Grass Sawfly in Wheat:  Once you start to see sawfly feeding (see previous blog for pics and details) the feeding damage is nearly done.  You only have about a 7 to 10 day window to do anything before the larvae finish their feeding.  That said, the grass sawfly infestations tend to be in pockets in the field, usually more focused along the fields edge.  A full field spray may not be necessary.  Plus make sure to follow the pre-harvest intervals for products registered in wheat since harvest is approaching.  See next paragraph for product info. Don’t confuse grass sawfly with wheat sawfly.  Some of you saw an article in CropPest this week regarding white heads in wheat and saw sawfly in the article.  That is wheat sawfly not grass sawfly.  Grass sawfly is very different and does not mine into the stalk of the plant.  It climbs up the plant from the outside and clips the stem as it sucks the sap from the plant. You will see clippings of stem and heads on the ground and may see the green larvae walking along the ground or stem of the plant.

Armyworm in Wheat-   The window is starting to close on the pre-harvest intervals (PHI) for some of the products available so keep that in mind when deciding whether to spray.  Also remember if the larvae are bigger than an inch, the control products don’t work as well.  Pre-harvest intervals can be found in Pub 812 at: http://bit.ly/jWpXjA One field was reported to have larvae that are naturally dying off from a virus that infected them.   It screws up their brain and sends them to the top of the plant to die there.  You can find shriveled up larvae stuck on the wheat heads.

Armyworm in Corn: As surprising as it seems, corn can grow out of armyworm damage.  They don’t feed on the growing point so the plant can start growing more new leaves once the feeding has stopped.  There may be a little yield drag but the cost of spray may not be economic.  The plants recovery does depend  a bit on the conditions the field goes through for the rest of the summer but pending no other stresses or lack of rain, they will pull through.  I only recommend seed corn or higher value corn fields to be sprayed if necessary.  But again, use the one inch rule.  If the larvae are bigger than an inch the control products may not be effective.

Black Cutworm in Corn:  Most of the pics I have received from growers and reps have been of larvae that were close to being full grown.  These not only are too difficult to control but should be finishing up their feeding soon.  Plus, the corn crop is advancing beyond the critical stage of 5 leaves.  These cutworm struggle to cut a corn stalk once they plant is at 5 leaf or beyond so focus your scouting in only those fields that were late planted and have not reached the 5 leaf stage yet.

Cereal Aphids: Twitter has been a new source of pest info from me.  Thanks everyone who are tweeting and retweeting info about what they are finding out there.  @KaptinKnapton found cereal aphids on spring wheat near Lanark above threshold levels.  Thresholds are 12 to 15 aphids per stem prior to head.  Scouting in that region is advised.  Natural enemies will be trying to move in to start feeding on them but they have to find the fields first.

I have info regarding soybean aphids too but I think I will wait to write another blog in the next few days to let you know what we are finding and what to expect this summer!

Grass Sawfly Larva (T. Baute, OMAFRA)

I was hoping this was a one time pest that showed up last year.  But there have been some wheat fields in the Highgate/Muirkirk to Kerwood areas that are seeing grass sawfly feeding.  It is easier to identify grass sawfly since they feed during the day.  They also have a characteristic feeding habit of clipping the wheat stems, sometimes in sections, leaving a small pile of short straws and heads on the ground.

What is odd is that the experts who I was in contact with last year said this pest only really becomes a problem when you experience warmer than usual April temperatures.  That is when the sawfly adult lays its eggs.  Do you remember April being warm this year because I certainly don’t?

The good news is that experience from last year taught us that this pest really doesn’t feed for that long.  By the time you see the damage it is already at least half way through its feeding period.  You are really only looking for a 7 to 10 day window of feeding so assessing threshold and implementing control, if needed, has to take place in a short time frame.  Also, the infestations tend to be very patchy in fields, often along the fields edge so spraying the entire field may not be economical.  Also keep in mind that you could have both armyworm and grass sawfly active in the same field and may need to assess the impact of both while making your management decisions.  Armyworm can also clip heads so don’t blame the wrong pest for the damage.

More pictures and threshold is available in previous blog entries that I have written at:  http://bautebugblog.com/tag/grass-sawfly/

1.  It was the abnormally warm April weather that we experienced that helped promote the sawfly problem.  The very warm spring favoured egg laying.

2. The feeding activity usually only lasts for about two weeks at most.

3.  There are 6 instars.  The last instar being over an inch long.

4.  They are usually found in clumps in the field, with higher numbers near the edge of the field.  One larvae can clip approx 10-12 heads.

5.  They recommend using the highest rate registered to control them as they are tougher than armyworm to control.

6.  They use the threshold of :  a) 0.4 larvae per linear foot of row or b) 0.7 larvae per square foot of surface area.  It is too late to treat if the number of clipped heads per unit area is 3 to 4 times the average worm count and most caterpillars are greater than 1 inch long.  If there is a combination of armyworm and sawflies, they lower the threshold.

However, as mentioned before, we feel that here in Ontario, if you are seeing 30 heads clipped per square meter or 3 per square foot, AND the larvae are present in the field, the cost of control may be worth it.  Counting larvae for some has been difficult but finding clipped heads has not been.  YOU MUST follow days to harvest intervals though.  Matador is no longer an option since its days to harvest is 28 days.  Lannate has a days to harvest interval of 20 days, Dylox is 21 days to harvest and Sevin is 14 days to harvest.

As the crop matures, the larvae will be less likely to feed.  It appears that they like to clip the stems to suck the juice from the plant.  As the stem starts to dry, the less likely they will want to clip.  They have  started to clip more of the stems than just at the base of the head.  Leanne Frietag of Cargill provided me with this picture of damage that she found.  As you can see, the sawflies have cut 4 to 5 inch sections along the stem of the plant.

Grass Sawfly Stem Clipping (Credit: Leanne Freitag - Cargill)

After speaking with Delaware, I am reassured that the end is near.  We are probably looking at only another week or less of feeding.  And now we know that if we get another warm April, we had better sweep the wheat fields in May and early June to see if sawfly larvae are present so that we can deal with them prior to this clipping.

This spring is already teaching us something new   Never a dull moment in the bug world.

Close up of Grass Sawfly Larva - T. Baute, OMAFRA

Scout fields to determine if sawfly is present.  All fields infested look fine until you part the wheat plants and look at the canopy floor.  Only then do you see the number of heads laying on the ground.  These larvae don’t seem to bother eating the leaves but instead climb the plant and clip the heads right off.

Wheat Heads Clipped by Grass Sawfly - T. Baute, OMAFRA

Side View of Grass Sawfly- Note the numerous chubby prolegs - T. Baute, OMAFRA

Finding the actual larvae is difficult.   They do feed during the day but blend in very well with the wheat.  Shake plants in a square meter to force the larvae to drop.  Then part the wheat plants and count the number larvae and clipped wheat heads laying on the ground.  The larvae curl into a C shaped when they first drop so look for a small green ball on the ground. Armyworm larvae may also be present (though they too are much greener than we are used to seeing them) but armyworm infestations seem to be low in general.  It appears that it really is the sawfly causing the majority of the clipping.

Thresholds based on the number of larvae per foot of row may be tough to determine.  Delaware’s threshold is 0.4 larvae per foot of row.  Taking into consideration the number of heads being clipped may be a safer assessment.  30 heads clipped per square meter or 3 per square foot should equal the cost of control.  Just make sure that the larvae are present still in the field.

We’re not sure how long the feeding will continue.  Most of the larvae I have seen are close to full size.  The larvae live for approx 30 days then burrow in the ground and stay in hibernation until next spring when they pupate and emerge as an adult sawfly.  So there will be an end in sight though I can’t exactly say when yet.

Chris DiFonzo from Michigan State University is also starting to see heads being clipped there.  She put together a great infosheet this weekend on which critters could be blamed for the clipping and how to tell them apart.

Sawfly larvae feed during the day so they should be easier to spot, though they do blend in well with the colour of the crop.  They can actually do more damage, according to the literature, than armyworm if in high enough numbers.  Thresholds are hard to peg down since this is a rare occurence but Delaware has a threshold of 2 larvae per 5 feet of row.  If significant head clipping is noticed I feel that control is necessary.  Treat them as you would armyworm, but spraying can take place during the day since that is when they are feeding.

As for the armyworm populations, larvae are still reasonably small in size (most at 1/2 inch to some at 1 inch) and populations do not seem as high as usual.  But given that they are still small, they have a lot more feeding to do.  So scouting this week is VERY IMPORTANT. The armyworm larvae seem to be more green and orange in colour than the darker brown versions we are accustomed to seeing.  Again, use the infosheet provided above to help confirm your ID.

Keep the comments coming in so that we can get a good sense of what the head clipping situation is like out there.