Effects of Cabbage Looper, 1 Imported Cabbageworm,2 and Diamondback Moth3 on Fresh Market and Processing Cabbage4

J. Econ. Enlomo!. 75; 742-745 (1982) Action thresholds for lepidopterans on fresh market cabbage in New York are proposed. These are based on foliage consumption rates of the cabbage looper, Trichoplusia ni (Hiibner) (Lepidoptera: Noctuidae), imported cabbageworm, Pieris rapae (L.) (Lepidoptera: Pieridae), and diamondback moth, Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), incorporated with actual larval counts of these pests. Once head formation is initiated, cabbage can tolerate ca. 0.5 cabbage looper equivalents per plant before head damage occurs. Larval populations can be permitted to build before head formation, without a loss in weight or marketability, provided they are treated with an effective insecticide at head initiation and treated thereafter at a low threshold. Head weight, rather than cosmetic appearance, is more valid criterion for processing cabbage but was not accurately predicted by larval counts. Over one-quarter of New York's 5,020 ha of cabbage is processed for sauerkraut, and the majority of this, as well as over 80% of the fresh market cabbage in the state, is grown in upstate New York (Anonymous 1979). Several thresholds for lepidopterans on fresh market cabbage have been suggested. Greene (1972) proposed an action threshold of 0.1 cabbage looper larva per plant to obtain 100% marketable cabbage, whereas Chalfant et al. (1979) proposed a visual damage threshold of one to two new holes per plant and Workman et al. (1980) proposed one to two windows (initial feeding sites) per plant to obtain an acceptable level of fresh market cabbage. All these studies were conducted in southern states where pressure from lepidopterans tends to be more severe than in the North. Their thresholds for fresh market standards, where cosmetic injury is of prime importance, may be unrealistic for our area. Additionally, there are no reported threshold studies investigating processing cabbage in which weight of usable cabbage is a more valid criterion. Growers who base treatment decisions on total larval counts may be misled because the main lepidopterous pests on cabbage, namely, the diamondback moth, Plutella xylostella (L.), imported cabbageworm, Pieris rapae (L.), and cabbage looper, Trichoplusia ni (Hiibner), t:onsume vastly different amounts of foliage. Thus, a total insect population may be high but not a threat to the crop because it is composed mostly of a species which consumes little foliage. Additionally, separate thresholds for each species do not take into consideration the occurrence of multiple subthreshold populations which, when combined together, are injurious. Likewise, action thresholds based on visual damage ratings may be inappropriate in some cases because some of the crop may already be lost. Larval counts provide a grower with lead time and help eliminate the loss in marketability. ILepidoptenl: Nactuidae. 2Lepidoptenl; Pieridae. lLepidoptenl; Yponomcutidac. 'ApproVed by lbe Director. N.Y. Slale Agric. Exp. Sin .• for publication as lournal Paper No. 3398. Received for publication 25 November 1981. llJcpanmcnt of Computer Services. We propose the use of larval counts, combined with the species' foliage consumption rate, of the major lepidopterous pests of cabbage for treatment considerations for fresh market and processing cabbage in upstate New York. Materials and Methods Experiments were conducted in 1979 and 1980 on the Robbins Vegetable Research Farm near Geneva, N.Y., where caggabe, cv. 'Roundup,' was transplanted 12 to I3 July 1979 and 7 to 9 July 1980. In 1979 and 1980, plots were composed of four 15-m rows and four 7.5m rows, respectively. Rows were spaced 0.9 m apart, with plant spacing of 41 em. Plots were bordered by two rows of com with 4.6-m alleyways between replicates. Twenty-five action threshold levels were replicated four times in a randomized complete block design. The treatment levels (x larvae per plant, regardless of insect species) before head formation (30 August 1979 and 3 September 1980) were: 0 to 0.4; 0.5 to 0.9; 1.0 to 1.9; 2.0 to 3.0; and untreated control. Once head formation was initiated, as evidenced by a ca. 7-cmdiameter firm leaf ball, the same five treatment levels were used in combination with each preheading level (an all-paired combination with 25 possible thresholds), except heading treatment levels were based on x cabbage looper equivalents (CLE) per plant rather than i larvae per plant. CLE standardizes insect damage by weighting each insect species by foliage consumption, i.e., I CLE = 20 diamondback moth larvae (B) = 1.5 imported cabbageworm (ICW) = I cabbage looper (CL) (Harcourt 1954). The rationale for this methodology was twofold: (I) younger plants are more susceptible to damage from feeding on their few frame leaves as well as their apical meristem, and the two pests present at this time (DB and ICW) both do similar feeding damage and are therefore a threat to delaying growth or aborting the developing cabbage head; and (2) once head formation has begun, the damage caused by any of these pests should be based on consumption of photosynthetic material, and each species consumes vastly different amounts