Impact of harvest operations on parasitism of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae)

Abstract

Wheat stem sawfly, Cephus cinctus, has been a long term pest management challenge for wheat producers in Alberta, Saskatchewan, North Dakota and Montana. Many studies have laid the groundwork for biological control of this pest. Two species of parasitoids, Bracon cephi and B. lissogaster, have been shown to effectively attack C. cinctus in wheat Triticum aestivum L. Their effectiveness as part of an integrated pest management plan, however, has been highly variable. A survey was conducted to assess the distribution of the two Bracon parasitoids. A two pronged approach was used to better understand parasitoid spatial dynamics and parasitoid promotion. First, intensive field sampling was performed to determine the overwintering location of parasitoid cocoons in wheat fields. Second, wheat stems were cut at varying lengths to stimulate harvest management techniques that producers could employ. B. cephi occurred at most locations where sawflies were an agricultural concern, although the level is sometimes very low. B. lissogaster was only found in wheat in Montana. The vast majority of overwintering cocoons (>80%) were consistently found in the bottom third of standing wheat stems when measured prior to harvest.This is what should be left standing because it protects critical overwintering habitat. Removing only the wheat heads at harvest resulted in maximum parasitoid conservation. Special efforts to conserve parasitoids in solid stem wheat should be concentrated in areas of each field supporting above average crop growth because these high biomass areas harbor more overwintering cocoons. In hollow stem wheat the area 7.5 to 30 meters from the field margin is the most critical to conserve. If the hollow stem wheat has lower levels of infestation (less than 60% infested) the area to be conserved should extend to the field margin. These recommendations should be incorporated with other management practices that encourage sawfly parasitoids such as: solid stem wheat varieties, avoiding the use of insecticide during parasitoid flight, larger block fields, minimum tillage and chemical fallow. The improvement of parasitoid activity should further be incorporated into a complete integrated pest management approach including solid stem wheat, crop rotation, larger block fields and trap strips.