Resistance to Catastrophic Stem Damage
Cannabis sativa normally has a dominant leader stem which produces a central stalk. As discussed in the following paragraphs, the species has an amazing capacity to recover from catastrophic damage to the main stem.
The European corn borer (Ostrinia nubilalis) or ECB (Fig. 6a), is a major Lepidopteran pest of C. sativa. Young ECB larvae eat hemp leaves until half-grown, then bore holes into the stems. A typical entrance hole resulting from an attack on the main stem is shown in Fig. 6b. The insect is indigenous to the Old World, where it apparently once reproduced mainly in association with Cannabis and its close relative Humulus (although also attacking many other plant species). It was not exposed to corn (i.e., maize, Zea mays), which is indigenous to the Americas, until post-Columbian times (“European hemp borer” would have been a better choice of name). In a study of ECB infestation of a large experimental field, Small et al. (2007) discovered that ECB damage to Cannabis increased the shoot weight of the plant by 20 %, concomitantly enlarging seed production, indicating that Cannabis is adapted to the insect. The expanded productivity observed was due to branch proliferation at the site of the attack (see Fig. 6c and d). Figure 6e shows silhouettes of a normal and an ECB-damaged plant, and it is evident that the increased number of branches resulting from the damage has produced more biomass and more seeds. (The insect preferred larger stems, but was
unaffected by THC content.)