Aphids are notorious as carriers of plant viruses, but they also have a helpful side with which growers need to be aware. This article will discuss both sides. First, the notorious.
Green peach and melon aphids are most known for the variety of viruses (more than 100 and 50, respectively) they can carry and spread to other plants. They are by no means alone in this capacity. Foxglove aphid is associated with more than 40 plant viruses and crescentmarked lily aphid with more than 30. Many of the other aphids found in greenhouse production carry one or more viruses.
Fortunately, this mostly has been a relatively minor problem, certainly compared with tospovirus (impatiens necrotic spot and tomato spotted wilt viruses) transmission by western flower thrips. Still, I have seen cucumber mosaic (CMV) transmitted by many aphids in maranta and lily necrotic fleck, and a combination of CMV and lily symptomless mosaic virus (LSV) in Easter lily. LSV is also aphid-transmitted; melon aphid is able to carry both viruses.
Propagators, outdoor growers, and those growing certain greenhouse crops like cucumber or lettuce, may need to be more vigilant. They may not only be exposed to virus-carrying aphids or producing a sensitive crop, but there is less tolerance for disease in these situations. Infected mother plants pass along the problem in vegetative cuttings and sometimes in seed. Under these conditions there is a low action threshold for aphids that can carry the threat of viruses in the crop.
I recently received samples of planted mint cuttings – unsprouted rhizomes, really – brought into a warm greenhouse for spring production. The grower had noticed aphids on the stems but wondered why the biocontrols released were having no effect.
We noticed aphids on the stems, but there were more on portions below the soil level. These mint aphids, or Ovatus crataegarius, are among several unexpected pests that can come from underground. You might think of them as the ‘Trojan Horses’ of the insect world.
Other aphids notorious for this submarine-like behavior include crescentmarked lily aphid on lily and gladiolus; tulip bulb aphid on iris, crocus, and tulip; several Pemphigus ‘root aphid’ species on aster, boltonia, flowering cabbage, and kale, and lettuce; onion aphid on chives; shallot aphid on many hosts; Prociphilus erigeronensis, a wooly aphid on many ornamental and other plants; and rice root aphid on dieffenbachia and other hosts.
This may explain why some aphids can appear on greenhouse crops during northern winters and in areas free of weeds or other possible sources of infestation. A few other pests can arrive this way such as bulb mite on lilies and bulbs; bulb scale mite on amaryllis, narcissus and related plants; gladiolus thrips in gladiolus corms; tobacco thrips in narcissus bulbs; root mealybugs on many hosts; citrus mealybug from stored bulbs; and fungus gnats in baled media. If you suspect receiving insects on your orders, get insects identified, if possible, and work with suppliers to resolve the issue. They are as interested in confirming, tracking, and eliminating these things as anyone.
To control these subterranean aphids usually one of the systemic products can be used, depending upon the crop. These include Flagship (active ingredient: thiamethoxam), Kontos (active ingredient: spirotetramat), Marathon/Discus N/G (active ingredients: imidacloprid + cyfluthrin. or Safari (active ingredient: dinotefuran). And as a drench, BotaniGard (active ingredient: spores of Beauveria bassiana, an inset-killing fungus) has also performed well against root aphids. As the plants grow, eventually the aphids move to above-ground plant parts.where they are exposed to foliar insecticides and biocontrols.
Beneficial food source
Now for the helpful side of aphids. In past articles, I have remarked on the use of “banker plants” in greenhouses that incorporate biological control. Some insects and mites, including many kinds of aphids, are highly dedicated to just one or a few types of host plants and will not establish populations in crop plants nearby. For example, banker plants such as barley or oats, can be used to maintain grain aphids that are an alternate food source for beneficial insects, such as ladybeetles or the aphid-parasitizing wasp Aphidius colemani).
This helps to not only keep the biocontrols within the crop, but it also potentially increases their numbers. Considerations include the cost and time to continually maintain a stream of banker plants and aphids, the additional space required, and the banker plants may develop pest or disease problems.
Growers who aehighly dedicated to the use of biocontrols are figuring this into production plans. By continually renewing the banker plants some problems are minimized. Banker plant ‘systems’ can be purchased from some biocontrol suppliers, or you might develop your own.
The UMass website discusses this in more detail (http://bit.ly/1guSPGL) using a grain (barley, wheat, or oats) with bird cherry-oat aphid to maintain Aphidius.
Corn or sorghum can be also used with corn leaf aphid and Aphidius. Asclepias and milkweed aphid have been suggested with Aphidius colemani. One grower uses turnip aphid on cabbage plants or vetch aphid on fava bean to maintain Aphidoletes, an aphid predator.
There are many possible combinations and they extend beyond aphid control, such as the papaya and papaya whitefly system with Encarsia transvena espoused by Dr. Lance Osborne. For more see http://bit.ly/1guSPGL. The key to all is that ‘banker plant’ insects do not recognize the crop so they shouldn’t become a pest there.
Furthermore, the banker plants and aphids are relatively easy and inexpensive to raise, and the system should work well under existing crop environmental conditions.
Daniel Gilrein is an extension entomologist at Cornell University Cooperative Extension of Suffolk County.
Reference: Blackman, R. L. & Eastop, V. F. 2000. Aphids on the World’s Crops. John Wiley & Sons, NY.