A Survey of Five Stem-Feeding Insect Pests of Wheat in the Northern Great Plains
The wheat stem sawfly, Cephas cinctus Norton, the wheat stem maggot, Meromyza americana Fitch, the Hessian fly, Mayetiola destructor (Say), the wheat jointworm, Tetramesa tritici (Fitch), and the wheat strawworm, Tetramesa grandis (Riley), have long been considered wheat pests in the northern Great Plains. This paper reports results of surveys for these pests conducted over 3 yrs in parts of Montana (10 counties), North Dakota (14 counties), South Dakota (1 county), Nebraska (4 counties) and Wyoming (1 county). Fields were randomly selected and a mean of 191 stems were sampled from each field. The percentage of stems infested with each pest was recorded. The percentage of parasitized wheat stem sawfly larvae also was noted. The wheat stem sawfly, wheat stem maggot, and Hessian fly were the most commonly encountered pests, recorded in nearly every county. Wheat stem sawfly density was twice that of wheat stem maggot (7% vs 3.2%) in infested fields. Wheat stem sawfly densities exceeded 10% in 4 counties: Da...
Jointworm Studies in Utah
Infestation of dry-farm wheat during 1931 by the second brood of the wheat strawworm, Harmolita grandis (Riley), was considerably lower, and that of irrigated wheat but slightly reduced as compared with conditions during the preceding year. During 1931 the wheat jointworm, H. tritici (Fitch), was found infesting wheat in three Utah areas, the principal injury occurring in the Tooele-Lake Point section. The rye strawworm, H. websteri (Howard), is frequently encountered in rye-growing areas of northern Utah, being more generally distributed than the rye jointworm, H. secalis Fitch.
Theory for Biological Control: Recent Developments
It has been argued that ecological theory has not been useful to the practice of biological control. The purpose of this article is to show how recent theoretical advances may reverse this situation. We first discuss four issues that have arisen in the development of theory for biological control. (1) Recent work has clarified and resolved earlier disagreements concerning the effect of aggregation of the parasitoid to local host density on the stability of parasitoid-host models; this work emphasizes that such aggregation increases the ability of the parasitoid to reduce pest density. (2) There has been disagreement over the conditions under which stability, in the mathematical sense and on a local spatial scale, is an appropriate goal for classical biological control of insect pests, and whether a metapopulation may sometimes provide a more appropriate framework. We also comment on (3) relative size of refuges, which has been proposed as a unifying concept, and (4) density dependence and ratio dependence. We then discuss recent models using a stage-structured approach, particularly those that compare potential biological control agents or agents that have been differentially successful in practice. We argue that this approach has already produced valuable insights into the factors operating in several field situations. It demonstrates the importance of identifying which pest stages are most injurious to the crop, since the natural enemy that wins in competition may not be the most effective at suppressing the crucial pest stage(s): in general, the winning parasitoid reduces the density of the host stage attacked by its competitor below the level at which the latter can maintain a positive growth rate when at low density. A useful criterion is that pest equilibrium density is suppressed most by the parasitoid species that needs the fewest host individuals to allow a female parasitoid to replace herself in the next generation. Caveats are that this may apply only under equilibrium conditions and that the pest stage most suppressed is the one we wish to control. We discuss the connection between successful biological control and evolutionary considerations.