We all know that healthy plants are resistant to bugs. Unfortunately, we all too often forget to ask ourselves why this is true.

In nature, weak organisms are typically attacked first. In the animal kingdom, predators see and smell the weakness of their prey. Gardeners have long suspected that diseased or dying plants put out some sort of “distress signal” that attracts scavenging insects to the scene.

Dr. Philip Callahan, a USDA entomologist from the University of Florida, has a theory about what this signal might be. Entomologists have long known that insects can detect infrared radiation from plants, and that they use this radiation to identify which plants are food and which are not. This is because different plants vibrate at different frequencies, and each insect is looking for a specific frequency to identify its next meal. A healthy plant vibrates at a different frequency than an unhealthy plant. The insect identifies the unhealthy plant as food and bypasses the healthy plant, which is producing infrared radiation in a manner that is less than appealing.

Another theory proposes that an unhealthy plant oozes nutrients in a last-ditch effort to restore balance within its systems. These nutrients collecting on the leaves of the plant become targets for insect pests.

But there is yet another possibility. Some gardeners have noted that the few insects who do feed on vibrant, healthy plants later act sluggish, as if they have been poisoned. Entomologists agree that insects have relatively simple digestive systems compared to mammals. As plants become healthier, they produce increasingly complex nutrients, better suited to the specially equipped digestive systems of ruminants and humans than to those of insects. When an insect feeds on a healthy plant, it will suffer from nutrient overload—in other words, poisoning.

Perhaps all of these theories are true to some extent. Perhaps the bugs know instinctively that one kind of infrared radiation means an easy meal and that another kind means food poisoning. Scientists have yet to find the complete answer.