One of the most intriguing aspects of biological evolution, at least for me, is the arms race between insect herbivores and plant chemical defenses. Plant-insect interactions should definitely make their way into Andrew Hendry’s Evolution Bucket List (read Andrew's post below). For example the interaction between the milkweeds and Danainae male butterflies involving pyrrolizidine alkaloids should find a spot inside the already crowded bucket. The alkaloids protect the milkweed from any generalist herbivore but are essential to the success of the specialist Danainae males. The male butterflies, that require the plant’s alkaloids as precursors for the biosynthesize of their own sexual pheromone, pass any excess of the compound to the female as a nuptial gift so that she can protect the eggs. Females assess the quality of males through their scent, which is indicative of their overall toxin load. In short, a set of compounds that very likely evolved to deter a set of herbivores is now essential for the success of others.
It is true that herbivory is not the only driver of chemical complexity in plants, many other environmental factors are responsible for the diversity of plant compounds (i.e. pathogens, draught, uv radiation, etc.). However, there are well-documented examples that support an escalation of plant chemistry in response to constant herbivore pressure (see Becerra et al. 2009 PNAS on Bursera). Chemicals that confer plants resistance against herbivory will be advantageous no doubt about it. The resistance can be achieved either by deterring the insect, affecting its development, directly poisoning it or recruiting natural its enemies. In return, the insect detoxification machinery is under constant selection, and as agriculture has taught us, insect resistance to particular chemicals can evolve fairly quickly.
Keeping the arms race between plant and insects in mind, I would like to pose a question. Can diet specialization be a way of taming the beast? In a particular plant-insect interaction, specialization of the herbivore to a single host could be advantageous to the host if it implies the insect could not survive without its host and this dependency would translate to an overall reduction of the damage inflicted. The rarer the plant the rarer the herbivore. Keep your friends close but your enemies closer, kind of a thing. The stochastic nature of many insect populations make generalist insects unpredictable. On the other hand specialists are completely in tune with their hosts and hence they are a lot more predictable.
Maybe the same question has already been asked in other interactions like those of parasites and their hosts, and hence this text just shows my ignorance to the subject. In any case, we are far from a comprehensive understanding of the mechanisms that sort insect assemblages to their hosts, and little is known about the steps towards host specialization.
Two factors are thought to be important in host specialization, 1) plant chemistry and 2) nutrient balance. Plant chemicals are not just defenses to overcome but also strong cues that are useful for making the right host choice (Bernays 2001). When there are more items to choose from, making the right decision is harder and takes longer. It seems that there is a tradeoff between the efficiency in choosing a suitable host and the amplitude of the diet (Bernays 2001). Specialists have less information to process in regards of finding their host while generalist have to fiddle with competing signals. A few studies show that when presented with hosts of low and high quality, specialists are always able to choose the high quality host, while generalists often make bad decisions. On the other hand generalists seem to have a better ability to cope with an imbalanced diet and are better at compensating a lack of a specific nutrient (Raubenheimer & Simpson 2003).
Comparisons of sister insect species that exhibit contrasting diet breadths is an interesting way of unraveling the route to specialization. But can specialization be beneficial in the long run to the plant host? Probably the answer to this question is case dependent and a HUGE caveat is the differences in time scales between the evolution processes that lead to specialization and the time scale of the interactions we are able to study and measure (ecological time scale).
Becerra JX, Noge K, Venable DL. 2009. Macroevolutionary chemical escalation in an ancient plant-herbivore arms race. Proceedings of the National Academy of Sciences 106: 18062-18066.
Bernays E. 2001. Neural limitations in phytophagous insects: implications for diet breadth and evolution of host affiliation. Annual review of entomology 46: 703-727.
Raubenheimer D, Simpson SJ. 2003. Nutrient balancing in grasshoppers: behavioural and physiological correlates of dietary breadth. Journal of Experimental Biology 206: 1669-1681.