Effects Of Changed Plant Chemistry On Insect Densities And Mortalities

There has been relatively little research into how CO2-mediated changes in plant chemistry affect insect densities and mortalities. This is because most plant—insect work has been done in laboratory conditions, where insects are fed foliage grown in elevated or ambient CO2 and insect weight gains, losses, and digestibility coefficients are measured. To predict the effects of elevated CO2 on insect densities, a population must be established on CO2-treated foliage. However, in the few cases where insects have been reared from first instars through to pupae and adults, a significant decrease has been found in resultant population sizes in over 30% of the cases. This is usually because nutritionally inadequate foliage kills the immature insects. In some species (e.g., leafminers), we can get a good estimate of host-plant-induced mortality. Here, larvae that die from nutritional inadequacy are entombed with the leaf and can be counted, permitting an accurate assessment of deaths induced by the host plant.

To study the effects of elevated CO2 on the interactions of insect herbivores with their natural enemies, such as predators and parasites, whole communities containing insect herbivores and their predators, parasites, and diseases are exposed to elevated CO2. Such community-wide exposure has proved to be very difficult to achieve in the laboratory. Only where whole communities of plants and insects are exposed to elevated CO2 in the field is it possible to fully address the effects of CO2 on natural enemies. Experiments like this are very costly to do because of the huge quantities of CO2 needed to arrive at a large enough increase in CO2 under field conditions. However, it is widely thought that the net result of increased plant consumption and slower growth by herbivorous insects in elevated CO2 is likely to result in increased exposure to natural enemies. For example, consumption of additional foliage increases the probability of ingestion of viruses or pathogenic bacteria, such as Bacillus thuringiensis, which can cause death. Once again, leafmining insects are a valuable study organism with which to examine the effects of elevated CO2 on attack rate by natural enemies. This is because the leafmines themselves leave a permanent record of the fate of the insect inside. Parasite larvae can often be found within a mine, or the emerging adult parasitoids leave characteristic small shotgunlike holes in the upper mine surface. A recent study by the author, at Kennedy Space Center, was able to examine attack rates of leafminers by parasitic Hymenoptera in field chambers under conditions of ambient and elevated CO2. The open-topped chambers contained the full complement of herbivores and their natural enemies on naturally occurring oak vegetation. Leafminer density was reduced inside the chambers, and leaf nitrogen content was reduced. The leafminers died more frequently inside the mines in elevated CO2 and the mine area was bigger, indicating that larval leafminers had to eat more. Attack rate by natural enemies, particularly parasitoids, was significantly increased inside the chambers in which CO2 was elevated. Perhaps the leafminers had created bigger, more obvious mines. Alternatively, their developmental time might have been slower in elevated CO2, exposing them to natural enemies for a longer time, or they might have been physiologically less well able to resist attack.

In other systems, aphids known to produce alarm pheromones show a reduced capacity to do so under elevated CO2. Once again, the result is an increased susceptibility to natural enemy attack. Finally, increased global temperatures are also likely to increase parasite and predator abundance as a result of increased population growth rates. This in turn could also lead to higher insect herbivore mortalities.

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