Guess the Pest! #7

Bill Cissel, Extension Agent – Integrated Pest Management;

Congratulations to Jeff Peat for identifying the grass sawfly in this past week’s Guess the Pest and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Jeff will also receive a FREE copy of A Farmer’s Guide to Corn Diseases. Click on the Guess the Pest logo below to participate in this week’s Guess the Pest! For Guess the Pest # 7, we will also be giving away A Farmer’s Guide To Corn Diseases ($29.95 value) to one lucky participant.


The correct answer to this past week’s Guess the Pest is Grass Sawfly. Grass sawflies damage small grains by clipping grain heads. They are often confused with true armyworms which also clip small grain heads. There are several reasons why it is important to be able to distinguish between grass sawflies and true armyworms:

1) Grass sawflies are more damaging than true armyworms because they prefer to feed on small grain stems as opposed to true armyworms that will typically feed on leaves before clipping heads. Also, grass sawfly damage usually occurs before the peak of armyworm damage.

2) The threshold for grass sawflies (wheat and barley – 0.4 linear ft of row) is lower than the threshold for true armyworms (barley – 1 per linear ft of row/ wheat-1- 2 –per linear ft of row).

3) Not all products that are labeled for true armyworm control will provide control of grass sawflies.

4) Insecticide rates also differ between the two species for some products.

There are several features that can be used to distinguish grass sawflies from true armyworm.

Grass sawflies larvae are active during the day and can often be found on the plants so “shaking” plants to dislodge larvae is necessary when sampling. They can be identified by their green color, large amber head, and 5-7 pairs of fleshy prolegs. Counting the number of prolegs is the most reliable way to determine if the “worm” is a grass sawfly or true armyworm.

Grass Sawfly Larva

True armyworms are active at night and can often be found curled around the base of plants or under crop residue during the day. Larvae have 4 pairs of fleshy abdominal prolegs not including the pair of legs at the very end of the abdomen. There also appears to be a large gap between the 3 pairs of true legs and the start of the fleshy prolegs.

True Armyworm Larva

If your field is at threshold for grass sawflies, here are several things to keep in when selecting which product to apply. Is the insecticide labeled for grass sawfly control? What is the days to harvest restriction (this varies among products)? Is the insecticide labeled on the crop (not all products are labeled for all small grains)?

Here is a link to our Extension Fact Sheet for additional information on identification, biology, and management of grass sawflies and true armyworms in small grains:

Here is a link to our Small Grain Insect Recommendations:

Guess the Pest # 7

What is this small grain disease? Think you know the answer…. Click on the Guess the Pest Icon below or go to to submit your best guess.

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