Potato Tuherworm Damage to Potatoes Under Different Irrigation and Cultural Practices

J. Econ. Entomol. 72: 261-264 (1979) Potatoes grown under furrow irrigation sustained over 58 times greater infestation by Phthorimaea operCIIlella (Zeller) and 3 times more green tubers than potatoes grown under sprinkler irrigation even though foliage tuberworm populations, soil type, and soil moisture levels were similar. Water usage on a per ha basis was more efficient under sprinkler irrigation. Under furrow irrigation, hilling the rows 4 times significantly reduced the percentages of greenheads, infested whites, and total infestation over those rows hilled twice. The method of vine removal affected the level of tuber infestation in rows hilled twice but not in rows hilled 4 times. Under frequent sprinkler irrigation potato tuberworm damage was low and not significantly affected by cultural p~actices. Soil cracking was more extensive under furrow than under sprinkler irrigation, and infestation was significantly correlated with the depth of soil cracks regardless of hilling practices. Experiments were conducted in 1977 at the University of California's Moreno Field Station, Riverside Co., with summer planted (Aug. 16) 'Norgold' potatoes grown on a uniform sandy clay loam soil. The planting consisted of 106 rows, IOJ m long, spaced 81 em apart (0.86 hal. This area was subdivided into halves, one of which was irrigated by a sprinkler system and the other by furrow. The 2 fields were separated by 25 buffer rows. Each field was set up as a 5x 5 randomized complete block. Each replicate (0.005 hal consisted of 4 rows, 15.2 m long, and plots were separated by 3 buffer The potato tuberworm, Phthorimaea operculella (Zeller), is a serious pest of potatoes in California (Bacon 1960). Most economic damage by this insect occurs when it infests potato tubers, but foliar infestations also may cause yield losses, especially when the insect mines in the stem and kills plant tissue above the site of infestation (Bald and Helson 1944). Early attempts at control of this insect emphasized cultural controls utilizing deeper seed planting, hilling the rows, and early harvest (Langford 1933). These controls alone are not always compatible with current production techniques and consequently the use of insecticides has played a major role in the management of this insect (Shorey et al. 1967, Bacon et al. 1972, Hofmaster and Waterfield 1972) although control of foliage infestation does not always prevent tuber infestation (Bacon 1960, Foot 1974). In warmer potato growing areas, irrigation is required to insure good potato yields, and O. G. Bacon (pers. comm.) indicated that sprinkler irrigated potatoes sustain lower potato tuberworm infestations when compared to furrow irrigated potatoes. Reported here are results of studies to determine levels of tuber infestation under different irrigation systems and cultural controls and to quantify the reasons for differences. Materials and Methods ~optern: Gelechiidae. 'Received for publication Dec. 12. 1978. © 1979 Entomological Society of America rows on the sides and 3 m of buffer on each end. Aldicarb (I5G) was applied at planting (3.35 kg AI/ha) for leafhopper control and 2 applications of nitrogen were used (202 kg/hal. Moisture levels at a depth of 30 em were monitored by 6 tensiometers in each field, and 6 soil samples/field were analyzed for soil type. Irrigation was begun 40 days after planting. Sprinkler laterals were spaced 12.2 m apart with 12.2 m between uprights, and 2.4 mm nozzles were used to give a light spray. For the 1st 3 wk of irrigation, sprinkler water was applied for 9 continuous h/week and furrow water was applied to every row for 18 continuous h/week. During the remainder of the growing season when tubers were setting, more frequent sprinkler irrigations with fewer h per set and longer continuous furrow irrigations were used. This schedule fluctuated with ambient temperatures but averaged 3 times/week with 4 hlset for the sprinkler and 24 h/week of continuous irrigation for the furrow. The 4 rows of each plot received different cultural practices for hilling and vine removal. Two rows were hilled twice and 2 were hilled 4 times. Hillings occurred at planting and 48,65, and 124 days after planting, with the last 2 hillings omitted on rows which were hilled only twice. The vines on one row of each hilling practice were killed with a rotochopper, which shreds the vines, and on the other with a commercial roller, which kills the vines more slowly but compacts the bed. Those rows that received the last hilling (7 days after the vines were killed) also were rolled at that hilling. Bed dimensions in all plots were recorded at each hilling by measuring the width at the top and bottom and the height of each row. All measurements were taken at the same site to determine changes in bed profile over the growing season. At harvest, the beds were sliced open and the positions of 10 tubers from each row were measured in relation to bed profile. The depths of tuber set from the top and nearest side of the bed surface were measured. The amount of bed cracking was determined at 109 days into the growing season prior to an irrigation cycle. The number of cracks in 90 em of each of the 4 rows/ plot ( 183 m total) was counted and their maximal length ,