INFLUENCE OF SWEETPOTATO WEEVIL' INFESTATION ON THE YIELDS OF TWELVE SWEET POTATO LINES'

Michael A. Mullen Stored-Product Insects Research and Development Laboratory

USDA, Agricultural Research Service Savannah, GA 31403

Abstract: The effects of infestation by the sweetpotato weevil, Cylas formicarius elegantulus (Summers), on yield in 12 sweet potato cultivars were compared. Significant reductions in yield were demonstrated by comparing weevil-free fields with infested fields. Only one cultivnr, W 101, showed no significant reduction in yield due to weevil infestation. The avcmge yield reduction of 69% WIlS probably cllused by a number of factors; however, mortality of infested plants was the major contributing factor.

I(ey Words: Swectpotnto weevil, yield reduction, sweet potato, Cylas formicarius elegantulus.

,I. Agric. Entomol. 1(3): 227-230 (July 1984)

Although moderate levels of resistance of sweet potato varieties to infestation of the sweetpotato weevil, Cylas formicarius elegalltulus (Summers), have been demonstrated, complete immunity does not exist (Mullen et al. 1980). Jones et al. (1983, 1984) released varieties, Resisto (fonnerly W 125) and Regal (formerly W 152), with moderate levels of resistance to the sweepotato weevil as well as a wide range of other insect pests and diseases. Little has been done to examine the effed of sweetpotato weevil infestation on yield. Cockerham et a1. (1954) observed that the effects of weevil infestation on yield were minimal. Similar results were reported by Talekar (1982) for C. f formicarius (Fabricius) in Taiwan. However. Pillai and Nair (1981) reported yield losses of 10 to 100% due to weevils. Subrarnaniam et al. (1977) and Pillai et al. (1981) reported losses of 35 ~ 70% depending on the season. Mullen et aJ. (1981) found that severe crown damage, especially in varieties like Centennial, resulted in significant reductions in yield.

This study was conducted at the Plant Introduction Station in Savannah, GA. to determine the effects of sweetpotato weevil infestation on yield of various sweet potato cultivars. Yields in infested and non-infested fields were compared.

MATERIALS AND METHODS

T\.."elve sweet potato lines were examined in each of two fields to determine the effect of weevil infestation on yield. Eight experimental lines and the commercial varieties, Jewel, Centennial, Resisto, and Regal, were used. Resisto (Jones et al. 1983) and Regal (Jones et al. 1984) are new commercial varieties with moderate levels of weevil resistance. Six plots were planted in each field and each consisted of five hills each of three or four experimental lines and either Jewel, Centennial or both. Each plot consisted of five rows and five columns with a total of 25 hills. The spacing between plants was 0.7 m. Each variety was represented in two plots

I COLEOPTERA: Curculionidllc 2 Received for Iluhlicmion 7 D!'cembeT 1983; Ilccepled 12 MaTch 1984.

227

228 J. Agric. Entomol. Vol. 1, No.3 (1984)

in each field for a total of 20 hills. In addition, in each field two plots of 25 hills each were planted in either Jewel or Centennial. The two fields were planted approximately 150 m apart. All plantings were from vine cuttings, and standard horticultural procedures were followed. Seventy·seven days after planting, 25 pairs of sweetpotato weevils were released at random in each plot in one field. No weevils were released in the other field.

The sweet potatoes were harvested 145 d after planting. The roots in each plot were weighed and evaluated for severity of weevil damage. The percentage of infested roots in each line was determined, and the severity of damage to the plant and to the crowns was determined and rated according to the level of weevil damage. The rating system for severity and for the crown damage was a 1 - 5 scale with a score of 1 indicating no visible damage and a score of 5 indicating severe damage. Mean yields of weevil-free and weevil· infested fields were compared with Student's t - test. A multiple regression analysis was used to compare the role of the various factors contributing to yield differences. Abbott's formula (Abbott 1925) was used to calculate mortality of sweet potato plants due to sweetpotato weevil infestation.

RESULTS AND DISCUSSION

The effects of sweetpotato weevil damage on total yield in mixed plantings of 12 lines are given in Table 1. No weevils or damage were found in the weevil-free field. A multiple regression analysis showed that the primary factor contributing to yield reductions was mortality of plants caused by weevil infestation. The severity of root infestation and the percent of infested hills weI1'! contributing factors. Crown infestation was found to be the least important factor. However, plant mortality was the result of the combined effects of crown damage and the severity of the damage. Because the roots are storage organs, the severity of root infestation was not considered as an important factor in yield reduction.

Weevil infestation caused significant reductions in yield of all lines except 'l,V 101. The high crown index of 4.9 indicates that \V 101 is tolerant of crown injury and thus crown injury had only a moderate effect on yield. Varieties, Resisto and Regal, which had previously been listed as having good levels of weevil resistance (Mullen et a1. 1982, Jones et aL 1980) produced large numbers of roots even when exposed to weevil injury. In both lines the severity and crown indexes were low. Cultivar W 151 had the highest yield with over] 200 g per hill, but the yield was reduced 85% by weevil damage. Damage to W 151 was severe, as indicated by a severity index of 3.2 and a crown index of 3.4. Root production in W 186 was eliminated in all but one hill when weevils were present. Only 13% of the roots produced by MK 8 were infested by weevils, but the total yield was reduced by 78%. This yield reduction in spite of low damage indicates that MK 8 is not very tolerant of weevil damage and that even low levels of infestation will result in reduced yield.

Pure stands of Centennial and Jewel had large reductions in yield (Table 1). Although the yield of Jewel in weevil-free and weevil-infested plots was smaller than in the mixed plots the percentage of reduction was similar. Reduction in the yield was similar in mixed stands and in pure stands. Centennial has been considered highly susceptible while Jewel has some resistance to weevil damage (Mullen et al. 1980, 1981).

Table L Effects of sweetpotato weevil infestation on yield in sweet potato plots.

Mixed stand

Yield (g/hill)" Percent hills with raots t

Percent Mortality due

Line Non

infested Infested Percent

reduction root

infest. Non

infested Infested to weevil infest. *

Severity Index§

Crown Index§

:s: c t""

Resisto (W 125) W 101 W 113 W5-75 W 186

729 a 238 a 259 a 216 a 381 a

280 a 182 b 36 a

101 a 17 a

62 23 90 53 95

47 21 80 27 60

90 60 50 50 70

60 60 30 50 10

33 0

40 0

86

2.1 3.1 3.0 1.8 3.0

1.8 4.9 2.8 2.0 3.0

t""

'"z en <•:!­" ~

W 151 1202 a 175 a 85 42 90 50 44 3.2 3.4 S W 131 490 a 95 a 81 33 70 40 43 3.0 3.3 0<: MK 8 283 a 62 a 78 13 60 30 50 1.8 2.3

0 0

W 185 Regal (W 152) Centennial Jewel

760 a 657 a 672 a 478 a

133 a 374 a 215 a 132 a

83 43 68 82

27 36 54 22

50 80 60 72

40 100 72 52

20 -2511 -201 28

3.1 2.6 3.7 2.3

4.3 1.9 3.8 2.3

~ 0

" en•0

:!­"0

Mean 513 ± 83 132 ± 29 69 39 67 49 25 2.7 3.0 0.-S

Pure stand n

Centennial 585 a 293 a 50 58 72 72 0 3.9 4.0 "c: Jewel 427 a 89 a 80 30 60 48

• Yield dirrl'Tence~ "ithin each line followed by th" ~lIme letter &t' difft-rent III the 95'7< l"vel based on the Student's t· teRI.

20 2.1 2.0 <> "•

t Ten hilJ5 per line Cltct'pt for Centennial and Jewel which hlld 25 hills eDeh. ~ Percent mortality due to weevil infestation as calculated by Abbott's formula. § 1 = No visible darnllge.

:; - High damllgc. ~ Survival of plllntSl in infestt'd plots was higher thlln in the control pk>tg.

'" '" '"'

230 J. Agric. Entomol. Vol. 1, NO.3 (1984)

Differences in yield resulting directly from infestation by the sweetpotato weevil are difficult to measure. Talekar (1982) found that the amount of damage, especially to enlarging roots, did not influence yield. However, he did not compare the yield of infested plots with that of non-infested plots. Talekar·s (1982) conclusion that the amount of damage has little effect on yield is substantiated by his findings that some severely weevil-damaged crowns were thicker. indicating that adventitious growth had replaced the damaged tissues allowing for proper development. The data presented here indicate that, although some lines are able to repair damaged tissues, severe damage to the upper parts of the plants, especially the crown, may result in mortality of infested plants with a corresponding reduction in yield throughout the field.

ACKi'lOWLEDGMENT

The author would like to thank Dr. Alfred Jones, Research Geneticist USDA-ARS, U. S. Vegetable Laboratory, Charleston, SC, for supplying the sweet potato cultivars used.

REFERENCES CITED

Abbott, W. S. 1925. A method of computing the effectiveness of an insecticide. J. Econ. Entomol. 18: 265-267.

Cockerham, K. L., O. T. Deen, M. B. Christian, and L. D. Newsom. 1954. The biology of the sweetpotato weevil. La. Tech. Bull. 483. 30 pp.

Jones, A., P. D. Dukes, J. M. Schalk, M. A. Mullen, M. G. Hamilton, D. R. Paterson, and T. E. Boswell. 1980. W-71, W-115, W-119, W-125, W-149 and W-154. Sweet potato germplasm with multiple insect and disease resistances. HortScience. 15: 835-836.

Jones, A., P. D. Dukes, J. M. Schalk, M. G. Hamilton, M. A. Mullen, R. A. Baumgardner, D. R. Paterson, and T. E. BoswelL 1983. "Resisto" sweet potato. HortScience. 18: 251­252.

Jones, A., P. D. Dukes, J. M. Schalk, M. G. Hamilton, R. A. Baumgardner, M. A. Mullen and D. R. Paterson. 1984. Release of 'Hegal' sweetpotato (variety release).

Mullen, M. A., A. Jones, R. T. Arbogast, J. M. Schalk, D. R. Paterson, T. E. Boswell, and D. R. Earhart. 1980. Field selection of sweet potato lines and cultivars for resistance to the sweetpotato weevil. J. Econ. Entomol. 73: 288-290.

Mullen, M. A., A. Jones, R. T. Arbogast, D. R. Paterson, and T. E. Boswell. 1981. Resistance of sweet potato lines to infestations of sweetpotato weevil, Cylas formj~arius efegantulus (Summers). HortScience. 16: 539-540.

Mullen, M. A., A. Jones, D. R. Paterson, and T. E. Boswell. ]982. Resistance of sweet- potato lines to the sweetpot.ato weevil. HartScience. 17: 931·932.

PiIlBi, K. S., S. S. Lal, and M. S. Palaniswami. 1981. Evaluation of newer insecticides and soil amendments for the control of sweet potato weevil, Cylas formicarius Fabricius. Entomon 6: 69-72.

Pillai, K. S., and S. G. Nair. 1981. Field performances of Borne pre~released sweet potato hybrids to weevil incidence. J. Root. Crops 7: 37-39.

Subramaniam, T. R., B. V. David, P. Thangavel, and E. V. Abraham. 1977. Insect- pest problems in tuber crops in Tami Nadu. J. Root Crops 3: 43-50.

Talekar, N. S. 1982. Effects of sweet-potato weevil (Coleoptera: Curculionidae) infestation on sweet potato root yields. J. Econ. Entomol. 75: 1042-1044.