Adult sawflies emerge from obligate diapause in late spring (May to June in Montana) and lay eggs in wheat or other grass stems (Ainslie 1920). Individuals live for about one week, but emergence of a local population usually lasts 3 to 4 weeks. Males emerge a couple days prior to the females (protandry). Females usually mate once while successful males may mate with numerous females. Both sexes produce pheromones that are used in mate detection. Plants also produce volatile chemicals that attract sawflies.

Sawfly larvae develop within the wheat stems and feed on parenchyma and vascular tissue. They complete four instars before girdling the base of their host stem and going into diapause. Generally, a single wheat stem will produce a single adult sawfly because the larvae are highly cannibalistic. The first larva to hatch will normally consume all subsequent eggs or larvae. Girdled stems fall over or lodge near the base of the stem; the larval sawfly plugs the end of the resulting stub with frass (feces). Larvae do not feed after they are in the overwintering stub. Sawflies overwinter as larvae, withstanding freezing temperatures down to -20°C. Pupation occurs the following spring. Adults emerge from wheat stubble from May to June. The wheat stem sawfly's primary negative effect on cereal yields is through grain lost to lodged stems that are not picked up by the combine during harvest. Sawflies also have a negative effect on wheat head weight and kernel numbers and may affect the protein content (Holmes 1977).

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Reproduction and Haplodiploidy of the Wheat Stem Sawfly

Like most Hymenoptera, wheat stem sawflies are haplodiploid. Male sawflies have only one set of chromosomes (haploid) while females have two sets of chromosomes (diploid). Female sawflies control the sex of their offspring by laying either a fertilized (diploid: female) or an unfertilized (haploid: male) egg. Evidence suggests that sawflies preferentially choose to lay female eggs in larger grass stems (Morrill et. al. 2000). Presumably, this is done to ensure high quality female offspring, which are larger than the males.