Don't Spray – Plants Can Defend Themselves!

September 15, 2013 lawanda Newspaper Columns

What happens when you see an insect chewing on your beans?  Do you immediately reach for some kind of spray?  You really don’t need to do that – as soon as the chewer landed on the plant and began snacking, the bean called for help.  It did this by releasing a complex of chemicals called “herbivore induced plant volatiles” or HIPVs.

HIPVs are released from leaves, flowers and fruits into the atmosphere or into the soil from roots and announce to predators of whichever insect is attacking the bean that they are welcome to come and get it.  The bean is so smart that if another type of insect comes and begins chewing, it will send out a different HIPV to call predators of that particular insect.  It is so exact an ability that the bean can determine the size and age of the attacker and call an appropriate predator to deal with it.  For example, when a small caterpillar is attacking, the plant calls out for a small insect predator.  If a big, fat caterpillar is ravaging the plant, a bird is called.  Some plants are so sensitive to attack that they will begin emitting HIPVs as soon as an egg is laid upon it.

Not only that, but all parts of the bean plant go on alert and begin emitting HIPVs.  And there’s more – the bean also alerts nearby bean plants and they begin calling for help as well.  That bean chewer doesn’t have a chance.

That is, it doesn’t have a chance unless you or your neighbors have used insecticides that have killed off all the good guys that might have come and rescued your plant.  In that case, you’ll have to step in and rescue the poor bean again, but it’s a downhill cycle filled with repeated use of dangerous chemical sprays.

When a plant calls for an insect to come and eat the insect doing the chewing, it is called an indirect defense.  Some plants take a more direct route.  As soon as they are attacked by a chewer, they make themselves inhospitable to their attacker. They do this by changing themselves physically or chemically to repel or even kill their assailant.

Sadly, some varieties of hybrid plants have lost the ability to emit HIPVs when the capability to do so was inadvertently bred out while breeders were going for size, taste, high yield or shipping capacity.  Now that scientists have become more aware of HIPVs, they will likely take care not to lose the HIPV ability in their development of new cultivars.  Still, the loss of HIPV ability is one more good reason to choose to grow open-pollinated heirloom plants.

HIPV emission isn’t limited to vegetables – trees, flowers, and shrubs also have the facility.  The effectiveness of HIPVs depends on how close the helpers are to the emitting plant, the proximity of other plants, the weather conditions, air pollution and the plant’s general health

Fruits and vegetablesGardening techniques and toolsMiscellaneousOrganic gardening

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