Effects of Grower Management Practices and Field Characteristics on Insect Damage to Sweetpotato (Ipomoea batatas (L. Lam.) Roots.

Abstract

On-farm research studies were conducted with North Carolina sweetpotato growers to determine relationships between grower management practices and field characteristics with insect damage to sweetpotato roots. Injury by wireworm, Diabrotica spp., and Systena spp. (WDS) complex, sweetpotato flea beetle, white grubs, and whitefringed beetle was categorized by the incidence of roots injured by those species and the severity of injury on damaged roots. Severity was determined by measuring the length of sweetpotato flea beetle tunneling, the number of WDS holes and the diameter of WDS holes. The incidence of the overall insect damage, or injury by any species, was also obtained. Field locations for these projects served as replications.

Questionnaire:

In 2002 and 2003, sixteen sweetpotato growers provided twenty-six field locations where roots were sampled, graded, and assessed for insect damage. Growers responded to a questionnaire about these field locations. The cultural practices, tillage practices, and pesticides used by growers, as well as the weeds, soil drainage class, and rainfall amounts in those fields, were related to the insect damage on the sampled roots.

Approximately 25% of North Carolina's planted sweetpotato acreage was represented by the project conducted in 2002 and 2003. Whitefringed beetle and white grubs caused the least amount of insect damage, 3.0% and 4.6%, respectively. Sweetpotato flea beetle damage averaged 18%, while the most damage, 29%, was caused by WDS. The cultural practices, pesticides used, and tillage practices varied considerably among the 26 field locations used for the questionnaire, indicating that, due to the high amount (average of 43%) of overall insect damage, current management practices used by sweetpotato growers are ineffective for controlling injury to sweetpotato roots by soil-borne insects.

Growers who rotated sweetpotato crops with cotton, delayed the planting and harvest time, subsoiled their fields, and cultivated more than 3 times during the growing season had higher amounts of insect damaged roots from their fields. Overall insect damage on roots from fields in which chlorpyrifos was used was high, about 30%, even though that amount was less than the incidence of damaged roots from fields where chlorpyrifos was not applied (75%). If growers used herbicides they had less WDS damaged roots, although the use of the EPTC herbicide resulted in more grub and whitefringed beetle damage. Roots from poorly drained soils had more WDS damage than roots from well drained soils. Rainfall was positively correlated to the amount of sweetpotato flea beetle damaged roots.

Time of planting and harvest study:

In 2003 and 2004, six sweetpotato growers provided thirteen field locations where sweetpotatoes were planted early (on or before 28 May) and late (after 16 June). Roots were sampled in each of the fields at approximately 90, 105, and 120 days after each planting time, graded, and scored for insect damage. Yield was also obtained.

An early planting and early harvest resulted in the least amount of sweetpotato flea beetle and grub damage, 16% and 1.7%, respectively, although grub damage was also low (1.7%) with a late planting and late harvest time. However, if growers planted late, sweetpotato flea beetle and overall insect damage was high (approximately 45 to 50%) regardless of when roots were harvested. The diameter of WDS holes was larger on roots harvested later in the growing season. The highest yield of number one grade roots, 21 t/ha, was obtained with a late harvest (120 DAP). These results suggest that growers will need to balance decisions between potential losses in yield, or more insect damaged roots, depending on the time that sweetpotatoes are planted and harvested.