Introduction

The fourth assessment report of the InterGovernmental Panel on Climate Change (2007) reconfirmed that the atmospheric concentrations of carbon dioxide (CO₂), methane, and nitrous oxide greenhouse gases have increased markedly since 1750. The report showed that these increases in greenhouse gases have resulted in warming of the climate system by 0.74° C over the past 100 years. The projected increase in temperature by 2100 was set at 1.8 to 4.0° C. Atmospheric concentrations of CO₂ have been steadily rising, from approximately 315 ppm in 1959 to a current atmosphere average of approximately 385 ppm. Even if the annual flow of emissions did not increase beyond today's rate, the stock of green house gases in the atmosphere would reach double pre-industrial levels, or 550 ppm CO₂, by 2050, and would continue growing thereafter (Stern 2007). This increase is likely to affect biota indirectly via climate change, and directly by producing changes not only in plant growth and allocation, but also in plant tissue chemical composition. Legumes, being more responsive to elevated CO₂ than other plants, will have a competitive advantage (Ainsworth and Long 2005) when grown under elevated CO₂. Within legume species, nodulating genotypes, or N fixers, are more responsive to elevated CO₂ than non-fixers (Ainsworth et al. 2004).

Many plant species respond to enriched atmospheric CO₂ by enhanced photosynthetic rates and increase in biomass, as well as alterations in leaf quality factors. Insect and host plant interactions change in response to the effects of CO₂ on nutritional quality and secondary metabolites of the host plants. In atmospheres experimentally enriched with CO₂, the nutritional quality of leaves declined substantially due to dilution of nitrogen (N) by 10–30% Coley and Markham 1998). Lower foliar N content due to elevated CO₂ causes an increase in food consumption by herbivores by up to 40% (Hunter 2001). Many species of herbivorous insects will confront less nutritious host plants under elevated CO₂, which may induce both lengthened larval developmental times and greater larval mortality (Coviella and Trumble 1999). Increased levels of CO₂ will enhance plant growth, but may also increase the damage caused by some phytophagous insects (Gregory et al. 2009). Reduced leaf N, and consequent compensatory feeding, is less likely in legumes, which may overcome N limitation by increased nodulation and N fixation. However, increased folivory in soybean, (Rogers et al. 2006) and increased damage by herbivores in some legume species, despite avoiding the N dilution, has been reported (Zavala et al. 2009).

Peanut (Arachis hypogaea L.), also known as groundnut, earthnut, and ground bean, is the world's fourth most important source of edible vegetable oil, and third most important source of vegetable protein. Peanut was grown on 23.91 million hectares worldwide, with a total production of 36.60 million tons, and an average yield of 1531 kg/hectare in 2009 ( http://faostat.fao.org). China, India, Nigeria, the United States of America, and Myanmar are the major peanut growing countries. India is the second largest producer of peanut in the world, with an average annual production of 5.51 million tons ( http://faostat.fao.org). Developing countries in Asia, Africa, and South America account for over 97% of world peanut area, and 95% of total peanut production. Production is concentrated in Asia (50% of global area, and 64% of global production) and Africa (46% of global area and 28% of global production), where the crop is grown mostly by smallholder farmers under rainfed conditions with limited inputs.

Peanut has traditionally been used as a source of oil; however, its worldwide annual protein harvest has reached nearly 4.5 million tons. Crude protein content of whole seed peanuts is estimated to be around 25%, followed by carbohydrates (16%), and monosaturated fats (24%) ( http://ndb.nal.usda.gov/ndb/foods/show/4779). Elevated CO₂ was reported to cause significant increase in total biomass at the final harvest of peanut crop, but decreased final seed yield in selected cultivars (Bannayan et al. 2009).

The peanut crop is attacked by many species of insects that cause damage ranging from incidental feeding to near total plant destruction and yield loss (Wightman and Ranga Rao 1994). Among the damaging species, the tobacco armyworm, Spodoptera litura (Fab.), is as a major pest, and can cause yield losses of 35–55%. Larvae feed gregariously on leaves, causing severe defoliation, leaving midrib veins only. Response of herbivory to elevated CO₂ is highly complex, and the interactions between legumes and insect-herbivores are unclear. The present study was aimed to elucidate the insect-herbivore (S. litura) and plant (peanut) interactions under elevated CO₂.

Materials and Methods

Biochemical analysis of foliage

Leaf tissue used in the feeding experiments was analyzed for carbon, N (C: N ratio), and polyphenols. To determine carbon and N concentrations, samples were dried at 80° C, and subsequently ground to powder. Leaf carbon and N were measured using a CHN analyzer (Model NA 1500 N, Carlo Erba Strumentazione, Italy) using standard procedures (Jackson 1973). Total soluble polyphenols (hydrolysable tannins, condensed tannins, and non-tannin polyphenols) were determined by the Folin-Denis method (Anderson and Ingramm 1993). For this method, leaf samples were dried at 40° C for 48 hours. Dried leaf samples were ground to powder, and phenolics were extracted with CH₃OH. The concentration of polyphenols in the extract was determined spectrophotometrically using tannic acid as the standard, and the results were expressed as percentage tannic acid equivalents.

Figure 1.

Variation in biochemical constituents of peanut foliage under elevated CO₂ conditions. High quality figures are available online.

Results

Larval growth performance

Relative consumption rate differed significantly among the three CO₂ treatments (F_5,10 = 5.41, p < 0.05). Relative consumption rate was significantly higher for S. litura larva fed plants grown at 550 and 700 ppm than for larvae fed plants grown at ambient condition (by 19% and 24% respectively), but did not differ between larvae fed plants grown at 550 vs. 700 ppm. The similar trend was reflected in the higher consumption (by 55% and 80%, respectively) of peanut foliage by larva (F_{5, 10} = 5.41; p < 0.01). The effect of elevated CO₂ (F_{5, 10} = 3.88, p > 0.01) on approximate digestibility of peanut foliage by S. litura was not significant across three CO₂ conditions 550 ppm, 700 ppm, and ambient, despite recording 16–20% higher approximate digestibility under elevated CO₂ conditions. The effect of elevated CO₂ on efficiency of conversion of digested food of larvae was significantly (F_{5, 10} = 12.42, p < 0.01) lower (by 35% in 550 ppm and 32% in 700 ppm condition) than in ambient. Efficiency of conversion of ingested food for S. litura larvae fed on peanut foliage under elevated CO₂ concentrations was significantly (F_{5, 10} =17.85, p < 0.01) reduced (by 25% in both the elevated CO₂ conditions) than ambient. Relative growth rate of larvae was lower significantly (by 9–10%) when fed on peanut foliage under elevated CO₂ (F_{5, 10} = 7.97, p < 0.01) than ambient (Table 1).

Table 1.

Impact of elevated CO₂ on larval indices of Spodoptera litura on peanut. Means in the same vertical column with different superscripts are significantly different by Tukey's test.

Table 2.

Impact of elevated CO₂ on growth parameters of Spodoptera litura on peanut. Means in the same vertical column with different superscripts are significantly different by Tukey's test.

Larval weights (0.64 ± 0.06 and 0.718 ± 0.09 g) also differed significantly (F₅, ₁₀ = 5.41; p < 0.01) with elevated CO₂ condition than ambient CO₂ (0.541 ± 0.02 g). The higher larval weights were recorded in 550 (18%) and 700 ppm (32 %) CO₂ conditions. The larval duration was extended significantly under elevated CO₂ condition than ambient (F_{5, 10} =4.57; p<0 .05). The fecal matter release by larvae was also significantly varied and was more in 550 ppm (25%) and 700 ppm (34%) than ambient (F_{5, 10} =5.41; p < 0.05). The pupal weights of S. litura were lower under elevated CO₂ conditions than ambient. The less fecund adults were noticed under elevated CO₂, and resulted in a total of 424– 427 eggs per female per day, as compared with 467 eggs per female per day under ambient (Table 2).

Discussion

The photosynthesis and growth of many plants are stimulated when plants are grown under elevated CO₂ condition, and reduction in leaf N content in plants grown at elevated CO₂, due to faster growth of the plant (Stitt and Krapp 1999), is well known. Biochemical analysis of peanut foliage a significant reduction (8%) of leaf N under elevated CO₂ conditions as opposed to ambient. It is understood that leaf N content of legumes decreased on average by only 7% under elevated CO₂, which was less than half the decrease exhibited by the non-legumes C3 plants (Cotrufo et al. 1998). Nitrogen is the single most important limiting resource for phytophagous insects (Mattson 1980), a decrease in foliar N of host plants could affect the development and survival rates of phytophagous insects.

Legumes capable of N fixation are less likely to suffer reduction in N, but may exhibit lower leaf N during early growth stages, as reported by Rogers et al. (2006) in soybean. In this study, biochemical analysis of foliage was done on samples collected from 30-day-old plants. It is possible that N fixation was not fully operational by that time. However, in the absence of specific data on nodulation, N fixation, and biochemical analysis of foliage at later stage of crop growth, no generalization can be made regarding the effect of elevated CO₂ on foliage N content of the peanut crop.

Increase in atmospheric levels of CO₂ can cause increases in plant growth rates, and changes in the physical and chemical composition of their tissues (Sudderth 2005). Phytophagous insects are indirectly affected by those changes in their host plants. In the present study, polyphenols were increased in peanut leaves under elevated CO₂ condition. Increase in phenolics and poly-phenolics concentrations in green leaves under elevated CO₂ is well known (Agrell et al 2000), and increase in phenolics have a negative influence on the development and fitness of chewing herbivore insects (Yin et al. 2010). In addition, most herbivorous insects appear to be negatively affected by elevated CO₂ because of the reduction in foliar N and increase in C: N ratio (Bezemer and Jones 1998). In our study, 13% percent increase in C: N ratio was observed under elevated CO₂ conditions. The reduction in protein content, and increase of C/N ratio in leaves under elevated CO₂ (Hunter 2001), imply a reduction in food quality that might have caused the higher feeding by larvae. The CO₂ mediated changes in the peanut foliage (i.e., decreased N and increased polyphenols) in the present study affected the growth and development of S. litura, causing higher consumption (54% in 550 ppm and 80% in 700 ppm CO₂ condition).

The increased larval weight (18% and 32%) with higher fecal matter release was observed under both elevated CO₂ condition over ambient CO₂. It was well known that most leaf-chewing insects exhibit compensatory increase in food consumption (Lee et al. 2002). Insects, when fed on elevated CO₂ grown plants, were shown to increase their individual consumption due to the poor food quality of these plants (Coviella et al. 2000; Hunter 2001). Similarly, the reduction of leaf N content of peanut foliage was noticed, and the feeding trials were conducted during the period of 30–60 days after sowing of the peanut crop. The present results were explained as a response of S. litura to reduced foliage quality, especially the variation in N and C: N ratio. Authors observed the similar trend of the effect of elevated CO₂ on S. litura in castor crop (Srinivasa Rao et al. 2009).

In this study, the insect growth performance indices also significantly varied between elevated CO₂ and ambient conditions. The relative growth rate of larvae fed on elevated CO₂ peanut foliage was significantly reduced. It is understood that larvae consumed and assimilated more but grew slower (lower relative growth rate), and took one to two days longer of extension to pupation than ambient. Low efficiency of conversion of digested food may result from a requirement of these insects to metabolize digested food in order to produce water (Lindroth 1993). Relative growth rates of Gypsy moth (Lymantria dispar) were reported to be reduced by 30% in larvae fed on Quercus petraea exposed to high CO₂ (Hattenschwiler and Schafellner 2004). However, it is generally believed that CO₂ induced changes in foliar phytochemical compounds play the most important role in the activities of phytophagous insects. The quantified review of information on published results of the impact of elevated CO₂ on insect pests through ‘meta analysis’ indicated that the elevated CO₂ increased relative consumption rate, total consumption, and developmental time, but decreased relative growth rate, conversion efficiency, and pupal weights. Smaller effect sizes for many response variables were reported (Stiling and Corneliessen 2007; Robinson et al. 2012; Srinivasa Rao et al. 2012).

In the present study, an increase in developmental time with decreased pupal weights was observed when larvae of S. litura were reared from hatching to pupation on elevated CO₂ grown peanut crop. A similar observation was reported on cotton leaf worm by Agrell et al. 2006. The fecundity of S. litura was found reduced under elevated CO₂ conditions, and might be due to lower pupal weights. Karowe (1992) observed that pupal weight in insects has a significant and positive correlation with fecundity in Lepidoptera, and a similar trend was noticed in the present study.

A response of an insect herbivore to elevated CO₂ levels was found to be species-specific, and might occur at different rates, potentially altering growth and development. A complete comprehension of these changes over generations will serve as major input in the development of insect population dynamics models.