Longevity and fecundity of Japanese beetle (Popillia japonica) on foliage grown under elevated carbon dioxide

Impact of elevated levels of atmospheric CO2 and herbivory on flavonoids of soybean (Glycine max Linnaeus)

Atmospheric levels of carbon dioxide (CO2) have been increasing steadily over the last century. Plants grown under elevated CO2 conditions experience physiological changes, particularly in phytochemical content, that can influence their suitability as food for insects. Flavonoids are important plant defense compounds and antioxidants that can have a large effect on leaf palatability and herbivore longevity. In this study, flavonoid content was examined in foliage of soybean (Glycine max Linnaeus) grown under ambient and elevated levels of CO2 and subjected to damage by herbivores in three feeding guilds: leaf skeletonizer (Popillia japonica Newman), leaf chewer (Vanessa cardui Linnaeus), and phloem feeder (Aphis glycines Matsumura). Flavonoid content also was examined in foliage of soybean grown under ambient and elevated levels of O3 and subjected to damage by the leaf skeletonizer P. japonica. The presence of the isoflavones genistein and daidzein and the flavonols quercetin and kaempferol was confirmed in all plants examined, as were their glycosides. All compounds significantly increased in concentration as the growing season progressed. Concentrations of quercetin glycosides were higher in plants grown under elevated levels of CO2. The majority of compounds in foliage were induced in response to leaf skeletonization damage but remained unchanged in response to non-skeletonizing feeding or phloem-feeding. Most compounds increased in concentration in plants grown under elevated levels of O3. Insects feeding on G. max foliage growing under elevated levels of CO2 may derive additional antioxidant benefits from their host plants as a consequence of the change in ratios of flavonoid classes. This nutritional benefit could lead to increased herbivore longevity and increased damage to soybean (and perhaps other crop plants) in the future.

Role of cysteine proteinase inhibitors in preference of Japanese beetles (Popillia japonica) for soybean (Glycine max) leaves of different ages and grown under elevated CO2

Elevated levels of CO(2), equivalent to those projected to occur under global climate change scenarios, increase the susceptibility of soybean foliage to herbivores by down-regulating the expression of genes related to the defense hormones jasmonic acid and ethylene; these in turn decrease the gene expression and activity of cysteine proteinase inhibitors (CystPIs), the principal antiherbivore defenses in foliage. To examine the effects of elevated CO(2) on the preference of Japanese beetle (JB; Popillia japonica) for leaves of different ages within the plant, soybeans were grown at the SoyFACE facility at the University of Illinois at Urbana-Champaign. When given a choice, JB consistently inflicted greater levels of damage on older leaves than on younger leaves, and there was a trend for a greater preference for young leaves grown under elevated CO(2) compared to those grown under ambient CO(2). More heavily damaged older leaves and those grown under elevated CO(2) had reduced CystPI activity, and JB that consumed leaves with lower CystPI activity had correspondingly greater gut proteinase activity. Younger leaves with higher CystPI activity and photosynthetic rates may contribute disproportionately to plant fitness and are more protected against herbivore attack than older foliage. Cysteine proteinase inhibitors are potent defenses against JB, and the effectiveness of this defense is modulated by growth under elevated CO(2) as well as leaf position.

Anthropogenic increase in carbon dioxide compromises plant defense against invasive insects

Elevated levels of atmospheric carbon dioxide (CO2), a consequence of anthropogenic global change, can profoundly affect the interactions between crop plants and insect pests and may promote yet another form of global change: the rapid establishment of invasive species. Elevated CO2 increased the susceptibility of soybean plants grown under field conditions to the invasive Japanese beetle (Popillia japonica) and to a variant of western corn rootworm (Diabrotica virgifera virgifera) resistant to crop rotation by down-regulating gene expression related to defense signaling [lipoxygenase 7 (lox7), lipoxygenase 8 (lox8), and 1-aminocyclopropane-1-carboxylate synthase (acc-s)]. The down-regulation of these genes, in turn, reduced the production of cysteine proteinase inhibitors (CystPIs), which are specific deterrents to coleopteran herbivores. Beetle herbivory increased CystPI activity to a greater degree in plants grown under ambient than under elevated CO2. Gut cysteine proteinase activity was higher in beetles consuming foliage of soybeans grown under elevated CO2 than in beetles consuming soybeans grown in ambient CO2, consistent with enhanced growth and development of these beetles on plants grown in elevated CO2. These findings suggest that predicted increases in soybean productivity under projected elevated CO2 levels may be reduced by increased susceptibility to invasive crop pests.