The endophytic fungi of certain grasses and other herbaceous plants have long been known to provide plants with anti-herbivore defense compounds, but there is little information about whether the endophytes of trees also engage in such mutualisms. We investigated the influence of the endophytic fungus Cladosporium cladosporioides on the chemical defenses of black poplar (Populus nigra) trees and the consequences for feeding preference and fitness of herbivorous insects and insect community assembly. Endophyte colonization increased both constitutive- and induced poplar defenses. Generalist Lymantria dispar larvae preferred and performed better on uninfected over endophyte-infected poplar leaves, most likely due to higher concentrations of salicinoids in endophytic leaves and the endophyte-produced alkaloid stachydrine. Under field conditions, the endophytic fungus also shapes insect community assembly in young black poplar trees. Our results show that endophytic fungi can play a major role both in defending trees against herbivorous insects and in structuring insect communities.

Diplodia sapinea causes Diplodia tip blight (DTB) and is recognised as an opportunistic necrotrophic pathogen affecting conifers. While DTB is associated with abiotic stress, the impact of biotic stress in the host on D. sapinea’s lifestyle shift is unknown. Observed co-occurrences of D. sapinea and Melampsora pinitorqua, causing pine twisting rust on Scots pine ( Pinus sylvestris), instigated an investigation into their interaction with and influence on the defence mechanisms of the host. We hypothesised that M. pinitorqua infections predispose the trees to D. sapinea by stressing the host and altering the shoot metabolites. Trees in a pine plantation in central Sweden were sampled over time to study pathogen biomass and host metabolites. The symptoms of both pathogens were consistent over years, and the preceding season’s symptoms affected the metabolic starting points pre-infection and M. pinitorqua’s proliferation. Symptoms of M. pinitorqua altered shoot metabolites more than fungal biomass, with co-symptomatic trees exhibiting elevated M. pinitorqua biomass. D. sapinea’s biomass pre-symptoms was independent of previous disease symptoms and infection by M. pinitorqua. Some trees showed tolerance to M. pinitorqua, with delayed rust infections and minimal DTB symptoms. This trait may improve the resilience of pine plantations, but more work is needed.

Monoterpenes in conifer oleoresin frequently occur as complex blends of 15-30 components, but the underlying ecological reasons for these mixtures are not yet known. We tested the toxicity of 12 individual Norway spruce monoterpenes and their mixtures to adult Eurasian spruce bark beetles, Ips typographus, and three major symbiotic fungi in the vapor phase. Individual monoterpenes had generally opposite effects on bark beetles compared to symbiotic fungi, with the compounds most toxic to beetles being the least inhibitory to fungal growth and vice versa. These results support the interaction diversity hypothesis, which posits that defense mixtures are maintained because the individual components target different attackers. Among the symbiotic fungi tested, one (Grosmannia penicillata) exhibited high tolerance to monoterpenes, and its growth was even stimulated by the monoterpenes most toxic to the beetle. Interestingly, spruce bark responds to G. penicillata inoculation by accumulating higher concentrations of specifically fungistatic monoterpenes.

Brassicaceae plants have the glucosinolate-myrosinase defense system, jointly active against herbivory. Glucosinolates (GLS) are hydrolysed by myrosinase to produce isothiocyanates as soon as herbivory begins. Isothiocyanates exert detrimental effects on the feeding insects. However, constitutive GLS defense is observed to occur at levels that do not deter all insects from feeding. That prompts the question of why Brassicaceae plants have not evolved a high constitutive defense. The answer may lie in the contrasting relationship between plant defense and host plant preference of specialist and generalist herbivores. One of the reasons plants are in this dilemma is that they do not know what kind of herbivore will attack them in any given year, and thus have to be prepared for different possibilities. GLS content increases the susceptibility to specialist insects because these are attracted to plants with a high GLS content and are capable of coping with the toxin. In contrast, generalists are deterred by the plant GLS. Although GLS can attract the natural enemies (predators and parasitoids) of these herbivores, enemies can reduce herbivore pressure to some extent only. So, plants can be overrun by specialists if GLS content is too high, whereas generalists can invade the plants if it is too low. Therefore, an optimal constitutive plant defense can minimize the overall herbivore pressure. To explain optimal defense theoretically, we represent the contrasting host selection behavior of insect herbivores and, in addition, the emergence of their natural enemies by a non-autonomous ordinary differential equation model, where the independent variable is the plant GLS concentration. From the model, we quantify the optimal amount of GLS, which minimizes the total herbivore (specialists and generalists) pressure. That quite successfully explains the evolution of constitutive defense in plants from the perspective of optimality theory.

Various herbivorous insects prefer plants of the Brassicaceae family as their hosts, although they are toxic. The two-component chemical defence system of the Brassicaceae against herbivores consists of glucosinolates (GLS) and the activating enzyme myrosinase. GLS hydrolysis by myrosinase leads to isothiocyanate (ITC) products, which are toxic and deterrent to many insect herbivores. Some insects that feed on Brassicaceae, however, have evolved specific adaptations (called counter-defences) against GLS. Two different types of counter-defences can be distinguished: a preemptive counter-defence that prevents the GLS from being hydrolysed to ITC due to metabolic redirection and direct counter-defence, where the ITC is formed, but then metabolized to a non-toxic conjugate. Preemptive counter-defence is believed to be more efficient due to the lower exposure to ITC, but this has not been well demonstrated experimentally. Here, we prove on theoretical grounds that preemptive counter-defence reduces exposure to ITC compared to direct counter-defence by studying the dynamics of GLS defence and counter-defence with two separate ordinary differential equation models. By quantifying the specific ITC concentrations that herbivores are exposed to during feeding with the two types of counter-defences, we show that herbivores with a preemptory detoxification system are less exposed to ITC. In addition, our models explain how the decline in the level of ITC is achieved by both counter-defences, which helps to understand the overall mechanisms and benefits of these techniques.