Insects Might Be Repelled By Higher Sugar Levels in Plants

Lately I’ve been reading about the relationship between sugar levels in plants and insect resistance.

Dr. Thomas M. Dykstra of Florida is perhaps the foremost expert on this topic. I learned about him through a recent video by gardener Travis Key of Lazy Dog Farm in Georgia. Dykstra is an entomologist who studies the way insects use their sense of smell.

Dykstra believes that insects can sense the sugar content in plants. Insects generally feed on dead or dying things. Healthy plants should have a high sugar content, which by the way makes them taste better to humans. But insects, since they prefer dead or dying things, rather eat plants with lower sugar levels.

For some insects, their guts cannot process plants with a high sugar content. Travis Key made the analogy in his video comparing this to humans: we cannot eat wood. Our guts cannot process lignin, which comprises the woody parts of trees.

The term “brix” refers to the concentration of sugar in plant sap. We can measure the brix using a tool called a refractometer. Each degree of brix equates to one gram of sucrose in 100 grams of solution. Plants with a brix level of 12 are generally considered healthy and high in sugar content.

Dykstra’s research indicates that grasshoppers tend to prefer plants with a slight dip in sugar levels – a brix level of about 10-12. Moths and butterflies (and I assume their larvae) prefer plants with a brix level no greater than 9. Sucking insects like stink bugs attack plants with a brix level below the 7-9 range. Aphids prefer sugar levels even lower, in the 6-8 brix range.

So according to Dykstra, we can avoid insect problems by growing healthy plants that are high in sugar. Unfortunately, there’s no magical fertilizer that instantly raises sugar levels.

The only way to increase sugar content in plants is to grow healthy soil – adding compost, worm castings, rock minerals, and organic fertilizer. Periodically apply molasses to the soil. This doesn’t increase the sugar content of plants directly, but it feeds the beneficial microorganisms in the soil. These microorganisms convert complex minerals in the soil into simpler nutrients that plants can process.

Also, avoid harsh chemicals and synthetic fertilizers, which can harm beneficial microorganisms in the soil.

Don’t let your soil completely dry out. This, too, will kill beneficial microorganisms.