Choose the most effective aphid controls

For plant entomologists, evidence that spring has arrived is the increased number of phone calls and plant samples being sent related to insect and mite activity. Thrips, fungus gnats, twospotted spider mites and aphids top the list of inquiries to me to date. Despite a relatively mild winter, clearly these pests did not originate outdoors, but more likely moved from other crops or in a few cases may have arrived with new plant material. In any case, there are problems to solve and choices to make in managing any of these.

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A traditional pest

Aphids are almost an annual tradition on greenhouse crops. Unlike some other pests, such as thrips, as a group aphids are usually relatively easy to control. So far this season, green peach, melon and crescentmarked lily aphids have been identified on spring crops.

Melon aphid has been the most common species on greenhouse plants and also the one eliciting the most complaints from growers about poor efficacy. These insects are usually dark green or nearly black, sometimes with pale-yellow individuals mixed in.

Melon aphid enjoys a very wide host range, although some strains or biotypes exhibit preferences for certain plants. For example, melon aphids have been collected from the field on catalpa or pumpkin that would just not “take” to impatiens after multiple attempts to infest the plants.

Melon aphid tolerates very low temperatures, a fact made apparent to one grower in late winter bringing herb stock plants into the greenhouse from unheated cold frames for spring propagation. Melon aphids often have a noticeable waxy coating, which may help protect them from insecticide sprays, and they have evolved physiological ways to detoxify some common pesticides. Michigan State University Pesticide Resistance Database has many reports of insecticide resistance in this species, primarily to organophosphate and pyrethoid insecticides.

Green peach aphid is also common in greenhouses with many reports in the Michigan State Database of resistance to the same chemical controls. They have a similarly wide host range including many popular greenhouse bedding plant and flowering pot crops.

Colonies of green peach aphids are mostly pale green with occasionally pinkish-orange individuals, although there seems to be a dark-red variant that some think may be a different species. Compared to melon aphid, I tend to receive fewer complaints about difficulty controlling green peach aphids, although they, too, are sometimes reported to have enzymes that detoxify some insecticides.

Both melon and green peach aphids are notorious for their ability to transmit certain plant viruses, although this is generally less of a concern in finished crop production compared with long-term maintenance of clean stock plants for propagation.

Crescentmarked lily aphid, named for a horseshoe-shaped dark band on the abdomen, was recently found on Easter lily on Long Island, N.Y. It has been reported to feed on other ornamental crops including begonia, calla lily, cineraria, Zantedeschia, ferns, pansy, fuchsia and cyclamen. Like the Trojan Horse, bulbs can carry these aphids within their layers. The aphids move above ground as foliage grows but may not be obvious at first, as the aphids initially remain low in the canopy. Some literature reports distortion of foliage from their feeding, although I have not observed this on Easter lily and have yet to find them on any other greenhouse crop.

Tulip bulb aphid is similarly insidious, conveniently carried on tulip, narcissus and crocus bulbs or iris rhizomes. Dusty gray in appearance, their heavy waxy coating and residence on vertical plant parts probably helps reduce impact of insecticide sprays that fail to stick or wet adequately.

Chrysanthemum aphid appears nearly every year on Long Island, but only on mums. They are relatively large, dark brown and often found on stems where they are more easily detected than the well-camouflaged green peach aphid.

Tobacco and potato aphids have appeared on calibrachoa and related plants. Tobacco aphid is closely related to green peach aphid with the same detoxification enzymes in some populations.

Foxglove aphid is distinguished by its pale-green color, dark-green blotch at the base of the cornicles (tailpipes) and larger size. It has become a fairly common pest in recent years, surprisingly on zonal and ivy geraniums. On many plants they cause moderate to severe distortion as they feed.

Other aphids. Other aphids I have observed include rice root aphid on Aechmea, dieffenbachia and other foliage plants; cowpea aphid on gerbera; plum leaf curl aphid on mums; bright yellow oleander (milkweed) aphid on oleander, Hoya and Asclepias; and the strikingly red Uroleucon obscuricaudatum on Heliopsis to name a few. In Florida, several new invasive aphid species have been sighted, including an Asian ficus aphid on Ficus and coriander aphid on fennel, dill and coriander.

Looking for aphids

Monitor for aphids by checking for the sticky honeydew they produce while feeding and their cast skins, which look like white bits of dust. Scan under leaves and on terminal growth and stems for the aphids themselves.

Sticky cards won’t provide an early warning for aphid infestations. However, if you are using them for other pests, watch for trapped winged aphids that could indicate a possible source to track down in the greenhouse.

Products to control aphids

On ornamental plants, aphids are usually easy to control with various spray materials and some systemics. Based on my own trials and experience, horticultural oil (Ultra-Fine, Saf-T-Side, and others) or insecticidal soap (M-Pede, Revoke) can be very effective but rely on good contact, which may be difficult with heavy canopy growth or leaves close to the growing medium surface. These products can’t be used on certain plants or flowers where there is a risk of injury because plants are under stress from lack of water or root damage, or applied too frequently.

Some caution is needed before applying oil to plants previously treated or in a tank mix with certain fungicides (e.g., Daconil and other chlorothalonil materials) or other products that may be incompatible. Oil and soap products have limitations, but in some crops or situations they may be especially useful, and resistance shouldn’t be an issue.

Some growers like using insecticidal soap at a very low rate (0.25-0.5 percent) tank mixed with other materials to enhance spreading and perhaps efficacy. Pyrethroids (Astro, Tame, Talstar, Scimitar, Mavrik and Decathlon) are sometimes effective, depending on type of aphid but they tend not to be a first choice for melon and green peach aphids.

Orthene TTO labels limit use to only a few greenhouse crops but it continues as a proven standby and provides some thrips control as well.

Avid and its generic versions can be used at the high (8 ounces) label rate for aphids. Expect moderate knockdown or better, as well as its usual miticidal activity. Weekly sprays with BotaniGard have given some control of green peach and melon aphids in trials at Cornell University (and even better whitefly control), but this product is probably best for low-level infestations. Use the wettable powder formulation on plants (tomato and poinsettia) that may be sensitive to the liquid formulation.

Endeavor, a U.S. EPA reduced-risk insecticide, has performed extremely well in Cornell University aphid control studies. It is also a useful material for whiteflies. In one test on heavily infested plants, the greenhouse benches were covered with a snowy layer of dead silverleaf whiteflies the morning after application. At higher rates there may be some whitish residue, but that has not affected sales.

Aria, labeled only for greenhouse use, has also been exceptional for aphid control. Some pansy cultivars have shown sensitivity to Aria, but results in trials and from grower reports are outstanding.

The introduction of neonicotinoid insecticides, starting with imidacloprid ( Marathon) in 1995, brought a new class of materials highly effective against aphids commonly seen in greenhouses. With very rare exceptions, this class of compounds also show a high level of safety to flowers and foliage and most leave little or no apparent residue as sprays. These products have familiar names: Marathon (and generic versions), Flagship, Safari, Celero and TriStar. Except for TriStar, which is also classed as reduced-risk by U.S. EPA, the labels also allow systemic (container drench or broadcast granular) uses to control aphids. TriStar can be used as a growing medium “sprench” for fungus gnats. Spray applications of these materials are very useful for quick knockdown, where infestations are heavy, if plants lack a good root system for systemic uptake and where cost or labor for granular or drench application a consideration.

Drench (or granular) applications provide weeks of control and are best applied after there is good root growth. I have observed efficacy five days after application with a Marathon drench to 4-inch impatiens. Generally it takes a week to show results on potted annuals or even longer on older plants and plants with poorly developed root systems. Safari is an exception, perhaps due to its high water solubility, showing results within just a few days following drench treatment. If using granulars, lightly water in several times after application and take care to avoid leaching pots for a couple of week when systemic control products are applied.

Natural enemies, if introduced early before infestations are apparent, can be quite effective against aphids. Occasionally we have great difficulty running aphid control trials due to parasitoid wasps that decimate trial colonies.

Banker plants

A fairly recent development is the use of banker plants to enhance biological control. Banker plants can be, for example, winter wheat grown in pots and infested with grain aphids that will not feed on most other greenhouse crops. Grain aphids provide an alternate food source for natural enemies when greenhouse pests are too few to sustain the beneficials and also create a new source of natural enemies.

University of Florida entomologist Lance Osborne has written about and used banker plant systems for whiteflies, mites and aphids (http://mrec.ifas.ufl.edu/lso/banker/banker.htm).

Work with biological control suppliers, and if possible, a cooperative extension specialist on integrating natural enemies into your production system. Consider release rates and timing, costs, pest and natural enemy monitoring in the crop, quality checks of the natural enemies and plans for dealing with compatibly of other pests and plant diseases.

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- Dan Gilrein

May 2008