Effects of SpinTor (Spinosad) on Cabbage Looper (Lepidoptera: Noctuidae): Toxicity and Persistence of Leaf Residue on Cabbage Under Field and Laboratory Conditions

Toxicity of SpinTor 2 SC (spinosad, a fermentation by-product based compound derived from a naturally occurring soil actinomyces bacterium, Saccharopolyspora spinosa) and its field- and laboratory-aged leaf residues were tested against the 2nd or 3rd instars of cabbage looper, Trichoplusia ni (Hubner), on cabbage. In field-aged leaf residue bioassays, the pyrethroid Karate gave 100% mortality from 0 to 12 d after treatment, whereas Spin Tor at all rates was highly toxic to 2nd instars, giving 100% mortality at 0 d after exposure. However, mortalities of T. ni larvae caused by SpinTor at 6d after exposure reduced to 26.7, 65.0, and 86.3% at 12 d aftertreatment at 0.51 kg (AI)/ha alone, and 0.051 and 0.105 kg (AI)/ha with a surfactant, Dyne-Amic, respectively. In the laboratory, leaf residue of SpinTor was highly toxic to 3rd instars for at least 36 d. At 4 d after exposure at 36 d after treatment, larval mortalities were still as high as 70-100%, and only SpinTor at 0.52 kg (AI)/ ha without adding Dyne-Amic provided lower mortality than the other 3 SpinTor treatments. Toxicities of SpinTor on 3rd instars were determined by 3 bioassays: larvae treated, leaves treated, and both larvae and leaves treated. LC 50 and LC 90 at 72 h after treatment were not significantly different between only larvae and both larvae and leaves treated. However, when only leaves were treated and the larvae were fed with treated leaves, the LC 50 and LC 90 were significantly greater than when larvae or both larvae and leaves were treated. These results indicate that SpinTor caused mortality both through direct contact and ingestion, and a combination of contact and ingestion provided greater toxicity to T. ni larvae than ingestion alone.