Little insect, big pest

They have many enemies – predators, parasitoids, severe weather, and even their own pathogens, but somehow aphids manage to prevail as agricultural pests. Produce ‘contaminated’ with their honeydew or sooty mold (which thrives on honeydew) is not marketable, and it can be difficult or impossible to remove infestations or residue by washing. A few kinds of aphids may cause growth to be distorted, and heavy populations can kill plants. Getting control late in production, while picking is going on, is almost out of the question for mature heads of lettuce or in dense canopies of greenhouse tomatoes. Some growers have had to discard an entire crop that could not be sold. A few types of aphids are notorious for spreading viral diseases, making them even more unwelcome.

There are more than 4,500 types of aphids, but fortunately just a handful are greenhouse pests. It is important to know which ones you’re dealing with, particularly if using biological control. (See article on page 14.) Identifying the species may help in pinpointing likely sources of infestation and explain why some controls work better than others.

Green peach (GPA) and melon (MA) aphids are the most common in greenhouses and among the most notorious for developing insecticide-resistant populations. Of the two, melon aphids seem to be more difficult to control. Both have very wide host ranges, including many greenhouse vegetable crops, though some strains may not ‘take’ to their otherwise usual hosts. The optimal temperature for GPA – at which the population grows most quickly – is 75°F, but for melon aphid it ranges from 70°F to 80°F. In northern areas MA may overwinter (as eggs) outdoors on catalpa or Rose of Sharon and GPA on various Prunus spp. (peach, apricot, plum). In the south, the aphids themselves overwinter on host plants.

1. Foxglove aphid can be distinguished by the two darker green patches at the base of their ‘tailpipes’.  This aphid seems to becoming a more important greenhouse pest in recent years.  2. & 3. Distortion on pepper from Foxglove aphid.  4. Green peach aphids on pepper.  5. Green peach aphids on cabbage. GPA is one of the most important aphid pests in greenhouse vegetable production.  6. Melon aphid (right) and parasitized aphid ‘mummy’ (brown, left).  7. Melon aphids under squash leaf. 

Photos courtesy Daniel Gilrein
 

Despite its name, potato aphid (PA) is common on dozens of host plants, including tomato, potato, and occasionally corn seem to be more important. They can be found on calibrachoa, a widely grown ornamental related to the tomato. PA can overwinter as eggs on rose, though the aphids themselves have been found overwintering on other plants. Its optimal temperature is around 68°F.

Foxglove aphid (FA) has long been a pest but complaints about this species have risen in recent years. It also has a wide host range and infestations often result in distorted growth. This species thrives under relatively cool conditions (50°F to 60°F) and may dominate from fall through early spring. It can overwinter outdoors as eggs on foxglove or some hawkweeds in cold climates, or as adult aphids on plants in warmer areas. There are some helpful on-line guides for distinguishing among these aphids and other species. You can also seek help from an extension specialist. Samples may include adults and winged individuals, if present. These can be placed in a vial with rubbing alcohol, if impractical to submit live samples.

Under some conditions aphids produce winged forms that can disperse into or throughout the crop, but populations may build to high levels before they appear, rendering yellow sticky cards unreliable for early detection (though they can still be helpful to show when aphids are moving into or around the greenhouse). Check leaves or petioles for the aphids themselves, early signs of shiny honeydew, or for their cast skins fallen on leaves below for evidence of infestation. Tapping branches over a white surface to dislodge the insects may also be helpful, as they can be difficult to see. Some sampling plans can be used or adapted for greenhouse crops.

Many growers of edible greenhouse crops are using biological control especially for managing aphids, which I highly recommend. It can be a bit complicated, but attentive and dedicated growers find it worthwhile. We’ve seen good results in our own and growers’ greenhouses. For example, parasitoid wasps are often used (they don’t bother humans) and include Aphidius matricariae for GPA, A. colemani for GPA and MA, A. ervi and Aphelinus abdominalis for FA and PA. Look for sesame-seed like tan (Aphidius) or black (Aphelinus) parasitized aphid ‘mummies’ that show the wasps are ‘working.’ Many aphids also die after falling off plants while escaping attacking wasps.

Predatory ladybeetles, lacewings, and the midge Aphidoletes aphidimyza can be used for many aphid species and may work best combined with Aphidius, especially for melon aphid. Be sure to check a portion of each shipment for survival, and release the natural enemies promptly on arrival. Before using biocontrols, be sure that environmental conditions (day length, relative humidity, and temperature) are appropriate and infestations are not already at high levels. Run cost estimates including product, shipping, and labor (for releases and monitoring) to compare with current practices. ‘Banker plants,’ such as barley infested with special aphids that don’t recognize the crop as a host, are useful for providing an alternate food source when pest aphid levels in the crop are low. Check with biocontrol suppliers for more information on using banker plants. The University of Massachusetts has a very helpful factsheet on the subject at http://bit.ly/1dBhc6l.

Insecticides can be useful, of course, and may even be necessary where the aphid populations are too high or conditions unfavorable for natural enemies. Some products qualify for use in organic production.

The Biobest (www.biobest.be) and Koppert (www.koppert.com) ‘side effects’ websites can help assess product compatibility with use of biocontrols.

Efficacy of fungal products appears to improve when a long period of very high humidity follows application.

However, be sure this will not lead to a foliar disease problem. A few products have labels for use on specific greenhouse-grown vegetable transplants. Be sure to read all product labels carefully to be sure intended uses are indicated.

If aphids are a chronic problem and the main source of infestation is outside, the greenhouse screening may be worth considering. This will entail some engineering and expense to retrofit to an older greenhouse but for long-term greenhouse vegetable production it may solve much of the issue – and keep other pests out as well.

Daniel Gilrein is a frequent contributor to Produce Grower and an extension entomologist at Cornell University Cooperative Extension of Suffolk County.