Impact of balsam fir flowering on pollen and foliage biochemistry in relation to spruce budworm growth, development and food utilization

Abstract

AbstractThe impact of balsam fir (Abies balsamea (L.) Miller) flowering on nutritional and allelochemical quality of pollen, current‐year and one‐year‐old foliage is studied in relation to spruce budworm (Choristoneura fumiferana Clem.) (Lepidoptera: Tortricidae) growth, development and utilization of food and nitrogen. In the laboratory, using fresh food from the field, we simulated conditions of low larval population density, in which there is no current‐year foliage depletion during the spruce budworm feeding period. Similarly, we simulated conditions of high larval population density when current‐year foliage depletion occurs.Because of the high nutritive value of pollen (high amounts of amino acids and minerals, especially nitrogen; low monoterpene content), insects from flowering trees reached the fifth instar five days earlier than those from non‐flowering trees, and had heavier dry‐ and nitrogen‐weights at the beginning of the fifth instar. At budbreak, switching from pollen to current‐year foliage negatively affected conversion efficiencies and digestibilities of food and nitrogen (AD; ADN; ECDN; ECI; ECIN). The switch from pollen to new foliage had a detrimental impact on fifth‐instar survival and on newly‐moulted sixth‐instar dry‐ and nitrogen‐weights. Moreover, during the fifth instar, balsam fir flowering reduced the nutritive value of current‐year foliage, which in turn, might have contributed to the reduced larval growth. Nevertheless, during the sixth instar, balsam fir flowering affected the biochemistry of current‐year foliage in ways that enabled larvae to compensate for their low fifth‐instar biological performance; larvae also managed to reach pupal dry weight similar to larvae reared on non‐flowering trees. Current‐year foliage from flowering trees contained less nitrogen, total soluble sugars and total monoterpenes. Those foliar characteristics enabled larvae to increase food and nitrogen consumption rates (RCR; RNCR), because of lower repellency and/or post‐ingestional feedback from monoterpenes.As for current‐year foliage, balsam fir flowering reduced nitrogen, total soluble sugar and total monoterpene contents in one‐year‐old foliage during the sixth‐instar feeding period. These characteristics enabled sixth‐instar larvae, fed on old foliage from flowering trees, to have high relative food and nitrogen consumption rates (RCR; RNCR). Larvae were also able to reach higher relative growth rates (RGR) and relative nitrogen accumulation rates (RNAR) compared to larvae reared on one‐year‐old foliage from non‐flowering trees. Finally, larvae on flowering trees had pupal dry weight similar to those from non‐flowering trees, but reached the adult stage nine days earlier.Regardless the foliage type consumed by spruce budworm larvae during the sixth instar, pollen consumption during early larval stages reduced total development time, and thus exposure time to natural enemies. This phenomenon might increase larval survival. Balsam fir flowering changed the biochemistry of one‐year‐old and current‐year foliages, but did not affect pupal dry weights of larvae reared on flowering trees compared to those reared on non‐flowering trees. Thus, at low population density, spruce budworm populations in balsam fir flowering stands might be favoured over those in balsam fir non‐flowering stands. In addition, when larvae consumed one‐year‐old foliage during the entire sixth instar, those on flowering trees are probably favoured over those on non‐flowering trees. However, because flowering trees produce less new foliage than non‐flowering trees, current‐year foliage depletion may occur earlier on flowering trees than on non‐flowering trees. Thus, at similar larval population density, larvae on flowering trees might have to feed on one‐year‐old foliage earlier than those on non‐flowering trees. In that case, spruce budworm populations on non‐flowering stands would be favoured over those on flowering stands.