 years ago only 1 out of a 1000 flea beetles would have been striped and the rest crucifer, however, the two fields we sampled today near Stirling AB were unusual. Approximately 48% of the flea beetle population trapped on sticky cards in the one field were striped flea beetles and a second canola field had ~71% striped flea beetles.”
Dr. Carcamo goes on to caution that the above observation was based on only a single week of sticky card monitoring. Stay tuned because the entire seasonal data set from these two sites will warrant careful scrutiny! Even so, this update is an excellent example of the importance of in-field scouting and the value of ongoing data collection!