Wendell L. Morrill

Parasitism of the Wheat Stem Sawfly (Hymenoptera: Cephidae) by Bracon cephi and B. lissogaster (Hymenoptera: Braconidae) in Wheat Fields Bordering Tilled and Untilled Fallow in Montana

Abstract We evaluated wheat stem sawfly, Cephus cinctus Norton, parasitism, infestation, and sawfly-cut stems in wheat fields bordering intensely tilled (no visible stubble residue), minimally tilled (>75% stubble residue visible), and untilled (chemical fallow, herbicide fallow management) summer fallow fields in north-central and south-central Montana. No difference in sawfly parasitism or sawfly-cut stems was found between fields bordering minimally tilled and fields bordering untilled summer fallow. Sawfly parasitism in fields bordering untilled summer fallow was greater than in fields bordering intensely tilled summer fallow at six of the eight sites examined. Sawfly-cut stems were greater in the field bordering intensely tilled fallow at four sites, with no difference in sawfly-cut stems between the intensely tilled and untilled field at the other four sites. Although it has never been reported, we have observed that many sawfly stubs are completely buried. Therefore, we measured the depth of sawfly stubs in four untilled fields in Broadwater County, MT. Two-thirds of the stubs were completely buried (206 of 300) with an average depth of 6 mm. Intensive tillage, which results in soil-covered stubble, is not an effective sawfly control practice, because sawflies typically overwinter below ground and upon emergence must dig to reach the soil surface. However, Bracon cephi (Gahan) and Bracon lissogaster Muesebeck overwinter above ground in stems and might be unable to dig to the soil surface if buried. The elimination of intensive tillage in favor of chemical fallow should result in greater sawfly parasitism over time. Producers replacing minimal tillage with chemical fallow should see no effect on sawfly parasitism.

Behavioural responses of wheat stem sawflies to wheat volatiles

1 Adult wheat stem sawflies Cephus cinctus, pests of cultivated cereals that also infests wild grasses, migrate into wheat fields where they oviposit in elongating, succulent stems.

Effect of Host Plant Quality on the Sex Ratio and Fitness of Female Wheat Stem Sawflies (Hymenoptera: Cephidae)

Abstract Larvae of the wheat stem sawfly, Cephus cinctus Norton, feed and complete their development within the lumen of wheat and other hollow-stemmed grasses. Stems with large diameters are preferred and are the most suitable hosts. The sex ratios of wasps are male-biased in small stems and female-biased in large stems. Sawfly fitness, as indicated by wasp size, fecundity, and longevity, increases with host stem size.

Wheat Stem Sawfly, Cephus cinctus Norton, Impact on Wheat Primary Metabolism: An Ecophysiological Approach

Abstract The impact of wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), on the photosynthetic capacity and primary metabolism of wheat, Triticum aestivum L., was evaluated in three different environments: environmental growth chamber, greenhouse, and field. C. cinctus elicited different photosynthetic responses in different environments. Wheat gas exchange parameters, such as photosynthesis, stomatal conductance, intercellular CO2, and transpiration in the growth chamber environment were negatively affected by C. cinctus feeding. Conversely, the same gas exchange responses were not observed under greenhouse and field conditions. This study shows the important role of environmental variables, such as ambient CO2 concentrations and light intensity, on plant responses to herbivores.

Trap Strip and Field Border Modification for Management of the Wheat Stem Sawfly (Hymenoptera: Cephidae)

The alternate-year summer fallow wheat production system predominates in the semi-arid prairie regions of Montana. These farms consist of the current crop and idle fields in which the previous years crop was located. Larvae of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), overwinter in post-harvest wheat stubble. Adults appear and disperse to new crops during the following growing season. Adults begin oviposition as soon as suitable hosts are encountered; therefore, larval infestations usually are concentrated along field borders. We tested several types of trap strips as intercepts to reduce dispersion of adult sawflies into fields. The most successful system was a fall-planted winter wheat trap that protected spring-planted wheat. These trap strips utilized the “border effect” as well as the higher attractiveness of the earlier maturing winter wheat. In another trial, losses were reduced by planting a semi-resistant solid-stemmed wheat cultivar within the border of a comparative...

Infestation of Wheat and Downy Brome Grass by Wheat Stem Sawfly and Subsequent Larval Performance

Abstract Wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), oviposition patterns and larval development in downy brome grass, Bromus tectorum L., and wheat, Triticum aestivum L., were compared in a commercially grown wheat field infested with downy brome grass in Montana. Seven weekly randomly selected samples of stems for each plant species were collected at points where both plants were growing together. The level of infestation in downy brome grass was almost two-fold higher than for wheat throughout the growing season. Larval mortality was not detected in either plant species early in the season, but mortality was more frequent in downy brome grass as the host plants matured. Mortality of late-instar larvae late in the season was significantly higher in mature downy brome grass than in mature wheat stems. The weight of these late-instar larvae from wheat was almost four-fold heavier than larvae from downy brome grass. Stem height and seed weight in wheat were significantly reduced by larval sawflies feeding. In contrast, stem height, stem diameter, seed weight, and seed number in grass stems were significantly greater in infested downy brome grass stems compared with uninfested plants. Our results suggest that downy brome grass, a serious weed in wheat cropping, may play an important role in the survival and dynamics of wheat stem sawfly populations in the northern Great Plains.

Mycoses of Wheat Stem Sawfly (Hymenoptera: Cephidae) Larvae by Fusarium spp. Isolates

ABSTRACT A complex of Fusarium spp., including F. pseudograminearum, F. culmorum, F. avenaceum, F. equiseti, and F. acuminatum, was isolated from field-collected larval cadavers of wheat stem sawfly at two locations for 2 yr. The Fusarium spp. isolates caused mortality in both diapausing larvae in a topical bioassay and in developing larvae feeding in infected stems in a greenhouse experiment. Larval mortality was >90% in both experiments at the highest dose. The pattern of correlation between integument discoloration, hyphal growth, and larval mortality showed that the Fusarium spp. isolates actively infect larvae and kill them, rather than colonizing larval tissue as secondary postmortem invaders. The versatility of Fusarium spp. as plant and insect pathogens enables colonization that results in disease in wheat plants and subsequent mortality of the wheat stem sawfly larvae developing within the same tissue.

Phylogeography of the Wheat Stem Sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae): Implications for Pest Management

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is a key pest of wheat in the northern Great Plains of North America, and damage resulting from this species has recently expanded southward. Current pest management practices are inadequate and uncertainty regarding geographic origin, as well as limited data on population structure and dynamics across North America impede progress towards more informed management. We examined the genetic divergence between samples collected in North America and northeastern Asia, the assumed native range of C. cinctus using two mitochondrial regions (COI and 16S). Subsequently, we characterized the structure of genetic diversity in the main wheat producing areas in North America using a combination of mtDNA marker and microsatellites in samples collected both in wheat fields and in grasses in wildlands. The strong genetic divergence observed between North American samples and Asian congeners, in particular the synonimized C. hyalinatus, did not support the hypothesis of a recent American colonization by C. cinctus. Furthermore, the relatively high genetic diversity both with mtDNA and microsatellite markers offered additional evidence in favor of the native American origin of this pest. The genetic diversity of North American populations is structured into three genetic clusters and these are highly correlated with geography. Regarding the recent southern outbreaks in North America, the results tend to exclude the hypothesis of recent movement of damaging wheat stem sawfly populations from the northern area. The shift in host plant use by local populations appears to be the most likely scenario. Finally, the significance of these findings is discussed in the context of pest management.

Host plant quality and male wheat stem sawfly (Hymenoptera: Cephidae) fitness.

The size of male Cephus cinctus Norton (Hymenoptera: Cephidae) adults did not significantly increase with the size of host stems in which larvae developed. Larger adult male sawflies had higher lon...