Insects will only eat that which is digestible to their system.

From the interview with Tom Dykstra:

John: I have friends who will attract every mosquito for dozens of yards around, and others who, for all practical purposes, are mosquito-immune. Can you describe the differences?

Tom: I hate saying this, but that is the difference between healthy people and unhealthy people. If you have digestible blood, the insect can use your blood. If you have healthy blood, the insect is not attracted to it. Does this mean you’re healthy all the time? No. People have certain states of healthiness and unhealthiness. But as long as your blood is digestible, it’s going to be attractive to an insect.

It’s the digestive system. Insects do not have very good digestive systems. Stuff has to come in digested to them because they cannot digest things. They just don’t have the enzymes. They’re garbage collectors. They eat muck. They eat garbage. They eat bad stuff. They eat stuff that we don’t want to eat. Take a look at cockroaches, for example, or fruit flies. They’re always around decaying fruit. They’re not around healthy fruit. These insects that are keying in on very specific plants. Or even, let’s say, the fruiting structures of a tomato—that’s what they are keying in on.

Mosquitoes need something that’s digestible. When they take a blood meal, it must be incorporated through the process of vitellogenesis: taking those nutrients in your blood and incorporating them into an egg. This occurs in about twenty-four hours—very, very quickly. So if they’re not getting digestible blood, they cannot, through the process of vitellogenesis, take those nutrients and feed their eggs. And that’s a problem, because obviously the mosquito wants to feed its egg. The only reason it takes a blood meal is to feed its eggs.

We do have these differences among individuals. Some are just more digestible than others, and they’re going to be more attractive. Grasshoppers have a choice as to what crop they eat, and mosquitoes have a choice as to what blood meal they wish to get. Through their antenna and all of their senses, they detect that someone is more digestible than another, and they’re going to go after that individual. Even though that may offend some people, I do feel an obligation to tell the truth the way that I know it, according to the information that has been given to me for the past twenty to thirty years of my life studying insects.

John: I very much doubt that you’re offending people. I would say that you’re simply communicating some very intriguing ideas that many of us have observed to be true in real life and have wondered about the differences. So thank you for sharing that.

Tom: I’m happy to do so. When you go to the supermarket and you see fruit flies flying around the tomatoes, some people think, “Oh, I don’t want these tomatoes—they have fruit flies in them.” But honestly, all you have to do is find that one tomato near the bottom that’s injured, and that’s where all the fruit flies are located—near the one that’s unhealthy. And that essentially equates to that which is digestible. A tomato that is degrading—that is decomposing—is being broken down. And when it’s being broken down, it’s digestible. Insects will only eat that which is digestible to their system.