Phytotoxicity and Pollination Disorders in Sweet Corn Due to Direct-silk Applications of Materials Used for Alternative Corn Earworm Control

Abstract

<jats:p>Corn earworm [CEW (<jats:italic>Helicoverpa zea</jats:italic>)] is one of the most important pests of sweet corn (<jats:italic>Zea mays</jats:italic>) in New England. Conventional management of this pest is achieved through repeated applications of chemical insecticides through the silking period. Organic growers, however, have few alternatives to prevent CEW infestation. Technology first developed in the 1930s and 1940s, using applications of mineral oil directly into the silk channel with an eyedropper, has been further researched in recent years using vegetable oils with and without pesticides, but pollination problems associated with these treatments have been observed. Several materials were evaluated for efficacy in controlling CEW populations and for phytotoxicity to the developing ear. Materials evaluated were corn oil, soy oil, carrageenan, corn oil mixed with <jats:italic>Bacillus thuringiensis</jats:italic> ssp. <jats:italic>kurstaki</jats:italic> (<jats:italic>Bt</jats:italic>), soy oil mixed with <jats:italic>Bt</jats:italic>, and carrageenan mixed with <jats:italic>Bt.</jats:italic> All treatments were compared with an untreated control. Treatments provided a range of 33% to 50% control of CEW infestation. The oil and <jats:italic>Bt</jats:italic> combinations provided some reduction in infestation compared with the untreated controls (33% vs. 100% infestation), but this level of control was inadequate for all wholesale markets and most direct markets. Additionally, oil-based treatments also caused significant injury to developing ears by reducing pollination quality, impacting the development of the kernels at the ear tip. This condition referred to as “cone-tip” is of concern since it may decrease marketability. The percent unmarketable ears due to cone-tips ranged from 0% to 13% for the untreated and carrageenan-based treatments. From 12% to 42% of ears were unmarketable due to the soy oil treatments. Corn oil treatments caused 10% to 50% cone-tips.</jats:p>

Impact of Applying Edible Oils to Silk Channels on Ear Pests of Sweet Corn

“…The effect of oil treatments on husk coverage was unexpected in this study, although phytotoxicity of various silk treatments on corn pollination, and cob and kernel development has been reported previously (Jackson et al 2007). The effect on husk coverage could be caused by either reduction of cob growth or increase in husk growth during the experimental period.…”

“…One of the well-known earworm control methods used by farmers as early as 1930s and 1940s was the application of a small amount of mineral oil into the silk channels at the tip of the corn ears either alone or in conjunction with an insecticide (e.g., 0.5Ð5% dichloroethyl ether) by using an eyedropper 4 d after the silk was Þrst observed on a developing corn ear (Barber 1938(Barber , 1939Pepper and Barber 1940;Carruth 1942;Jackson et al 2007). The distillation properties of these oils were similar to contemporary mineral oils with median equivalent n-parafÞn values 25 (nC25), possibly 27 (nC27), and in some instances possibly Ͼ30 (nC30).…”

“…Research in the last few decades has examined alternative pest management tactics, including planting dates , corn husk features (Wiseman et al 1977), and silk maysin level (Waiss et al 1979, Snook et al 1993, as well as the application of edible oils on corn ears with fresh silks (Cook et al 2003(Cook et al , 2004, or by using a mixture of oils with microbial insecticides . The recent studies have used edible oils instead of mineral oils that were used in 1930 Ð1940s in the New England states, because certain mineral oils no longer meet the present food safety standard (Jackson et al 2007). FAO (1970) and WHO (1974) have stated the mineral oils are safe to be used, and no further studies are needed, which was based on toxicological and other related studies.…”