Background & Rationales

White spruce and spruce budworm

White spruce (Picea glauca) is a perennial tree species distributed across Canada with high economic value (Lu et al., 2014). It provides millions of dollars of income via multiple products, such as lumber, pulp, and paper (Bérubé-Deschênes et al., 2016 & Petrinovic et al., 2009). Spruce budworm (Choristoneura fumiferana (Clemens)) is one of the most harmful herbivorous insects in North America, which leads to the defoliation of millions of hectares of trees in the year of 2009 in Canada (Chang et al., 2012). 

The spruce budworm has a one-year life cycle with six key steps (Fuentealba et al., 2021). The adults emerge and mate in summer and lay eggs, then the eggs are hatched immediately, and then the first instar larvae search for spruce trees as a shelter in the fall (Fuentealba et al., 2021). This insect overwinters as second instar larvae and emergencies before forming new buds (Fuentealba et al., 2021). Third-instar larvae then stay at the young needles of their hosts and feed through fourth, fifth, and sixth instar larvae until pupate in mid to late June (Fuentealba et al., 2021). 

Recent studies show that the next spruce budworm outbreak will occur in the next 10-20 years, and future climate scenarios will increase the damage (Chang et al., 2012; Fuentealba et al., 2021; Navarro et al., 2018). Unfortunately, traditional control of spruce budworms via aerial spraying reduces only around 60% of the damage, so forest managers must find another control method (Chang et al., 2012).

Fungal endophytes and their potential as biological control agents for spruce budworm

Fungal endophytes are mutualistic microorganisms of plant species that cause non-detectable damage to the host plants (Stefani & Bérubé, 2006). Fungal endophytes were first defined around one century ago, and studies in the past twenty years have been focused on the diversity and distribution of fungal endophytes among different hosts (Stefani & Bérubé, 2006).  As it causes non-detectable damage to hosts, the basic knowledge of the abundance and distribution of fungal endophytes among different tree species is not well studied (Stefani & Bérubé, 2006). As a result, understanding the abundance and distribution of white spruce fungal endophytes is crucial for providing some basic information on future forest management practices.

Recent studies indicate that the mechanisms of how fungal endophytic chemical compounds can be applied to deal with pathogens and pests are unknown (Mane et al., 2018; Segaran & Sathiavelu., 2019). However, studies do find the potential of fungal endophytes as a biological control agent for spruce budworms. Fungal endophytes can produce rugulosin that reduces the larvae development of spruce budworms (Miller et al., 2002). As shown in the figure-3, fungal endophytes indirectly promote plant terpenes' biosynthesis via four steps, and terpenes are plant defensive compounds that reduce the damage to spruce budworms (Caron et al., 2013 & Chen et al., 2021). Thus, it is possible that fungal endophytes can work as a biological control agent dealing with spruce budworms if we can find a strong correlation between fungal endophytes and terpene concentration. However, there are six steps before releasing the fungal endophytes as a biological control agent, and this project is the very first step (Briese, 2000). There are difficulties in applying fungal endophytes, including host specificity test, control evaluation, and industrial volume production, and future studies may focus on these aspects.