Why insect energy budgets do not balance

Summary

• (1) Energy budgets are based upon the equation: C=P+R+FU, where C=consumption, P=production, R=respiration and FU=egesta. In studies of insect energetics, where all four parameters have been measured, the completed budget rarely balances. This can often be traced to inaccuracy in the R term which, when measured with a respirometer (R _m ), is almost invariably less than when calculated as C-P-FU(=R _c ).

• (2) The R _c·R ^{Emphasis>/−1} _m values of ‘wild’ insects (av. 2.58) are consistently larger than those of ‘laboratory’ insects (av. 1.25).

• (3) Suggestions that incorrect oxycalorific equivalents, caused by inappropriate RQ values, result in large inaccuracies in R _m are shown to be invalid.

• (4) The possibility that the C term in the energy budgets of phytophagous insects has been miscalculated because of a failure to account for leaf respiration is discounted because it is equally possible, at least in properly conducted experiments, for leaves to gain weight by photosynthesis. Furthermore, this factor cannot account for imbalances in the budgets of nonphytophages.

• (5) The gut contents of phytophagous insects can weigh as much as the gut-free body. Failures to account for this factor lead to over-estimates of P.

• (6) It is concluded that there is little reason to doubt evidence published elsewhere that ‘flask-effects’ are the major cause of the discrepancy between R _m and R _c .