HPIPM:Wheat Stem Sawfly
Author: Frank B. Peairs[1], Gary L. Hein & Michael J. Brewer
Background
The wheat stem sawfly has been a key pest of spring wheat in North Dakota and Montana for more than a century. It was not a severe problem in winter wheat because the earlier maturing winter wheat was not attractive for egg laying and larvae were not able to complete development. Since the 1980s, however, winter wheat in the northern plains has been damaged more severely by the sawfly. In the central High Plains wheat stem sawfly has not been a pest of significance, presumably because of the predominance of winter wheat and lack of spring wheat. However, over the last decade serious infestations have occurred in southeastern Wyoming, in adjoining counties in Nebraska and, most recently, in northeastern Colorado. Its increase may be associated with conservation tillage practices or other changes in the production system.
Identification (and life cycle/seasonal history)
The adult wheat stem sawfly (Figure 1) is a wasp-like insect about 3/4 inch long. It has smoky colored wings and a shiny black body with three yellow bands across the abdomen. When present in the field the adults are often seen resting upside down on the wheat stem. The sawflies will be active in the field when temperatures are above 50° F, with calm winds. They are not strong fliers and generally fly no further than the nearest wheat field or other suitable host grasses. This often results in the greatest damage in field margins closest to fields containing wheat stubble from the previous season. The adult emergence and flight continues for 3-6 weeks, with adults beginning to emerge in May and can still be present in early June. Females lay eggs immediately upon emergence and typically live about 1 week. They preferentially select the largest wheat stems available and insert eggs into the first available internode or when a stem is fully developed, below the uppermost node. If sawflies are abundant, eggs may be laid in smaller stems, and multiple eggs may be laid in a single stem. However, only one larva will survive in each stem due to cannibalism. Females lay an average of 30-50 eggs, depending on the size of available host stems. Eggs are difficult to detect because they occur inside the stem.
Sawfly larvae are always found within the stem and will assume an S-shaped position when taken out of the stem. They move slowly down the stem as they feed, for approximately 30 days. Sawfly larvae (Figure 4) are cream colored, have a broad head, and are ½ to ¾ of an inch in length when fully grown. When they are mature they move down towards soil level and cut a V-shaped notch around the interior of the stem. They then seal the interior of the stem just below the notch with frass and move down near the crown. The upper stem often breaks just prior to harvest at this weakened notch. The remaining stem containing the overwintering chamber is referred to as the “stub” (Figure 3). The larvae overwinter in the stubs, slightly below soil level, before pupating in early spring. They produce a clear protective covering that protects them from excess moisture and moisture loss.
The presence of wheat stem sawfly can be verified by splitting the suspected stem from top to bottom and examining the interior of the stem. If the stem is packed with a sawdust-like material, the stem was infested with a wheat stem sawfly larva. The sawfly larva will likely still be in the stem in a chamber just above the crown. Another clue to the presence of infested wheat is the occurrence of a darkened area on the stem just below the nodes. This symptom results from the internal feeding of the sawfly and can be used to detect the level of infestation without having to split each stem. As wheat approaches harvest, the stems of the infested tillers may begin to lodge. The proportion of infested stems that lodge depends on weather and plant conditions. Both the lower end of the loose stem and the remaining stub (Figure 3) have a distinct uniform cut at the break site, and both ends will have a saucer-shaped appearance with the hollow stem packed with sawdust. Adults can be sampled with a standard insect sweep net, and a pheromone is available.
Plant Response and Damage
The most dramatic impact of the wheat stem sawfly is the lodging of damaged stems (Figure 2) and the subsequent losses from not being able to completely harvest these stems. This damage is very apparent at harvest time and will be easily observed by the combine operator. However, not all infested stems will break off and lodge. Yield loss of five to ten percent due to unrecoverable wheat heads are common. In addition to losses from lodging, sawfly larvae cause physiological damage of 10 to 20 percent to the infested stems.
The wheat stem sawfly can use several hollow stem wild grasses as hosts, including quackgrass, smooth brome and wheatgrasses. The sawfly will not damage corn or broadleaf crops. Cereal crops other than wheat are not adequate hosts for the wheat stem sawfly to complete its development even though eggs may be laid in the stems of these grasses.
Management Approaches
Cultural Controls
Tillage reduces wheat stem sawfly survival, however, its impact on overall sawfly abundance and on damage to the next wheat crop is variable. Shallow tillage after harvest lifts the crowns and loosens the soil around them. This maximizes larval exposure to the late summer dryness and winter cold, increasing mortality. Intense tillage that buries stubble also reduces sawfly survival, but to a lesser degree. Intense tillage may interfere with important biological control agents and will increase the risk of soil erosion. No-till has been linked to many of the recent wheat stem sawfly problems in the region. However, the advantages of controlling the sawfly with tillage must be weighed against the considerable benefits of no-till.
Planting attractive varieties of trap crops such as barley, oat or triticale along the edge of wheat fields may be effective in decreasing damage and reducing the number of sawflies the following year. The sawflies will oviposit in the trap crop, but the larvae will be unable to complete development. Additionally, the trap crop can be removed as hay to increase sawfly mortality. This method is especially effective when the sawfly abundance is low to moderate and significant infestations are limited to the field margins. However, when sawflies are abundant, females may move past the trap crop and into the wheat to oviposit, resulting in significant damage.
Planting wheat in larger blocks as opposed to narrow strips is another cultural practice that may reduce sawfly damage potential. This minimizes the amount of field border adjacent to stubble where sawfly adults will be emerging, and thus, the part of the field most vulnerable to infestation. Sawflies are not strong fliers and tend to fly only until they reach a stem that is suitable for egg laying, which is the basis for this practice. Though the soil erosion benefits of planting in narrow strips may be reduced, larger fields are still a viable option if erosion is addressed by no-till practices.
Host Plant Resistance
Solid stem varieties of spring wheat have been successful at reducing the amount of damage from the wheat stem sawfly. However, this resistance is influenced by environmental conditions, and yield potential may be lower for these varieties. No winter wheat varieties adapted to the central High Plains region have solid stems; however, Montana has recently released the first solid-stemmed winter wheat varieties. However, preliminary data from eastern Wyoming suggest that these varieties are not competitive in yield with commonly used adapted varieties.
Biological Control
Several natural enemies to the wheat stem sawfly have been noted in the northern plains, and these are thought to be important mortality factors. The presence and effectiveness of natural enemies in the central High Plains has not been determined.
Chemical control
Currently available insecticides, targeting adults to prevent egg laying, are ineffective and cost-prohibitive due to the prolonged flight period. An exception may be the use of Thimet 20G soil insecticide, which has a 24(c) label in Montana. Using solid-stemmed cultivars and cultural controls are currently the most effective alternatives.
The information herein is supplied with the understanding that no discrimination is intended and that listing of commercial products, necessary to this guide, implies no endorsement by the authors or the Extension Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or equipment not listed is neither implied nor intended. Due to constantly changing labels, laws and regulations, the Extension Services can assume no liability for the suggested use of chemicals contained herein. Pesticides must be applied legally complying with all label directions and precautions on the pesticide container and any supplemental labeling and rules of state and federal pesticide regulatory agencies. State rules and regulations and special pesticide use allowances may vary from state to state: contact your State Department of Agriculture for the rules, regulations and allowances applicable in your state and locality.
