Background
The authors write that castor "is an important non-edible oilseed crop grown in many parts of the arid and semi-arid regions of India," and that the castor semilooper (Achaea janata) and the tobacco caterpillar (Spodoptera litura) "occur during early and late stages of growth of castor, respectively," during which periods they state that castor oil yields can be reduced "by 30-50% by the semilooper alone," and that the tobacco caterpillar "can cause yield losses of 25-40%."

What was done
To determine what foliage-mediated effects atmospheric CO2 enrichment might have on these pernicious insect pests, Rao et al. allowed larvae of both species to feed on castor foliage grown in present-day air (presumed to contain 350 ppm CO2) and in air enriched with CO2 to concentrations of 550 and 700 ppm, during which stage of the insects' life cycle they made various measurements pertinent to their specific quest for knowledge.

What was learned
The six scientists found that "compared to the larvae fed on ambient CO2 foliage, the larvae fed on 700 and 550 ppm CO2 foliage exhibited greater consumption." However, they say that the efficiency of conversion of both ingested and digested food into larval biomass "decreased in the case of larvae grown on 700 and 550 ppm CO2 foliage," so that they "grew slower and took longer time (two days more than ambient) to pupation," which latter phenomenon would allow significantly more time (~13%) for them to be preyed upon by still higher orders of creatures - many of which are considered to be much less of a threat to crop production than are insect larvae.

What it means
In the case of castor beans, the oil of which has many industrial applications, it would appear that in addition to the productivity enhancement likely to be provided by the stimulation of photosynthesis driven by atmospheric CO2 enrichment - an approximate 34% increase in response to a 300-ppm increase in the air's CO2 content (Grimmer and Komor, 1999; Grimmer et al., 1999) - a substantial increase in atmospheric CO2 would likely also curtail yield losses currently caused by the castor semilooper and the tobacco caterpillar.

References
Grimmer, C. and Komor, E. 1999. Assimilate export by leaves of Ricinus communis L. growing under normal and elevated carbon dioxide concentrations: the same rate during the day, a different rate at night. Planta 209: 275-281.

Grimmer, C., Bachfischer, T. and Komor, E. 1999. Carbohydrate partitioning into starch in leaves of Ricinus communis L. grown under elevated CO2 is controlled by sucrose. Plant, Cell and Environment 22: 1275-1280.