Baute Bug Blog

I had the fortune of speaking with specialists at the University of Delaware (Galen Dively and Joanne Whalen) yesterday and learned more about these grass sawflies.  Delaware finds them in many of their fields every year and scouts early each season to see if they are going to be a problem.  Here is what I’ve learned from that conversation:

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.

Several wheat fields were found with grass sawfly north of London from Hyde Park to Kintore and St. Marys.  Damage averaged anywhere from 3 heads clipped per square meter to 30 heads clipped per square meter!  Reports have also come in from Rodney, Glencoe, Molesworth, Brussels and now northern Huron County.

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.

There have been a number of reports of heads being clipped in wheat.  Some of it can be blamed on armyworm, but some cases (at least in Rodney) have been due to larvae known as grass sawflies.  These larvae are green to green-yellow in colour with a yellow brown head.  They look like caterpillars but are actually larvae from the order Hymenoptera or wasp group.  Unlike armyworm, they do not have stripes along their body but do have 8 pairs of chubby prolegs starting directly after their 3 pairs of true legs all the way to their last body segment.

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.