The adult wheat stem sawfly 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. and when conditions are calm. They are not strong fliers and usually only fly until they find the nearest wheat field. This often results in more serious problems occurring at the field margins closest to the adult emergence site which is the previous year's wheat field. The adults begin to emerge in May and can still be present in early June. The females will begin to oviposit five days after they emerge. They will select the largest stems, and insert a single egg just below the node. If populations are high, smaller stems will be selected and multiple eggs per stem will be laid. However, only one egg will survive in each stem.
Sawfly larvae feed within the stem after hatching and gradually move down the stem, feeding as they move for about 30 days. The larvae are cream colored and 1/2 to 3/4 inches long and have a broad head. They will always be found within the stem, and will assume an S-shaped position when taken out of the stem. When mature the larvae move to the area in the stem near the soil line and cut a V-shaped notch around the stem, weakening it at that point. The larvae then plugs the stem at the notch and moves down to near the crown where it remains until it pupates the next spring. It produces a clear protective covering around it that protects it 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 that extends down the stem near the bottom of the stem, 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 area 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. This results from the feeding activity of the larvae in producing the V-shaped notch in the stem. The stem is weakened at this feeding site and will break off when stressed by wind or rain. Both the lower end of the loose stem and the remaining stub 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.
The most dramatic impact of the wheat stem sawfly is the lodging of damaged stems 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. In addition to losses from lodging, sawfly larvae cause physiological damage of 10 to 15 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.
Tillage can be used to reduce wheat stem sawfly larval survival through the winter and spring. The objective of summer and fall tillage is to bring the stubs containing the larvae to the surface so they will be maximally exposed to the dry conditions in the late summer and the cold through the winter. Blading after harvest and/or before winter will accomplish this by lifting the crowns and loosening or removing the soil around them. Studies in Nebraska have shown this to result in about a 50 percent reduction in sawfly emergence the following year. In contrast, spring tillage should bury the stubble so that the adult sawflies will have a problem emerging from deeper soil levels.
The use of a trap crop (barley, oats or rye) along the edge of winter wheat strips may be effective especially when populations are low to moderate. These trap crops will be attractive to the sawflies for oviposition, but the larvae will not be able to complete development. However, if sawfly populations are heavy, trap crops may not be enough to satisfactorily reduce damage. Enough sawfly adults will move past the trap crops to infest the wheat.
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 does seem to 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. Preliminary data from eastern Wyoming
indicates these varieties are not competitive in yield with commonly used
adapted varieties.
Several natural enemies to the wheat stem sawfly have been noted in the northern plains, but none of these is identified as a major factor in reducing the population. The presence and effectiveness of natural enemies in the central high plains has not been determined.
Insecticide control has been tried in areas where sawflies have been a severe problem. This has proven to be an ineffective option, because of the extended period that the adults are present and control is needed. Effective control efforts would require close monitoring to determine the timing of sawfly presence and very likely repeated applications to maintain effectiveness for most of the period adults are active.
Categories: Small grains, Insects, Mites, Wheat stem sawfly
Date: 04/16/2004