Do insects target sick plants and stay away from healthy ones?

Question

I was watching an organic gardening video, and Dr. Jana Bogs, who holds a PhD in horticulture, said that aphids and other insects go after plants that are nutrient deficient because weak plants produce smaller fragmented proteins and carbohydrate molecules that insects can easily digest. Healthy plants, she said, produce larger molecules that are good for humans but insects cannot digest. 
In my garden it just seems random and the horn-worms want to eat my tomatoes to the ground, regardless of how healthy the plants look. So, is there science to back up what she's saying?

Noor Najma · Accepted Answer

Basically, that is not true esp with aphids coz they love tender new leafs to feed on and healthy plants have plenty of that. Same can be observed with many other insects. You may observe sometime that the density (per area / inch) of aphids on the leaf(s) of healthy plants is lower than the density on weak plants; that occurs coz the expansion/growth rate of leafs of healthy plants is greater.

Lana · Answer

To my knowledge and experience insects will attack ANY plant that
they like to munch on. Whether the plant is sick or healthy. I have
honestly never heard of insects attacking sickly plants first. I think
they go after whatever they can find. Healthy or sick.

Do insects target sick plants and stay away from healthy ones?

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