Alfalfa Weevil-Feeding Resistance 



R. H. Ratcliffe 



Beltsville Agricultural Research Center, Beltsville, Md., 



Methods are available for laboratory selection of germplasm 

 with alfalfa weevil (Hypera postica (Gyllenhal)) larval anti- 

 biosis and nonpreference to adult feeding and oviposition. 

 Laboratory methods for evaluating larval antibiosis have 

 been used successfully in selecting plants with a high level 

 of resistance in several Medicago species (4,78). Present 

 levels of antibiosis and nonpreference in M. sativa, however, 

 are too low to recommend use of laboratory methods for 

 cultivar evaluation. These methods do not accurately iden- 

 tify tolerance to larval feeding, which is the principal mecha- 

 nism of resistance in M. sativa evaluated to date (3). In field 

 tests, weevil-resistance selections have been separated 

 consistently on the basis of larval-feeding damage. Thus, 

 field testing in areas with natural weevil infestations is the 

 only practical method for evaluating cultivars for weevil 

 resistance. Tolerance may be related to fall and spring 

 growth characteristics plus the plant response to weevil 

 injury. 



Test entries should be seeded in broadcast or drill-row plots 

 at least 4.5 m 2 . Spaced plants have been unsatisfactory for 

 alfalfa weevil evaluations. At least four replications of each 

 entry, including resistant and susceptible check cultivars, 

 should be included in each trial. Larval feeding damage is 

 evaluated by estimating the percentage of leaves eaten (per- 

 cent defoliation) on the basis of total leaves available. Per- 

 centage defoliation is estimated for the entire plot. Data for 

 each entry should then be converted and presented as per- 

 cent defoliated and compared with the resistant check, 

 using a base of 100 for the check. An example of this proce- 

 dure is shown in table 2. More than one evaluation date is 

 necessary to determine the performance of most entries 

 accurately. 



Table 2.— Alfalfa cultivars classified for tolerance to alfalfa 

 weevil larval feeding by percent defoliation in 

 relation to the check cultivar, Team, Raleigh, N.C., 

 specified dates, 1970 



Cultivar 



Defoliation (pet of Team) 1 

 April 22 May 1 May 6 



Average 



Arc 



Saranac 



Atlantic 



Percent 



106 

 186 

 224 

 236 



Percent 



73 

 172 

 195 

 213 



Percent 



90 

 146 

 151 

 151 



Percent 



90 



168 

 190 



Cherokee 



197 



Percent 

 defoliation of 

 Team 



28.3 



36.7 



51.7 



38.9 



'Team = 100. 











Conditions that influence plant growth and insect infestation 

 levels cannot be controlled in the field; therefore, at least 

 2 years of data are obtained on each set of plots. The varia- 

 tion because of differences in population levels of alfalfa 

 weevils (both seasonal and between locations) is reduced 

 when tests are located in areas of known high weevil infes- 

 tation. This reduction, however, often necessitates testing 

 many lines or cultivars outside their area of development and 

 adaptation, which can influence tolerance ratings. Recom- 

 mending one or two specific test locations for alfalfa weevil 

 testing, therefore, is difficult. Entomologists and plant 

 breeders involved in developing resistant cultivars will need 

 to determine appropriate test locations based on weevil 

 pressures and cultivar adaptation. 



Techniques similar to those used for alfalfa weevil feeding 

 resistance can be used for evaluating Egyptian alfalfa weevil 

 feeding resistance (84). 



Pea Aphid Resistance 



E. L Sorensen 



Kansas State University, Manhattan 



Several methods and resistance criteria have been effective 

 for evaluating resistance of alfalfa cultivars to the pea 

 aphid, Acyrthosiphon pisum (Harris) (10, 37, 67, and 81). 

 Plant resistance to pea aphid has been shown to be similar 

 at various stages of growth in both laboratory and field. We 

 use a seedling test under controlled conditions. 



Numbers of aphids needed to evaluate resistance can be 

 increased in the greenhouse on succulent susceptible 

 alfalfa or on broadbeans, Vicia faba L. (Faba vulgaris 

 Moench). Broadbeans, however, should be used with cau- 

 tion because aphids may become conditioned to them, 

 which could in time influence their reaction on alfalfa. 



Evaluation tests are conducted in either a growth chamber 

 or greenhouse. Greenhouse tests are most practical, but 

 aphids tend to congregate at the sides or ends of flats as 

 temperatures and light intensities change. Tests conducted 

 in growth chambers at 20°C, 55 to 65 percent relative 

 humidity with a 16-h photoperiod at 13,455 lux of light, are 

 more precise than greenhouse tests. 



Seedlings are established in rows in greenhouse flats. A 

 minimum of four 50-plant replications is recommended. 

 Scarified seeds are planted on steam-sterilized soil and cov- 

 ered with 1.3 cm of sand. Flats are placed in aluminum trays 

 for subirrigation. Seedlings are counted in the cotyledon 

 stage and immediately infested by shaking large numbers 

 of aphids over them. Additional aphids are added as needed 

 to maintain a high population. Plants and insects are en- 

 closed in 25-cm-high glass cages with plastic screen tops. 

 When most plants of the susceptible check cultivar are 



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