Forest insect : a delicate balance between a primary and secondary insect

许多食草昆虫栖息在森林中，以树叶、树皮、木材、根、种子和其他植物部位为食。其中，以内皮(韧皮部)、形成层和木质部(木质部)为主要食物来源的树皮和钻木虫。一般来说，当被认为是昆虫生长和发育的基质时，大多数木质组织的营养价值比叶组织差。因此，为了弥补这种糟糕的营养条件，一些种类的木蜂(沙蜂科)在产卵期间向树木中引入了线虫真菌，然后腐烂木材并为发育中的幼虫提供营养，还有其他昆虫，如栖息在木材上的文冠甲虫(Scolytidae)，也开始完全依赖真菌作为它们的食物来源。由于取食树皮和木材的幼虫在发育过程中无法离开寄主树木的木质组织，由于木质栖息地的高度营养异质性，它们最终成虫的身体大小往往比自由生活的昆虫变化得更大。在许多针叶树中，人们都知道树脂是抵御树皮甲虫和钻木虫攻击活树的重要因素。因此，树皮和木材钻探昆虫的发育和生存似乎受到寄主树木条件的密切影响，如营养水平和树脂流动。

Numerous plant-eating insects inhabit forests, feeding on leaves, bark, wood, roots, seeds and other plant parts. Among them, the bark- and wood-boring insects utilize mostly inner bark (phloem), cambium, and wood (xylem) as their food resources. In general, most woody tissues are nutritionally poorer than leaf tissue when considered as a substratum for insect g rowth and development. Hence, to compensate for such poor nutrient conditions, some species of woodwasps (Siricidae) introduced Amylostereum fungi into trees during oviposition, which then decays the wood and provide nutrients to the developing larvae, and still other insects such as ambrosia beetles (Scolytidae), which inhabit wood, had also come to depend solely on fungus as their source of food. Because bark- and wood-boring larvae are unable to exit the woody tissues of their host trees during development, their eventual adult body size often varies considerably more than free-living insects due to high nutritional heterogeneity in the woody habitat. In many coniferous trees, it is known that resin is an important factor for defense against bark beetles and wood-borers that attack living trees. Thus, development and survival of bark- and wood-boring insects appeared to be closely affected by host tree conditions such as nutrient levels and resin flow.