Gardening Harmful insects Plant diseases

Insects That Spread Plant Diseases

Some pretty innocent-looking insects can transmit some pretty powerful plant diseases. Ill.:

Insects that feed on the sap or scrape or munch on the leaves of our garden plants sometimes have much more serious consequences than just a bit of leaf damage. They may well be carrying an incurable plant disease that will cause more damage than the insect itself ever did.

This isn’t so surprisingly, really. It’s well known that mosquitoes transmit malaria, dengue fever and Zika virus to people. That other insects do the same to plants is a similar process.

Plant viruses and their relatives, viroids and phytoplasmas, are mostly transmitted by insects that inject them into plant tissues as they eat. However, there is no treatment for viruses in the home garden except to pull out and destroy infected plants. That’s why it’s important to act quickly when a plant is attacked by any insect in the hopes of removing the pest before it has time to spread its deadly cargo.

Spittlebugs look fairly innocuous, but are a major vector of plant diseases. Photo:

Among the insects that commonly transmit viruses are aphids, flea beetles, leafhoppers, whiteflies, thrips and spittlebugs (froghoppers).

20190109C DieterO, Wikimedia Commons.JPG
Mosaic virus is one of the more visible viruses. Photo: DieterO, Wikimedia Commons

Viruses (and other related diseases) sometimes have visible symptoms: for example, a specific discoloration of the leaf (mosaic or marbling) or deformed foliage or flowers, but most often not … except the plant weakens and becomes less productive. The two classic cases are strawberries and raspberries. Both are very productive for 2 to 5 years, then go so far downhill due to multiple viral infections that the only logical solution is to destroy them and start anew with “indexed” plants (plants confirmed to be free of viruses).

A good way of reducing the attacks of virus-carrying insects in the home garden is to maintain a good biodiversity in your plantings. Monocultures, where a single plant species is grown over a large area, attract and retain predatory insects of the crop being grown. When plants are grown in mixed plantings, though, these insects have a harder time finding their favorite host and your plants are therefore less often infested with debilitating diseases.

Nasturtium used as a trap crop. Photo:

If you add a trap crop to your garden, that is, a plant the insect pest likes even better than the crop you want to protect, such as the nasturtium (Tropaeolum majus), an excellent trap crop for aphids, prevention can be even more effective: just yank out the trap crop at the first signs of infestation, before the insect can spread to neighboring plants.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

1 comment on “Insects That Spread Plant Diseases

  1. Pingback: For your entertainment | Strafford County Master Gardeners Association

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