Characterizing Insect Resistance 



Background 



More than 100 insect species have been recorded as injuri- 

 ous to alfalfa, according to App and Manglitz (1). In a survey 

 of the United States, however, only 11 alfalfa insects were 

 identified by research and extension specialists as serious 

 over large areas. Additional insects were identified as local 

 problems. This publication is restricted to the 11 widely dis- 

 tributed insect pests. 



Insect pests described here injure the alfalfa crop in several 

 ways. Some reduce plant yields and stands by consuming 

 portions of the plant or by sucking plant juice. Others influ- 

 ence yield and quality by injecting toxins into the plant that 

 cause stunting and distorted growth. Some insect pests 

 attack the flowering parts or developing seed and thus re- 

 duce seed yields. Insects also can serve as vectors of plant 

 viruses and mycoplasmas or provide avenues of entrance 

 for disease organisms. Insect mobility and the many types 

 of feeding injuries they produce often make breeding for 

 insect resistance more difficult than breeding for some 

 types of disease resistance. The recent identification of bio- 

 types for some alfalfa insects further complicates breeding 

 for insect resistance. This is especially true for the pea 

 aphid and spotted alfalfa aphid. 



Considering the complexities of insect, host plant, and envi- 

 ronmental interactions, it is not surprising that reliable eval- 

 uation procedures are available for only the alfalfa seed 

 chalcid, alfalfa weevil, Egyptian alfalfa weevil, potato leaf- 

 hopper, pea aphid, blue alfalfa aphid, and spotted alfalfa 

 aphid. Unfortunately, only the pea aphid, blue alfalfa aphid, 

 spotted alfalfa aphid, and alfalfa seed chalcid tests can be 

 conducted in the laboratory or greenhouse. Most insect 

 screening and evaluation procedures depend on natural 

 field infestations of the target insect, which often creates 

 uncertainties about levels and uniformity of infestations 

 among test years. Field evaluations also create the problem 

 that damage by the target insect may be confounded with 

 disease injury and with injury caused by other insects. 



Although more difficulties appear to arise with insect resist- 

 ance evaluations than with disease evaluations, the same 

 principles should apply in evaluating cultivar resistance to 

 both types of pests. Dependable results require that rela- 

 tively uniform insect infestation levels be developed and 

 repeated in subsequent tests. A desirable practice is to 

 check periodically the identification of the test insect spe- 

 cies because similar-appearing species may confound the 

 results. Whenever possible, cultivar evaluations should be 

 based on scoring individual plants rather than solid-seeded 



plots. A numerical-rating system based on specific levels of 

 damage or on insect population counts per plant should be 

 used, and rating values should be consistent from one test 

 to the next. The percentage of resistant plants, ASI, or aver- 

 age population counts per plant should be used to compare 

 cultivars statistically with standard seed lots of resistant 

 and susceptible check cultivars. 



Insect host-plant resistance often can be expressed using 

 such specific terms as "nonpreference," "tolerance," and 

 "antibiosis" (72). Although this information may be difficult 

 to obtain, specific terminology should be used, when possi- 

 ble, in describing cultivar evaluations. This practice will pro- 

 mote a better understanding of insect resistance and may 

 be useful in planning pest-management programs. 



