With so many herbivorous insect species, you may wonder what can a plant do to deter insects 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 oviposition as they make plants surfaces difficult to access. The defence provided by trichomes is not only physical though, as many of them have glands that produce or store chemical deterrence 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 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 and herbivore attacks. This defence 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 plant, 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 moult. Trees, especially coniferous trees, have a defence 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 defence. 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 and open wound. Plant resistance that deters insect feeding is called non-preference because the defence mechanism reduces the insect's preference for using a particular plant without impacting the insect's metabolism. While non-preference resistance can include chemical defences, it is often associated with physical defences. Chemical defences 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 oviposition activities or interrupt digestive processes in insects. Plant resistance that operates through metabolic disruption is referred to as antibiosis. Many secondary plant metabolites form 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 defence 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 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. We spoke with our resident expert on plant chemical defences, Dr. Nadir Erbilgin. He describes the ways plants deal with insect attacks and how plant defensive chemistry can have far-reaching impacts on the natural world around us. I'm a professor in Forest Health or Forest Entomology and Pathology basically, or Chemically Ecology at the University of Alberta, Department of Renewable Resources. What I do, I'm a Chemical Ecologist in training, especially a bark beetle ecologist. 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. I study forest insects, but mainly I focus on the bark beetles because of they are really ecologically important species throughout the world. They are the major tree killing insects pretty much everywhere in the world. Fifty percent of my research really focuses on mountain pine beetles, where I look at what are the key chemicals, host chemicals that drive mountain pine beetle tree interactions. In this case, I look at these chemical classes called monoterpenes this is one of the classes of terpenoid chemicals. Recognition is the really I think the most important part of evolution. If the host plants cannot recognize their enemy, they're in big trouble. That's exactly what's happening right now with loss of trees, exotic pathogens and insect are coming in killing North American plants, why? Because North American plants don't recognize them, what it is. That's the reason they cannot react to it. If they do, they can produce defensive compounds. Roles of chemicals really depend on their concentrations. Monoterpenes, for example, at high concentrations can be highly toxic to insects, but at lower concentration, they 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 when we're dealing with bark beetles. Interaction is very interesting. Some of the chemicals that could be toxic to the insects, but may not be toxic to the symbiotic fungi or vice versa some of the compounds that we found, highly toxic due to the fungus, but is not so much to the bark beetles. It's really varies by environment, varies with age, varies with the plant species and there are a lot of variations even within a plant species. Within a species because of phenotypic differences, environmental differences, differences in age as well as some genetic differences. We have really very little understanding. Their exploitation is really depending on their evolutionary interactions, Let's given example for mountain pine beetle. Mountain pine beetle may use some of these compounds to produce pheromones. Alpha-pinene is one of the most abundant compounds in lodgepole pine, for example and they use a precursor, an alphapinene precursor, one of the female's aggregation pheromone transverbenol. So far we've covered the constitutive defences, which is present in the trees all the time, but induced defences usually happen when the plants are attacked by bark beetles or defoliators or even pathogens. There is some commonality among these defensive responses, but there are also differences when the trees are attacked by stem versus foliage, their responses are different because of they have evolved with organisms that- they recognize it actually they recognize the threat. Plant response, plant defences are strongly tied to the environment. One of the biggest environmental factors people started is drought. Drought is really the major limiting factor for plants, especially in places like Alberta because we are really drought-prone province. Several factors affect plant-insect interactions, one of them is drought as I mentioned before. If the plants are stressed because of limited access to water, they have less defensive capacity. Why they are important, I will say two reasons. First, they attack the very vital plant tissues, in this case, the phloem. They carry the carbon from foliage to the remaining portion of stem and roots. We have a xylem. Xylem mainly deals with water transportation, even though they is good evidence that they may also transfer nutrients. So this is very vital. If you cut off this phloem, 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. So really are bark beetles perfect tree-killing insects? Absolutely tree-killing organisms, but they don't achieve it by themselves. They're always associated with other organisms. In these cases, symbiotic fungi. This what happens, when the bark beetle attack the trees, they feed on the phloem, they cut off it. At the same time, they introduce fungi into the phloem, the fungi grow from the xylem. What happens, they're cutting off both nutrient flow and water flow at the same time and the trees basically, can defend that for a certain amount of time, but after that, they cannot because there's no more resources. In order to produce defensive compounds, they have to access resources. I like mayflies, my favourite insects is really mayflies, nothing to do with bark beetles but really beautiful insects. They have aquatic larvae, really vicious too. But the adults, in summertime usually they appear after rain in May, millions of them if you are lucky, you can see in just a single day, thousands of them, millions of them emerge. They are really beautiful, they have big white wings, very beautiful tails it's like a wedding or the gowns that people wear. It's so beautiful everywhere you can see, and they don't live long just live one or two days, and they die short after. Mayfly is my favourite insect. Strange though it might seem, plants can even form symbiotic relationships with non-herbivorous insects, to defend against herbivory. An example of this close cooperation between species occurs in ant-plants or myrmecophytes. These specialized plants have a mutualistic association with ants. The plant often provides shelter for the ants in specialized cavities 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 wastes on or around the plant. Finally, the ants can help the plant with seed dispersal, and are 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 behavioural defence mechanism in 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 be startled by the plants movement. The leaves also look like a less sizeable 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 deterrent 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 defences to protect themselves from herbivores. While plants may manipulate defences 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 extremely complex. Despite these defences, most insects still feed on plants. We'll look at how insect herbivores get around these defences and exploit plant tissue by examining feeding guilds in the next lesson.
