species of plants may be preferred by some kinds of insects. The 

 beet leafhopper, which transmits curly top virus disease to sugar 

 beets and other crops, prefers weeds, particularly Russian thistle 

 and mustards on recently abandoned fields and overgrazed 

 ranges in the western United States. Plants in the later stages of 

 succession, such as Bromus tectorum and the perennials, are not suit- 

 able hosts. Thus the destruction of the first weed invaders by 

 this insect may hasten the establishment and growth of the more 

 permanent species. ^^^ 



DiflFerences of Species in Susceptibility to Parasites 



Differences between species may be caused by the resistance 

 capacity of the host, the degree of virulence and abundance of 

 the parasite, and by environmental conditions. The chestnut 

 blight is an outstanding example of a high degree of virulence of 

 a parasite in a nonresistant host, resulting in the virtual elimina- 

 tion of the chestnut from the deciduous forests of eastern North 

 America (Figures 1-24, 1-24A, 1-24B). 



The most widespread pathogens in grasses, especially rusts, at- 

 tack leaves. Other important diseases are root and crown rots, 

 head and leaf smuts, ergots, and other seed disorders.^^ Variation 

 between strains in resistance to pathogens is one of the criteria 

 for the selection of plants for cultivation and breeding; for ex- 

 ample, the susceptibility of the Merion strain of Kentucky blue- 

 grass to stripe smut {Ustilago striiformis) is much greater than that 

 of other varieties and selections. '^^ Common insect parasites on 

 grasses are the spittlebug in humid regions, mites in the Pacific 

 Northwest, and thrips, chinch bugs, grasshoppers, and white 

 grubs in various areas. ^^ 



Effects of parasites include the reduction of vigor and conse- 

 quent loss of competitive capacity, decreased range of ecological 

 amplitude, loss in capacity to use the resources of the environ- 

 ment, and decreased yields of herbage and seed. The association 

 of species and management practice may influence the degree of 

 parasitism; for example, the spread of crown gall was less rapid 

 on alfalfa when grown in association with smooth bromegrass 

 than when it was grown alone. ^^ Blind seed disease on ryegrass 

 in Oregon can be effectively controlled by deep and early plow- 

 Ecological CHairacteristics o£ Species &. Populations • 51 



