Horticultural Oil Sprays to Control Pests of Landscape Plants: An Industry Survey

Questionnaire Results

Of the 3,500 questionnaires distributed, 35% (1225) were returned; 698 (57%) of the respondents did not or have never used horticultural oil and, of those, 1 % did not realize that the product existed. Replies were received from 45 states, the District of Columbia, and five Canadian provinces. By Green Industry divisions, 25%were arborists, 30% landscapers, 37% nurserymen, and 8% were in other categories. Of the 527 using oil, 284 (54%) routinely apply it as a general purpose spray.

Improved pest control was reported by 54% when another pesticide was added to oil; 40% indicated no experience with combinations. The most commonly added pesticides, (in decreasing order), were ethion, diazinon and malathion. A few respondents were using a relatively new premix containing Trithion. Others acknowledged that they were experimenters and had tried Orthene, Dursban, Cygon, Pentac, Kelthane and Thiodan with the oil. One respondent in Oregon uses 2% oil all season long; not for its pest control attributes, but because he feels that it enhances the effectiveness of the synthetic pesticides in a tank mix.

Respondents used horticultural oil most frequently as a spring season treatment. Regardless of the pest, about 65% of the applications were in the spring, about 16% in the fall and 11 % in the winter months (common in Pacific Coast states and Florida). Very few applications (8%) were made as a summer treatment. Selected pests and data on timing of oil applications are presented in Table 1. A few individuals reported a routine practice of spraying oil on woody plants three times per year (spring dormant, summer, and fall dormant).

Oil was used most frequently to control scale insects and mites. The survey contained a section that asked for an evaluation of the efficacy of the oil applications (Table 2). Oil applications for scale control were considered to provide acceptable to complete control (86% of the responses), and applications for mite control were considered as acceptable to complete (89% of the responses).

From replies to the section on the quantity of oil used per year, the senior author conservatively estimates that the professional landscape industry uses about 750,000 gallons annually. This does not include oil used by homeowners.

Applicators obtain information on how, when and where to use horticultural oil from the Cooperative Extension Service (33%), label instructions (32%), their supplier (13%), and 21% from other sources.

Phytotoxicity

The survey included a section for recording experiences with oil on 27 landscape plants (Table 3). Of the plants listed, there were no reports of damage to sycamore/London plane. All other plants were reported to have been damaged to some degree (e.g., for dogwood, 98% reported no injury, 2% slight injury, 0% moderate injury, and 0% severe injury; and for sugar maple, 56% no injury, 13% slight injury, 13% moderate injury, and 17% severe injury).

The plants that were most frequently reported to be damaged were: aucuba, cryptomeria, hickory/pecan, Japanese holly, Japanese maple, sugar maple, and silver maple (Table 3). Phytotoxicity was also reported on spruce (mostly Alberta, blue, and white).

Phytotoxicity was rated in general as being limited to marginal burn of leaves from spring and summer treatments and twig dieback from dormant treatments. Summarizing all phytotoxic evaluations for all plants the responses were: 83% no injury, 7% slight, 6% moderate, and 4% severe. One respondent observed that highly pubescent leaves were more likely to be injured; another reported that red and silver maple sprayed immediately following bud break resulted in stunted foliar growth. Responses in regard to the dilution rates used were (gallons oil/100 gallons water): 17% at 1.5 gallons of oil, 51 % at 2 gallons, 29% at 3 gallons and 2% at 4 gallons.

There were major differences in phytotoxicity reports within and between regions. In the Southeast, for example, there is an interesting discrepancy regarding the sensitivity of Japanese and Burford hollies. A few respondents indicated no phytotoxicity, but an equal number reported severe defoliation after an application of oil. Several respondents continue to associate summer phytotoxicity with high temperatures, particularly in the north central states. This perception is probably based, in part, on the label admonition to avoid spraying when the ambient temperature is above 85° F. In contrast, one landscape firm in northern Texas uses oil sprays regularly in the summer, even spraying when it is 100°F. One respondent from Colorado reported that label rates were too high for Colorado conditions and usually resulted in foliage injury. He suggested that increased ultraviolet light (associated with higher elevations) may enhance oil-caused phytotoxicity in the Rocky Mountain region.

Misconceptions and Other Uses

While used mostly as a pesticide, numerous references were made to oil’s purported value as a sticker/spreader for other pesticides. Some use oil largely because of customer acceptability. “A slight oil sheen adds greatly to the customer’s visual approval of the spray job,” is a statement made by one respondent. One individual sprays oil with Benlate to control Diplodia tip blight (a foliar and cone disease) on Austrian and mugo pines and says that it has held the disease in check. Another respondent says that oil sprays encourage Diplodia on Austrian pine. Several positive comments were made about oil applications removing sooty mold from leaf and twig surfaces.

Through the survey, we found several innovative and unique uses, some of which strained even the most liberal interpretation of the label; others were overt misuses. Observations made by most respondents are not supported by published research, but in some instances their conclusions seemed logical. Some of these were included above with the hope that new research will be initiated to answer many of the questions and eventually establish guidelines for using oil. Even though horticultural oil is one of the safest pesticides, we would caution the innovator that all of the federal and state pesticide regulations apply.

Conclusions

Since 57% of the respondents do not use oil, more research and/or advertising needs to be done so that horticultural oil is considered more frequently as a pest control option. The vast majority of professionals who use horticultural oil believe they obtain suitable pest control without causing phytotoxicity. Extension personnel have very little current information to disseminate, as most guidelines for oil use are based on work with older oil formulations that are no longer used. Future research must explore proper application timing in relation to phenological development of both plant and pest species, in order to: 1) define the conditions that appear to predispose plants to injury and 2) document efficacy.