Although there are a great number of insects feeding on willows, three main species are of concerns for willow short rotation coppice in Quebec. The first is the willow leaf beetle (Plagiodera versicolora Laicharteg.), one of the most common insects found on willows. The willow leaf beetle is a small (4 — 6 mm long), metallic-blue beetle widely distributed around the world. In Quebec, adults emerge from their overwintering quarters under the loose bark and feed on young willow foliage in spring. Egg laying begins in mid-June. Females lay yellow eggs grouped on the undersides of the leaves. The young larvae emerge a few days later and begin feeding on both sides of the leaves and eating the tissue between the veins, thus skeletonizing the leaves and, depending on the extent of the attack, in all probability leading to a reduction of plant growth. In Quebec, this insect has been frequently observed feeding on leaves of clones of Salix viminalis and to a much lesser extent on most common commercial varieties of S. miyaheana (SX64 and SX67) and S. sachalinensis (SX61). To date, the reported threshold of damage caused by this insect has never been high enough to justify any type of control. However, in case of severe attack, non-toxic products based on Bacillus thuringiensis, shown to be effective in eliminating this pathogen, can be used [60].

The other predominant insects found feeding on willow trees and shrubs are two aphid species: the giant willow aphid, Tuherolachnus salignus (Gmelin) and the black willow aphid, Pterocomma salicis (L) [61].

The giant willow aphid. is one of the largest aphids ever recorded, measuring up to 5.8 mm in length [62]. It feeds almost exclusively on willow, but has very occasionally been recorded on poplar (Populus spp.). The species is strongly aggregative, forming vast colonies on infested trees. These colonies can cover a significant portion of the 1-3 year old stem surface of a willow tree. Laboratory experiments with willows grown in soil and in hydroponic culture have revealed that this species can reduce the above-ground yield of biomass willows, have severe negative effects on the roots and reduce the survival of both newly planted and established trees [63]. Other preliminary studies carried out in the UK have shown that this insect’s feeding behavior is affected by chemical cues from the host. Researchers found that one of its most preferred willows was S. viminalis [64]. Although large colonies of this insect have recently been found on several willow varieties in Quebec, it is not yet possible to estimate its threat to willow plantations in this region (Figure 5).

Figure 5. Giant aphids feeding on willow. This insect is often found forming large colonies at base of the stem.

The black willow aphid, Pterocomma salicis (L) may actually pose a threat only if severe, frequent attacks occur. Several studies have shown that this species is less damaging than the giant willow aphid, with a less persistent negative impact on willow growth. In Quebec, high density populations of this species have recently been found at the end of June on a willow plantation in the upper St. Lawrence River valley (Huntingdon), mainly on S. miyabeana (SX67 and SX64); it did not seem to feed on S. viminalis.

Other less damaging insects have been found on willow plantations in Quebec. Calligrapha multipunctata bigsbyana adults and larvae may feed on willow leaves without destroying leaf veins, with consequences quite similar to those of Plagiodera versicolora. Willow flea beetles of the genus Crepidodera (C. nana and C. decoraalso feed on Salicaceae leaves [65], and are easy to recognize by their brilliant metallic and bicoloured upper surface; blue or green head and pronotum tinged with strong bronze, copper or violet; and unicolorous blue or green elytra. This beetle feeds on either the upper or lower leaf surface, consuming the epidermis and tissue below, but not on the opposite side. After desiccating, the tissue falls out, resulting in a leaf with a bullet-hole appearance. Varieties of willows developed in Europe, based on pedigrees with Salix viminalis or S. viminalis x S. schwerinii, are susceptible to potato leafhopper (Empoasca fabae Harris), which causes serious damage to this species and its cultivars or hybrids. Willow shoot sawfly (Janus abbreviates Say) larvae have recently been found in Quebec, carving deep tunnels on young willow S. miyabeana SX64 shoots where they cause wilting, change of colour (brown or black) and eventually drooping of shoot tips. It has been observed that in some cases 30% of individuals of SX64 in Huntingdon showed at least one shoot affected by this insect. However, only repeated and severe attacks in young willow plantations may adversely affect tree growth.

Willow can be injured by several diseases [66]. Willow leaves may be sensitive to Alternaria spp., Melamsora spp. and Venturia spp., whereas Cryptodiaporthe spp., Glomerella spp. and Valsa spp. are found to affect stems and branches and Armillaria spp., Fusarium spp. and Verticilium spp. roots [67]. However, the most widespread, frequent and damaging disease in willow plantations is leaf rust, caused by Melampsora spp. In northern Europe, leaf rust is considered a major factor limiting growth of short-rotation coppice willow [68]. It can cause premature defoliation, poor cold acclimation, premature leaf senescence, and a predisposition to abiotic stress (e. g., competition and drought) in host trees, along with secondary disease organisms, and it may reduce yields by as much as 40% [69]. One of the main alternative solutions to spraying fungicides proposed in northern Europe is growing willow in inter — and intra-species mixtures [70]. If a variety dies out of a mixture due to disease, competition or some other factor, the remaining varieties can compensate for the loss [71]. In some willow plantations in Quebec, severe attacks of Melamsora spp. have been detected mainly on a specific commercial clone S301 (S. interior 62 x S. eriocepala 276), which seemed to be more vulnerable to rust than any other clone studied in the area [29]. Few rust attacks have been reported for most commercial clones, however, chemical or biological disease control is generally not required.