Wednesday, August 17, 2016

Soap destroys plants' defenses

How to fight aphids on milkweed?

We plant milkweeds to encourage the monarch butterflies, not only for the nectar, but also because milkweed is the ONLY larval food for the monarch caterpillars. But milkweed also attracts aphids...
Non-native scarlet milkweed (Asclepias curassavica)*. Notice all the yellow aphids on the stems.
I was focused on the monarch when I took the above photo, but it's obvious that my milkweeds had become infested with aphids. I just allowed the aphids to stay. Eventually some ladybugs came in, but the plant is better off without any "treatment" from the gardener.

Often the "expert" advice is to spray homemade concoctions with soap or detergent to get rid of the aphids. Don't do it! 

Water beads up on plant surfaces because of their waxy, hydrophobic cuticles.
The beading water also washes dust from the plant surfaces, 

Soap dissolves the waxy cuticle, which is the plant's defense against desiccation, pathogen infestation, UV radiation, and pest attacks.

Cuticle formation is an adaptation that plants had to make to survive on land--out of the water environment. Its formation takes quite a bit of energy as the outer cells exude this complex mixture of waxes and cutins. New studies have shown that the cuticle is much more than just a barrier to water loss. The cuticle is a barrier to fungal and bacterial pathogens; it protects the plant from UV radiation; and it deters some herbivore activity.

The casual and oft repeated suggestion to use insecticidal soap and other soapy mixture is short-sighted, because the plant's cuticle will be dissolved. The plant, which may already have been stressed by the aphids, may not have the spare energy to form a new cuticle. Without its cuticle, the plant will wilt more often and without its normal turgidity, even more types of pests will be able to attack it. Without its cuticle, the plant may be sunburned and the burned tissue is an opening for fungi and bacteria to enter. In the end, the soap-treated plant is MUCH worse off than before its treatment.

FYI, dawn, the most widely recommended soap
for homemade insecticides is not benign.

Soaps and detergents disturb the balance in working ecosystems

Soaps and detergents serve as poisons in your landscape's ecosystem. Yes, they may wipe out the aphids, but in addition to leaving your plants vulnerable, they will also wipe out or chase away their predators. The lady bugs (both the larval and adult stages), assassin bugs, praying mantises, predatory wasps will not have their food. And the predators for those bugs such as dragonflies, lizards, bats, and insect-eating birds will not have their food either. Your yard's ecosystem is a tangled web and your intervention against one member will have an impact on that whole web of relationships.

If you must intervene...

If the infestation is really intense, use only water to rinse away the aphids and other bugs. This way the plant retains its cuticle and you can reduce the aphids on the plants without killing them or their predators.

Your goal, as a sustainable gardener, is to help your 
yard become a working ecosystem. 

* This photo was taken several years ago when I was still growing this non-native species of milkweed, because it was readily available at the local Home Depot. I don't grow this milkweed any more because it doesn't die back soon enough in the winter and the monarchs get off cycle, stay too long into the winter, and pick up a deadly bacterial disease. If you do have the scarlet milkweed, you can keep the monarchs safe by cutting it back to the ground at the beginning of December and keeping it cut back until February.

For further information:

A poison is a poison is a poison!

The formation and function of plant cuticles

Dish soap can damage your plants on Garden Myths website by Robert Pavlis

Green Gardening Matters,
Ginny Stibolt


  1. Thanks for the info. Big aphid infestations past 2 years. I will switch back to water only.

  2. You're welcome. Good luck with the aphids.