Balsam Fir Foliar Chemistry in Middle and Lower Crowns and Spruce Budworm Growth, Development, Food and Nitrogen Utilization

Abstract

The impact of current-year foliage from the middle or lower crown section of balsam fir trees on larval growth, development, nitrogen, and food utilization was studied with laboratory rearing experiments, gravimetric analyses, and foliage chemical analyses. Pupal dry weight, pupal nitrogen weight, and total development times were not affected by feeding on current-year foliage from either the middle or the lower crown section. However, the nutritive and, particularly, allelochemical profiles were significantly different between crown levels and influenced spruce budworm food utilization, depending on development stage. Newly molted fifth instars feeding since post-diapause on middle crown foliage had lower dry weight and nitrogen weight than those fed lower crown foliage. This apparently resulted from the high monoterpene content in the mid-crown foliage. At the end of the fifth instar, however, insects from both crown levels had similar larval dry weights and nitrogen weights. Larvae fed middle crown foliage compensated with several mechanisms that led to increased relative nitrogen accumulation rate (RNAR) including increased efficiencies of conversion of ingested and digested nitrogen (ECIN, ECDN). Sixth-instar development time was nearly two days shorter for larvae fed middle crown foliage compared to those fed lower crown foliage. A higher relative consumption rate (RCR) due to the high content of two phagostimulatory components (soluble sugars and β-pinene) may account for reduced sixth-instar development time. Larvae from the middle crown section had higher relative growth rates (RGR) and RNAR than those from the lower crown. Pupal development time was also longer in the lower crown. The existence of spruce budworm flexibility to adapt to foliar biochemistry variations by developing compensatory mechanisms enables larvae to exploit the whole tree crown.