Home & Garden Columns

Think Twice Before You Reach for the Bug Spray

By Ron Sullivan
Friday July 21, 2006

It’s midsummer, more or less, and the other inhabitants of the garden are showing up in numbers. Aphids and whiteflies and thrips, oh my! The first flush in spring gave rise to another generation or two, multiplying all the way, and most of the birds have about finished raising their first and maybe second broods for the year, so fewer insects are being turned into babyfood.  

But it’s not time to panic and start throwing insecticides around. Some of our most charming predators are emerging into visibility, and killing everything that nibbles on the plants will kill them, too. 

Now, if you have an infestation and you grab the spray and start shooting indiscriminately, you’ll kill off most—only most —of what’s bugging you, plus anything else that’s in range, including the things that are eating the pests. 

That’s elementary. You’ll also be killing the decorative insects like butterflies, by way of collateral damage. I’m including “safe” sprays, too—the average insecticide is not particular, even if it’s safer for the likes of us mammals. 

The catch is that, like predators on any scale, the useful insectivorous critters don’t multiply as fast or as prolifically as the vegetarian pests that are chewing or sucking the vigor out of the garden. 

At worst, the bugkillers will have become concentrated as each predatory bird or arthropod or even mammal eats many individual insects. By the time they’ve recovered from poisoning or just from short rations, the herbivores have had two or three litters and those litters have littered.  

Anyone at the base of the food web (to mash a metaphor) is likely to be a determined breeder. Some aphids, for a pertinent example, breed asexually over the summer and don’t bother with complications like mating until they’re ready to shut down their whole enterprise for the winter. 

That’s right, folks, little girl-aphid clones are what’s overrunning your beans and posies. Everything Lucas does in Star Wars got thought up and done already by that original trickster, Nature. (If you really want to scare yourself and gross yourself out too, read Carl Zimmer’s excellent Parasite Rex.) 

With such a big prey base, the ladybugs and mantids and spiders will breed more prolifically too. It takes a little time, but insect and arachnid generations are a whole lot faster than ours. 

If you’re willing to accept some holes and puckers in your leaves now, you’ll spare yourself lots of work later and you’ll spare your garden’s friendly inhabitants too. It helps a lot to persuade your neighbors likewise; their frequently-sprayed yards can be reservoirs of pests.  

There’s one set of exceptions to the indiscriminate-killer insecticide: Bacillus thuringensis (“Bt”) sprays or pellets. These contain a subspecies of microorganism that’s bred specifically for the life form it’s aimed at—caterpillars (but remember, that’s any caterpillar) or mosquitoes. 

They kill the pest in its infancy, so it doesn’t survive to bite or breed. I like the stuff for small watergardens—tubs, pots—better than the “mosquitofish” the county hands out for free, which are becoming pests themselves. More on that problem next week. 


Ron Sullivan is a former professional gardener and arborist. Her “Garden Variety” column appears every Friday in East Bay Home & Real Estate. Her column on East Bay trees appears every other Tuesday in the Berkeley Daily Planet.