[MUSIC] With so many herbivorous insect species, you may wonder what can a plant do to deter insect feeding. Well, plants use a variety of defences for protection against herbivory. As plants can't run away from their attackers, physical and chemical plant defences are common. And some, even recruit natural enemies of the attacking insect herbivore, to provide indirect protection to the plant. Physical plant defences are morphological characteristics, that inhibit exploitation of the plant by insect herbivores. Small hairs on the leaves and stems of the plants called trichomes can deter small insects from feeding and over position as they make plant surfaces difficult to access. The defense provided by trichomes is not only physical though, as many of them have glands that produce or store chemical deterrents, that further ward off insect herbivores. Leaf toughness is another plant trait that can protect plants against herbivory. The toughness of leaves can physically hinder feeding by leaf chewers or insects with piercing sucking mouthparts. This is usually accomplished through increased lignin and cellulose content in the leaves, compounds which are tough and indigestible to many insects. The toughness of leaves can also change with plant phenology throughout the growing season. Phenology refers to the periodically occurring events in plant and animal life, cycles that are influenced by variations in climate, and in environmental factors especially temperature. Many insects that defoliate trees are most active in the Spring when leaves are more nutritious and less tough. Some plants like grasses, have silica in their leaves that can act as a physical defence against insect herbivores. The presence of silica within plant cell walls fortifies the mechanical barrier against pathogen in her before attacks. This defense is particularly effective against insects that bore into plant tissue, such as stem boring moth larvae. As silica slows the entry time of insects into the plant, the more time the insect is forced to remain outside the planet, the greater its exposure to external biotic and abiotic threats. Silica deposition also reduces digestibility, and increases the abrasiveness of plant tissues which can have severe consequences for the digestive system of some insect herbivores. For example the mandibles of rice stem boring moth larvae become damaged after they feed on rice plants with high silica content this can inhibit feeding activity at least until the next molt. Trees especially carnivorous trees have a defense against insect herbivores that is both physical and chemical in nature called resin or pitch. If you've ever played the violin or even just got your hands sticky by touching the bark on a conifer tree, you'll already be familiar with this particular defense. In addition to chemical properties that make resin toxic to insects, resin can physically push burrowing insects out of the tree and block pathogens from entering an open wound. Plant resistance that deters insect feeding is called non-preference, because the defence mechanism reduces the insects preference for using a particular plant without impacting the insects metabolism. While non-preference resistance can include chemical defences, it is often associated with physical defences, Chemical defenses used by plants involve compounds called secondary plant metabolites. These chemicals are not involved in plant growth or any regular plant metabolic processes. Instead, they can be used by the plant to defend against herbivores. Secondary metabolites can directly deter insect feeding and over position activities or interrupt digestive processes and insects. Plant resistance that operates through metabolic disruption is referred to as antibiosis. Many secondary plant metabolites formed the basis of commercial insecticides because they are toxic to insects. Furthermore, the natural defensive traits of plants can be enhanced in crop plants through selective breeding. Secondary plant metabolites can also act as an indirect defense against herbivorous insects. Plants can change the bouquet of chemicals, that they release after attack by an insect herbivore. These chemicals can signal the presence of an insect herbivore. And attract natural enemies of the herbivores, such as predators, or parasitoids. The chemical profile released by plants can differ by its species and type of herbivore damage it experiences. Recent research has shown that a single type of plant can release different compounds when fed upon by different insect herbivores. In these cases, the production of specific plant chemicals may even be induced by compounds in the insect's saliva. >> I'm a professor in forest health or forest entomology and pathology basically or chemical ecology at the University of Alberta Department of Renewable Resources What I do, I'm a chemical ecologist in training, especially the bark beetle colleges. I look at the chemical interactions between the plants and bark beetles in general in North America as well as in Europe and some part of my research involves in Asia. Studying any forest insects, but we mainly focus on the bark beetles, because they are going to make the important, ecologically important species throughout the world. They are the major tree killing insects pretty much everywhere in the world. 50% of my research would have focused on mountain pine beetle ] where I look at what are the key chemicals, host chemicals that drive mountain pine beetle-tree interactions. In this case I look at this chemical classes called monoterpenes. This is one of the classes of terpenoids, the chemicals. Recognition is the really, I think the most important part of evolution. If the host plants cannot recognize their enemy, they are in big trouble. That's exactly what's happening right now with lots of trees, exotic pathogens and insects are coming in killing North American plants. While, because not American plans don't recognize them as what it is, that's the reason they cannot react to it. If they do, they can produce the defensive compounds. Roles of chemicals really depending on their concentrations monoterpenes for example at the high concentrations can be highly toxic to the insects but the lower concentration day could be benefiting the insects. Bark beetles in general are associated with a number of symbiotic fungal organisms. In addition to the bacteria and yeasts and some nematodes, but we know much about the plant the fungal interaction in when we dealing with the bark beetles interaction is very interesting. Some of the chemicals that could be toxic to insects but may not be toxic to their symbiotic fungi or vice versa. Some of the compounds that we found, highly toxic to fungus but is not so much the bark beetles. It really varies by environment, it varies with age, varies between plant species and there are a lot of variations even within a plant species, within a species because of phenotypic differences, environmental differences, the differences in age. As well as some genetic differences that we have really very little understanding. Their exploitation is really depending on their evolutionary interactions, let's give an example for mountain pine beetle. Mountain pine beetle may use some of these compounds to produce pheromones one of the most abundant compound in pine for example. And they use a precursor, awful pine and precursors to female aggregation from trans-verbenol. So far we've covered that the differences which is present in the trees all the time. But induced to fast as usually happens when the plants are attacked by a bark beetle or defoliant there's or even the pathogens. There is some commonality among these defensive responses but there are also differences. When the tree is attacked by stem versus foliage, their responses will different because of they have evolved with the organisms that they recognize that actually they recognize the threat. Plan defenses stronger tied to their environment, one of the biggest environmental factor people studied, drought, drought is really the major limiting factor for plants. Especially, in places like Alberta because, we are really drought prone province. The several factors affect planting second is one of them is drought as I mentioned before. If the plants are stressed because there is limited-access to water, they have less defense of capacity. Why they are important? I will say two reasons, first, they attack very vital plant tissues in this case phloem. They carry the carbon from foliage to the remaining portion of stem and the roots. We have a xylem, xylems mainly deal with water transportation even there are good evidence that they may also transfer nutrients. So this is very vital, if they are cut off this flow, what's going to happen, right? You're cutting off the nutrient supply for keeping the trees green that's one factor. And the second thing is they are associated with this staining fungi. Is it really bark beetles are perfect tree killing insects? Absolutely, they are tree killing organisms but they don't achieve by themselves. They always associated with other organisms, in this case is symbiotic fungi. This what happens, when the bark beetle attack the trees, they feed on the phloem. They cut off it and at same time to introduce fungi into the phloem, the fungi grow from the xylem. When does happens they're cutting off both nutrient flow and water flow at the same time, and the tree gets basically, can defend a certain amount of time. But after that they cannot because there is no more resources. In order to produce defensive compounds, they have to access resources. I like mayfly, my favorite insect is really mayfly. It's nothing to do with the bark beetles. These are beautiful insects, they have aquatic larvae. Really vicious too, but the adults, some of the time usually they appear after rain in May. Millions of them if you're lucky, you can see just in a single day thousands of them, millions of them emerges. They're really beautiful, they were big white wings very beautiful tails. It'is like a millions of I don't know, it's like a wedding or the gowns that people wear. This is so beautiful everywhere you can see it and they don't live long just live on one or two days, and die short after, mayfly is my favorite insect. >> Strange though it might seem plants can also form symbiotic relationships with non herbivorous insects, to defend against herbivory. An example of this close cooperation between species occurs in specialized plants called myrmecophytes or ant plants. Myrmecophytes are plants that live in mutualistic association with ants. The plant often provides shelter for the ants in specialized cavities is called domatia. They can also provide ants with extra food sources in the form of proteinaceous growth called beltian bodies, as well as carbohydrate sources from nectar reservoirs. These are known as extrafloral nectaries because they exist outside of the floral structures of the plant. The plant also receives many benefits from its relationship with the ants. For instance, the ants can protect the plants from various insect herbivores and working in concert, can even attack large vertebrate herbivores. The plant also obtains extra nutrients from the decomposition of the ants and their waste. On or around the plant. Finally, the ants can help the plant with seed dispersal and I'm motivated to collect the seeds by the presence of nutritious structures on the outside of the seeds that the ants eat. The ants then carry these seeds into underground nests where they consume the nutritious structures and leave the seeds to sprout in their new environment. If you thought the ant plants were cool, wait until you hear about the mimosa plant. It is also called the sensitive plant and for good reason. This plant exhibits a behavioral defense mechanism is which the leaves fold rapidly in response to mechanical stimulation. By closing its leaves suddenly the plant can cause a large insect to fall off of the plant,or to be startled by the plants movement. The leaves also look like a less sizable meal when folded, and therefore it will be less appealing to herbivorous insects. Furthermore, sharp spines on the plant stem are exposed when the leaves are folded, which serve as a deterrence against large herbivores. While some other plants are capable of rapid movement such as the Venus flytrap, the mimosa plant may be one of the very few plants that uses rapid movement to defend against herbivory. That said there remains no conclusive evidence that this plant response evolved for this specific purpose. Plants produce an incredible range of defenses to protect themselves from herbivores. While plants may manipulate defenses in response to a single type of herbivore, multiple herbivores may feed on a single plant at the same time, making the potential plant insect interactions in extremely complex. Despite these defenses most insects still feed on plants. We'll look at how insects herbivores get around these defenses and exploit plant tissue by examining feeding guilds in the next lesson.
