Evaluation of Summer Oil Spray on Amenity Plants

Efficacy against Aphids

Of the 24 formal and 9 informal spray oil trials conducted during the summer of 1987, 15 involved various species of aphid. Plantings of European euonymus often supported heavy populations of Aphis fabae, commonly averaging more than 100 individuals per leaf. Terminal portions of many mock orange shoots were frequently seen solidly encrusted with the same aphid species. Leaves of red oak trees often supported about 40 individuals of Myzocallis granovski on many upper surfaces. Myzocallis sp. aphids were also abundant, about 50 per leaf undersurface, on the foliage of witch hazel and were also found, at a population density of 30 per linear inch, on ornamental pear. It is interesting to note that neither of these plants has been previously reported as a host for this specific aphid. Red pine, the sole conifer, supported a large (about 25 insects per needle) population of pine needle aphid (Eulachnus agilis). Aphid infestations were also evaluated on 3 other woody ornamentals: Macrosiphum liriodendri at 50 per upper leaf surface on tulip tree, Eucallipterus sp. (possibly E. tillae) on little leaf linden at 40 per leaf, and a species of Aphis found solidly encrusting the terminal shoots of shrubby viburnum. Informally evaluated (not actually counted) were 4 additional heavy aphid populations on woody ornamentals: Aphis citricida on bridal wreath, Macrosiphum rosae on rose, Aphis cltricola on a semi-dwarf ’Empire’ apple tree, and what is probably Sitomyzus rhois on staghorn sumac. Infestations of Aphis sp. were also found on a pair of herbaceous plants, garden amaranthus and oriental poppy.

In all of these cases, complete elimination of the colonies was obtained within a 24-hour period. During both laboratory and field trials, through several replications, both winged and wingless forms were killed rapidly with a 3% oil spray. Seen through a microscope at higher magnifications, individual aphids still had their mouth parts embedded in host plant tissues, implying that the oil was not an irritant. In none of these evaluations was there any indication that oil had damaged the foliage in any way.

Efficacy against Scales

Six trials evaluated the effect of Sunspray 6E on scale insects. Since one of the primary objectives of this study was to evaluate the ovicidal potential of this spray oil, as well as its effect on adults and crawlers, an effort was made to identify test populations before a large number of eggs had hatched into first instar crawlers. Heavy infestations of armored scales on a pair of conifer species, pine needle scale (Chionaspis pinifollae) on red pine and juniper scale (Carulapsis juniperi) on American arborvitae provided an opportunity to do this.

The juniper scale population was just starting to hatch out with only a few crawlers present at the time the 3% oil spray was applied. Within 24 hours all crawlers on treated needles appeared to be dead. At the end of the following week nearly all eggs on the control needles had hatched and crawlers were abundant, whereas no additional eggs on treated foliage had hatched and no crawlers remained alive. Spray oil seemed to be immediately and highly effective against the life stages of these two scales.

The population of pine needle scale, representing a mid-season (early to middle July) second annual generation, was located and treated well prior to the first emergence of the reddish crawlers. Of the 255 female scale insects examined on 10 “population” needles, 85% were found to contain reddish eggs beneath the scale covering. Needles brought back into the laboratory for examination at higher magnification following application of spray oil showed that the oil had rapidly and readily migrated under the scale covering, nearly all of which contained a large number of eggs. Two weeks after spraying, most of the eggs on the untreated control needles had hatched and essentially all now supported more than 100 crawlers per needle. The reduction in crawler population on the treated needles was dramatic, ranging from no individuals to a high of 20 and averaging fewer than 10 crawlers/needle, thus indicating a better than 90% reduction in egg hatch and suggesting that this spray oil has very good ovicidal activity against scale eggs of the second generation.

A heavy pre-emergence population of euonymus scale Unapsis euonymi) on tree eunoymus provided supportive evidence for the ovicidal effectiveness of Sunspray 6E. Two days after oil treatment, control twigs supported an abundant active crawler population averaging 26 per linear inch while the treated twigs had no living crawlers. In addition, eggs on treated twigs had enlarged, become transparent and easily ruptured if touched; the overwintering female adults had darkened and started to decompose.

Three other scale infestations were treated with oil at a point when egg hatch was well advanced and nymphs were abundant. Two of these were lecanium scales, the European elm scale (Gossyparia spuria) on American elm and redbud scale (Lecanium corni) on redbud. An additional population of cottony maple scale (Pulvinaria innumerabilis) on blackhaw viburnum was also treated. All populations were counted at more than 100 individuals and all showed nearly complete control of crawlers within a 2-day period. No additional egg hatch on treated foliage was observed in the following 5 days. In none of these evaluations against scale populations was there any indication of oil-induced damage to foliage.

Efficacy against Mites

Of the 6 trials of spray oil in controlling mite infestations, 5 involved species of spider mite: spruce mite (Oligonychus ununguis) on hemlock, red oak mite (0. bicolor) on Chinese chestnut, maple mite (0. aceris) on sugar maple and linden mite (Eotetranychus tiliarum) on silver leaf linden. One eriophyid species, privet rust mite (Aculus ligustri) of privet was also investigated.

Conifers were represented by a pair of mature northern hemlocks found, by the middle of June, to be heavily infested with spruce spider mites. Ten needle populations on separate tagged branches were found to average 4 adults and 14 eggs each. A recount of these populations 24 hours after spraying with 3% oil showed no living adults, and a follow-up at 1 week showed that none of the eggs had hatched. Mite populations on control branches continued to thrive and expand.

Results from oil-treated spider mite populations typically exceeding 100 individuals per leaf on deciduous ornamentals followed a similar pattern. Within 24 hours after treatment no living mites could be found on treated foliage and at the end of a week post-treatment no additional eggs had hatched. Under higher magnification the normally smooth-skinned eggs had darkened, shrunken and become wrinkled and irregular in shape. Repeat population counts on oil-treated leaves confirmed the observation that there had been essentially no emigration of mites away from the leaves following treatment with oil. An additional informal trial suggested that 3% oil was also highly effective in controlling spider mites on a greenhouse specimen of large-flowered tuberous dahlia without phytotoxicity.

Mites of the family Eriophyidae were represented by a heavy mid-August infestation of pivet rust mites on a planting of border privet. Each leaf typically supported a population of considerably more than 100 individuals, probably several hundred, and much of the planting showed severe russeting of foliage. As in previous trials involving spider mites, a thorough spray of 3% oil gave nearly complete control of privet rust mites within a day or two. Pest populations were also treated with 1% and 2% oil sprays in a first attempt at evaluating the degree of control attainable using lower concentrations of oil. Although still preliminary, the results of these trials suggest that acceptable control may be obtainable at oil concentrations substantially less than 3%.

Efficacy against Other Pests

As the 1987 field season progressed, several other insect populations were located and treated with a 3% spray of Sunspray 6E. One of these was a heavy infestation of euonymus webworm (Yponomeuta multipunctella), the larva of an ermine moth, on tree euonymus. This is a species of ornamental typically free of most holometabolous insect pests that in this case had been nearly defoliated by the caterpillars. Due to some uncertainty as to how well the web would be penetrated by oil spray, this trial was run in both field and laboratory. In one instance webs were sprayed from the outside, while in a parallel study caterpillars were extracted from their webs, treated with oil on a raised wire screen, and returned to fresh, unsprayed euonymus leaves. Unlike many other pests, the treated caterpillars were not immediately inactivated by the oil, although within 24 hours they had become noticeably lethargic, had not resumed eating, and were webbing only weakly. At the end of a week after treatment, untreated caterpillars had produced well-developed webs containing abundant frass and all had spun elongate, dense cocoons. During this same period, roughly half of the treated population had died outright, while the remainder had spun thin, poorly developed cocoons. Lack of frass accumulation suggested that none of the treated caterpillars had resumed eating. During the following 2 weeks, essentially the entire control population emerged as small, white moths, while not a single webworm from the treated population emerged. Although considered preliminary, these findings suggest that in such lepidopteran larvae as euonymus webworm, horticultural oil may act as a physiological toxin and/or an anti-feedant. Similar results were obtained when spray oil was evaluated against another moth larval form, a leafroller (Archips sp.) on red osier dogwood.

Sunspray 6E was also evaluated for efficacy against the larvae of two Hymenoptera (sawfly) species, Amauronematus sp. on bristly locust and Pristiphora geniculata on mountain ash. The oil appeared to act as a strong antifeedant on the locust sawfly larvae, but had essentially no effect on the sawfly infestations of mountain ash.

Horticultural oil has been suggested as an effective means of controlling mealybug infestations on evergreen species, and our trials of Sunspray 6E seem to bear this out. Several decorative yew plantings were found to support heavy populations (about 20 per linear foot of twig) of taxus mealybug (Dysmicoccus wisteriae). Within a day or two of applying 3% oil, 80-100% of the insects physically contacted by the spray had been killed. The treatment seemed to have been as effective against adults in their waxy coverings as against naked first instar crawlers. Efficacy of oil against mealybug eggs is unknown. Of the several yew branches treated, none showed any indication of having been damaged by the oil application.

A small but severe infestation of greenhouse whitefly (Trialeurodes vaporariorum) on some floriculture stock provided an informal opportunity to evaluate the efficacy of spray oil against this nearly universal pest. A single spray treatment of 3% oil was found to be a rapid and highly effective control measure for whitefly infestations on New England aster and summer phlox. On neither was there any indication of foliar phytotoxicity.