spit??  Sounds more than odd, but that is what researchers at Cornell, University of Goettingen, and National University of Colombia discovered.  They were studying the effects from saliva of the Guatemalan potato moth larvae (Tecia solanivora) had on a commercial potato plant.

Their results included that when the larvae infected fewer than 10% of the tubers, the plant produced marketable yields that weighed 2.5 times more than undamaged plants.  Even when up to 20% of the tubers were infected, yields still doubled.  How about when half of the potatoes were infested?  Yields were equal to plants without infection!

This is something you might want to investigate further http://www.sciencedaily.com/releases/2010/05/100527131704.htm

Wonder if any of the crops in the Midwest respond in a similar fashion to invasive pests??