The basic nutrient requirements are largely the same for all animals. What, among other things, makes the animals different from each other is the ability of each species and phase to satisfy these requirements with what they eat. Versatility is a fundamental requirement for nutrition. Even animals which appear to feed on nutrient-poor or unbalanced diets compensate in different ways, for example by feeding heavily or by symbionts in the intestines.
Animals cannot live off too pure foods. Insects cannot live in white sugar. It is simply too much of a unilateral food. Sometimes you can find booklice in sugar bags, but if you look closely you will see that the insects are sitting high up on the bag’s interior, where the combination of sugar, paper and humidity has fostered the microscopic fungi that booklice feed on. The insects that live in dry flour do not thrive very well in the flour with low grind levels wherein it mainly consists of starch. At higher grind levels, the flour will contain several amino acids and vitamins, insects will perform correspondingly better, they will develop quicker, get more offspring. Potato flour is probably the worst food that you can offer to food pests. There are no known pests in potato flour unless it is stale or contaminated. Potato flour is not just a unilateral diet but potato flour has physical properties that remove the wax from the insect exoskeletons, after which they die due to fluid loss through the leaky exoskeleton.
The insect body also has an impact on what foods they can exploit. Booklice and sugar mites can live on the surface of flour, but are too fragile to dig into the flour. Those booklice that are seen on top of an otherwise untouched bag of flour are thus the total number of booklice in that bag of flour. Flour mites and confused flour beetles are better suited to dig into flour and are not always seen on the surface.
In densely packed grain live only insects that are small enough to travel in the gaps between the kernels. All other large insects must stay on the surface.
As pedestrians insects like mites are unsurpassed. They can walk to virtually everything and in all angles with gravity. However, they cannot pass a thin layer of talcum powder or an even thinner layer of petroleum jelly. It prevents them from getting a hold with their feet.