||While the cost of detoxifying plant compounds is difficult to
quantify, there is one way to determine whether detoxification is cost-free to an
insect. The principal means by which parsnip webworms cope with furanocoumarins in
its host plants is via detoxification by cytochrome P450 enzymes.
Because these enzymes are composed primarily of amino acids (they also contain iron),
having amino acids locked up in P450s could be costly when protein is scarce. To test this
idea, we fed larvae different dietary levels of protein and observed three fundamental
life processes--growth, detoxification capacity, and silk spinning. Reductions in dietary
protein quickly led first to declines in growth, then in silk spinning, and finally in
detoxification capacity. The decline in detoxification capacity of larvae fed diet
containing no protein appears not to be associated with a breakdown in functions (i.e.,
impending death), because the larvae still managed to grow, albeit at only a fifth of the
maximum rate. As further evidence that the ability to detoxify furanocoumarins is
still intact, even at low protein levels, larvae fed protein-poor diet still manage to
increase (induce) detoxification capacity.