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THE BIOLOGY OF BEAN BRUCHID ( obtectus Say) FEEDING ON MATURE SEED OF TEPARY BEAN AND COMMON BEAN Richard E, Shade and Richard C. Pratt, Entomology and Horticulture Departments, Purdue University RIISP Program. W, Lafayette, IN, 47907.

Acanthoscelides obtectus Say (Ao) (bean bruchid beetle) causes widespread destruction of stored common beans (P^. vulgar i s L.), High levels of biological resistance to Ao in jP. vulgaris have been definitively established only in noncultivated P.. vulgaris accessions (Schoonhoven et_ al, 1983). Observations of stored tepary bean (Phaseolus acutifolius A. Gray) have indicated them to be more resistant to bruchid infestation than are peas, pinto beans or cowpeas (Nabhan and Teiwes, 1983). Birch ejt al^ (1985) have reported tepary bean to possess resistance to Callosobruchus maculatus (cowpea bruchid). In a preliminary study, we observed tepary beans to resist damage by Ao, a major pest of Phaseolus species. In the following study we describe the biology of bruchid feeding on six P.. acutifolius accessions vs. seven P., vulgaris accessions.

A randomized complete block design was used for infesting this study. Five consecutive dates of infestation were considered as five blocks. For each block, 20 seeds (10 seeds/4 dr vial) of each accession were infested with ca. 100 one day old eggs (ca. 50 eggs/4 dr vial). The bimodal emergence curve from tepary 310-803 prompted us to divide the emerging adults into two distinct classes (a and b). We do not know if this emergence pattern should be attributed to an environmental or a genotypic effect on portions of the seed lot.

The developmental time of Ao was significantly affected by the bean species on which feeding occurred. All tepary accessions were more resistant to bruchid feeding (developmental times ranging from 45-7 5 days) than were any of the common bean accessions (developmental periods from 32-38 days). Also, within each species variability among accessions was also indicated. Red kidney bean (RKB) was the most susceptible, and tepary PI 310-803 (a and b) were the most resistant to bruchid feeding, as expressed by developmental time. RKB is used to maintain the stock cultures and therefore may be more susceptible due to enhanced feeding specificity.

Mortality percentage figures include mortality during hatching, establishment of the larvae, and feeding of the larvae within the cotyledon. A comparison of the mortality percentages was performed by using a chi-square test. These tests indicate mortality was significantly greater when larvae were feeding on tepary bean rather than when feeding on com.mon bean seed.

By combining the delay in developmental period and the percent mortality we are able to make certain approximate predictions regarding the extent of damage which will occur to seed of the various accessions during six months of storage. The instantaneous rate of population increase was estimated by r = In Ro/T (Price, 1975). The net replacement rate was calculated by using a constant 32,5/eggs per female (Szentesi, 1972), and the percentage total mortality obtained from the experiment. Such estimates are useful to the plant breeder in determining the potential contribution of various types of resistance, and hence in selection of resistant parents for a breeding program. A delay in developmental time of e.g. 5.5 days, coupled with increased mortality (43% increase relative to the RKB control) suggests useful levels of resistance might be found within P.. vulgaris. When compared 46 to the population of adults expected to infest the RKB control after 182 days (10,000) all teparies would incur far less damage (range of expected adult number = 34-579).

The emergence rate ( 3) of adult bruchid from common bean seed was generally faster than from tepary bean seed. The slowest emergence rates from tepary accessions were significantly different from the fastest rates recorded from the common bean accessions. The rate of adult emergence between 10 and 90% emergence, ranged from 7.11 to 20.09%/day. Therefore, 80% of the adults on PI 310-803 emerged over an 11.3 day period while 80% of the adults on Red Kidney Bean emerged over a 4,0 day period. Significant differences among the emergence rates within each species were also observed.

As mean developmental times increase, mean rates of adult emergence decrease. A linear correlation between mean developmental time and rate of adult emergence was significant (1% level) correlation coefficient was obtained (r = .76). A more significant (0.1% level) correlation coefficient, r = .85, was obtained when a linear correlation was run between mean developmental time and the number of days over which 80% (between 10 and 90%) of the adults emerged.

Within a twelve day period after Ao adult emergence, females mate and lay all of their eggs. When seeds are harvested with low infestation levels of Ao, the longer period of adult emergence may affect sychronization of mating and ultimately reproductive efficiency. When low rates of adult emergence were observed, a greater mortality percentage was also noted (r=0.81, P < 0.01). Also correlated with increased mortality percentages was the greater time required for development of Ao. (P < 0.05). Fates of adult emergence between 10 and 90% for all accessions were linear, as coefficients of determination (r ) obtained in calculating each rate of emergence ranged from .94 to .99 and were significant at the 0.1% level.

Incorporation of 4.3% crude albuminic lectin fraction from tepary bean into RKB artificial seed caused a 2 day delay in initial adult emergence as well as in emergence of 50% of the population relative to the RKB control. A slower rate of emergence also was noted. These prelim.inary data suggest the basis of resistance in tepary bean may in part be associated with the albuminic (lectin-rich) fraction.

References

Birch, N., B.J. Southgate and L.E. Fellows. 1985. Wild and semi-cultivated legumes as potential sources of resistance to bruchid beetles for crop breeder: a study of Vigna/Phaseolus. In: "Plants for Arid Lands" (G.E. Wickens, J.R. Goodin and D.V. Field, Eds.) George Allen and Unwin.

Nabhan, G.P. and H. Teiwes. 1983. Tepary beans, O'Odham farmers and desert fields. Desert Plants 5(1):15-37.

Price, P.W. 1975. Ecology. John Wiley and Sons, Inc. New York.

Schoonhoven, A.v., C. Cardona and J. Valor. 1983. Resistance to the and the Mexican bean weevil (Coleoptera:Bruchidae) in noncultivated common bean accessions. Jour. Fcon. Entom. 76(6): 1255-1259.

Szentesi, A, 1972. Studies on the mass rearing of Acanthosce lides obtectus Say (Col., Bruchidae). Acta Phytopathol. Acad. Sei. Hung. 7:453-463.