Sugar metabolism and carbohydrate synthesis are at the very foundation of plant health, but we generally don’t learn much about them in agronomy or even entomology. The types of sugars and the relative concentration of different sugars contained within the plant seem to be foundational in determining susceptibility/resistance to many herbivorous insects.

Here are a few excerpts from Harold Willis1 I found interesting:

The role of sugar in insect attack of plants is fascinating. Based on research done on various insect and plant species, apparently insects like moderate amounts of plant sugars and are attracted to plants containing them. But high concentrations of sugars are avoided by leafhoppers, grasshoppers, and the European corn borer2

Alfalfa was found to be resistant to pea aphid when its stem tissues had a more acid ph and higher levels of sugar (pentoses) and pectic substances (larger carbohydrate molecules formed by linked sugars). Pentose sugars are formed from hexose sugars which are the original products of photosynthesis. Alfalfa plants that are normally susceptible to aphids will become resistant if the above-mentioned cellular changes occur3

A possible reason that some insects avoid high sugar plants comes from research by G Fraenkel. Some sugars and sugar alcohol combinations (glucoside and mannoside) interfere with normal utilization of other sugars, and so are toxic to insects (mealworms)4. The inhibitory sugars are found mainly combined with other molecules in plants, but if digested by insects and in the presence of the sugar glucose, their toxic effects occur5.

Our knowledge of plant immunology has progressed well beyond this research in the ’40s and ’50s, but the practical application has fallen well short. I describe how we have applied these principles in our plant health pyramid infographic and on YouTube here.

1. Willis, H. Crop pests and fertilizers – is there a connection?

2. Thorsteinson, A. J. Host Selection in Phytophagous Insects. Annu. Rev. Entomol. 5, 193–218 (1960).

3. Emery, W. T. Temporary Immunity in Alfalfa Ordinarily Susceptible to Attack by the Pea Aphid. Journal of Agricultural Research 73, 33–43 (1946).

4. Fraenkel, G. Inhibitory effects of sugars on the growth of the mealworm, Tenebrio molitor L. J. Cell. Comp. Physiol. 45, 393–408 (1955).

5. Dethier, V. G. & Rhoades, M. V. Sugar preference-aversion functions for the blowfly. J. Exp. Zool. 126, 177–203 (1954).