tomDear Tom –

I seem to have trouble with aphids in cold frame crops in March and April each year. Do you have suggestions? Thanks

Bill in Old Fort

Dear Bill –

Aphids are a pest for us too. We see them in early lettuce and on tomatoes in our heated greenhouse. On lettuce they are very difficult to remove and on tomatoes, they can get bad enough that we need to wipe each tomato, adding hours to tomato harvest.

Part of the problem is that aphids like cool weather better than their natural enemies and they also reproduce more quickly. I find they are less of a problem when weather warms and their enemies have a chance to catch up and keep aphid populations in check.

Here are a series of recommendation but our best success has been with beneficial insects released proactively which I describe below.

  • Start Clean – Be sure your structure is clear of aphids to start the season. A break of a few weeks with no plants in the coldframe may help, if you can afford that loss in production. If you can manage a plant free period, consider shutting down vents to trigger outbreaks before your crop arrives and then flame or till under your potential problem. Also be sure your transplants are clear of aphids. Particularly with purchased transplants, check them carefully and dunk them in insecticidal soap if you see any aphids at all.
  • Preventive vs. Reactive – Daily scouting will be helpful. Try ten spots in your coldframe and look under ten sets of leaves. Hot spots sometimes show a sooty deposit which is actually a fungus living on the “honey dew” exuded by aphid colonies. The white husks of earlier instars are also easy to spot. Both of these symptoms may suggest that it’s almost too late, so act quickly with controls. Some consultants believe that finding even one aphid is too many. It’s in their interest of course but they strongly recommend that you use their products before you see any problem. If you know you will have aphid problems every April, it may make sense to take action in March. If you do find hot spots in your structure, insecticidal soap is usually recommended but I have never been able to solve the problem with just soap. Consider removing plants in and around hot spots. That approach is easier for me to do with lettuce than long season crops like tomatoes.
  • Natural enemies – I have tried lady beetles, a fly called Aphidoletes and a wasp named Aphidius. Here are some suppliers that I have used:
  • Beneficial Insects – Understanding that the following comments are farmer opinions and that I have little formal training in entomology, I intend to use Aphidoletes in 2016 using two or three releases at 7-10 day intervals. The multiple releases help avoid a boom and bust cycling of beneficial populations so they are hatching more or less continuously.

I find that lady beetles work for a while but then they disappear, even when the weather in the greenhouse is much better than outside. Lady beetle larvae are probably better since they cannot fly. They also are said to eat more aphids than adults. I have not tried the larvae.

I used the parasitic wasp Aphidius last year along with Apidoletes. My experience in 2014 is that this combination is completely effective but based on advice from other growers, this year I plan to try just Aphidoletes to save on control expenses. This film is a little dramatic but the Aphidius photography is good:

Aphidoletes larva “managing” an aphid outbreak. Note the shriveled shells close to the orange larva. Source:

Aphidoletes larva “managing” an aphid outbreak. Note the shriveled shells close to the orange larva.

Apidoletes are much more effective than I am at finding aphids, particularly early on. The survival of their species depends on this ability. Supplier directions suggest releasing the flies away from the hot spots so they will hunt for smaller outbreaks in route to the larger populations. They also recommend against refrigeration which causes the beneficial flies to lose interest in hunting for some reason.

Shipping costs about as much as the controls in many cases. Overnight or second day air is worth the expense to keep your bugs healthy. Work with your mail carrier to leave the mailbox open if the supplier intends to use USPS.

Banker plants are widely used in large greenhouses. Apparently some aphids insist on specific hosts – cereal aphids for example only feed on grasses and not on broadleaf plants. The basic concept is to raise grain rye in a pot and then introduce cereal aphids into your greenhouse or coldframe on this patch of grass. With that “bank” of aphids to support the beneficials, a reproducing population of controls can be maintained. “Grow your own” beneficials to save on repeated shipments has some appeal. Check the IPM Labs link for more on this concept.

I hope this helps. Thanks for your question.

— Tom

Ask Tom © 2014 Tom Elmore & Organic Growers School

Author: Tom Elmore

Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.

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