Ultimate Survivors: How Plants Combat Pests
Naturally, the topic for this post came to mind because of our newly-exterminated soft scale infestation. But the whole episode got me wondering--how do plants defend themselves?
A summary with a fun name is available from WSU:
. The first thing that caught my eye on the list is what the University refers to as "Roadblock." Thorny, hairy, sticky or spiny leaves that make the plant less attractive to its predators. (
The photo at left is a painfully obvious illustration of one of those features, but I used it anyway because it was available
page includes a large number of plant species that deer simply don't like to get their tongues on--rough, hairy--not the juicy salad they would prefer. Evidently these same deterrents work on species besides deer.
At the molecular level, we shouldn't be surprised to learn that plants have their own immune systems. Unlike
, plant immune systems can't learn new tricks. The basic apparatus for fighting invaders is there, however, setting up interceptors to reduce the success of pathogens. This helps them when confronted with pathogens that cause disease. Like us, plant immune systems are more effective if the plant is healthy before getting hit by the pathogen. More on that momentarily.
Some plants appear to also be able to call for help from external sources. Some do it through the roots, while others secrete chemicals that call in armed defenses like parasitic wasps. Within the realm of a healthy ecosystem, other agents come into play that help to protect plants, even if that is not their goal. Birds, for instance, can really decimate an insect population--provided those insects haven't been poisoned, and therefore killing the bird (it may take more than one poisoned bug to do it). Lady bugs and other beneficial insects work the same way--it only takes a few effective predators to wipe out a whole pile of the bad guys (like aphids), provided the predator is not poisoned. Remember that a significant population of "bad bugs" will have to be present in order to sustain the needs of a passel of predators. A few dozen aphids on one plant may not be enough to attract attention. So you may have to fight the natural impulse to take matters in to your own hands for a while in order to allow the predators to move in to your home territory. Try to remember that it is a complex picture, and while many of us may be master gardeners, we are not the designers of all that makes up our environment. (
Photo above right: house wren hauling in yet another kill to her brood
Some insects that can cause us alarm--like yellow jackets and wasps--are voracious eaters of other insects. Throughout most of the year they are carnivores and will generally ignore you if given the option. Moving in to fall they will begin adding carbohydrates to their diets. You will see these insects collecting nectar as if they were butterflies. When their diets change, so do their temperaments--they are more easily "irritated." Think of them as teenagers on Mountain Dew and approach with caution!
When you think of all the allies that plants have, it's easy to see why we should all try to avoid poisons. One of the most effective portions of Integrated Pest Management (IPM) is the "Leave It Alone" part. Simply giving nature time to do its thing can work out really well for you if you can quell the urge to fix the problem too soon. While IPM is usually discussed in the context of agriculture, it is equally valid in your own backyard. The primary goal of IPM is the reduction or elimination of pesticides, as these potent chemicals have a host of economic and ecologic baggage.
Think of your plants the way you would your own health care. You really don't want to start chemotherapy unless you've got an aggressive cancer. The better plan is to build your health, rather than constantly poisoning yourself in the hope that you'll kill off everything that's trying to get to you before you kill yourself with the chemo. So putting your plants in the situation they are healthiest in (right plant, right place) is the first step. Providing a setup that encourages their allies is the second step (landscape plant diversity, mulching). And finally, providing additional nutrition or hydration when the need arises is all your plants ask to help them fight their daily battles. Watch them do their thing!