Damage Relationships and Control of the Tobacco Splitworm (GELECHIIDAE: Phthorimaea operculella) in Flue-cured Tobacco

Abstract

The potato tuberworm, (Phthorimaea operculella Zell.) is the most serious of all potato (Solanum tuberosum L.) pests worldwide (Fenemore 1988; Saur 2004; Kroschel 1995). It also goes by older synonyms, Bryotropha solanella, and Gelechia solanella (Meyrick, 1885). In tobacco, the insect is known as the tobacco splitworm, and can be found in most tropical and subtropical areas worldwide. It was first detected in the U.S. in 1856 in California (Graf 1917). Biology The tobacco splitworm is an oliphagous insect confined to species in the family Solanaceae, including both cultivated and feral plants (Broodryk, 1971, Fenemore 1988). The cash crops attacked by P. operculella include Solanum tuberosum, Nicotiana tabacum, Solanum melongena, and Solanum lycopersicum (Fenemore, 1988), while some of its weed host species include Solanum dulcamara, S. rostratum, and Datura stramonium (Potato Association, 2008). The moth is distinguished from other solanaceous-feeding moths by a costal hairpencil on the hind wings of the male (Meyrick, 1885). Females lay 60-200 eggs in as little as four days, and eggs hatch within three to six days. Eggs are typically laid on green foliage in potato, as larvae prefer to be leaf and stem miners rather than tuber miners, but can be laid in soil or on stems (Potato Assn, 2008). Larvae will pupate in a variety of sites, including walls, windows, screens, plant debris, leaves, soil, ect. (Ferro 1993). Adults usually emerge within a week. In New Zealand, the pupal stage overwinters (Potato Assn, 2008). In the Pacific Northwest, it is possible that all stages but the adult are able to overwinter, although the overwintering behavior is not understood as well in the United States (Potato Assn 2008). Pest Status As a pest of potatoes, the tobacco splitworm not only does minor to moderate damage in the field through larval leaf mining, but can cause severe crop loss in stored potatoes through larval tuber mining (Das 1995). There is zero tolerance for tuberworm in fresh potatoes (Potato Assn 2008). Control methods in potatoes are mostly cultural, such as deep planting, sealing soil cracks, covering tubers with soil, eliminating waste products and maintaining a clean environment, as well as keeping a close eye on stored product (Ali 1993; Coll et al. 2000; Hanafi 1999; Potato Assn 2008). Other practices include using insecticides, mass trapping with pheromone traps, microbial control through the PoGV Granulosis Virus (Baculoviridae) and the bacterium Bacillus thuringiensis, and biological control through the Braconid wasps Apantelels subandinus and Braconi greeni, and the Encyrtid egg parasitoid wasp Copidosoma koehleri (Baggen 1998; Hanafi 1999; Mohammed et al. 2000). Little is known about Phthorimaea operculella as a tobacco pest since most previous research has been in the context of its status as a potato pest. In tobacco, P. operculella is a leaf miner, producing translucent patch mines in tobacco leaves, reducing the value of the leaves. Frass ensnared in these mines also contaminates harvested leaves. Large populations can destroy leaves and plants. The feeding site for this leaf miner makes it difficult to control. Applied insecticides must be systemic to affect the larvae which are protected by the leaf lamina. There are no economic thresholds for the tobacco splitworm, nor have there been any assessments of damage relationships prior to this study. Insecticidal control with currently registered materials has largely been ineffective which occasionally results in season-long outbreaks. Often populations will reoccur in the same field over multiple years, as seen in potato fields (Hanafi 1999). The incidence of tobacco splitworm damage has increased in recent years, creating a need to identify efficacious insecticides (P. Semtner, personal communication) . Infestations can be patchy and sporadic, and range from mild to severe, depending on the year. This insect can be a serious problem for tobacco farmers with no tools to deal with an unfamiliar pest. The tuberworm can rapidly increase to uncontrollable populations because of its high fecundity, short generation time, and because it appears to be favored by dry weather. The insect’s cryptic nature often allows several generations to cycle undetected, and then the insect suddenly appears in overwhelming numbers. Splitworm populations appear to persist from year to year in previously infested fields, depending on the summer weather and crop rotation. Tobacco splitworm populations are generally higher in eastern North Carolina, where the climate is warmer than the western areas of the state. Eastern North Carolina is in plant hardiness zones 7b and 8a, while North Carolina’s overall range is 6b-8a (US Nat. Arboretum, 2003).