Insect ecology
Insect ecology is the interaction of insects, individually or as a community, with the surrounding environment or ecosystem. This interaction is mostly mediated by the secretion and detection of chemicals in the environment by insects. Semiochemicals are secreted by the organisms in the environment and they are detected by other organism such as insects. Semiochemicals used by organisms, including to interact with other organism either of the same species or different species can generally grouped into four. These are pheromone, synomones, allomone and kairomone. Pheromones are semiochemicals that facilitates interaction between organisms of same species. Synomones benefit both the producer and receiver, allomone is advantageous to only the producer whiles kairomones is beneficial to the receiver. Insect interact with other species within their community and these interaction include mutualism, commensalism, ammensalism, parasitism and neutralisms.
Insects play significant roles in the ecology of the world due to their vast diversity of form, function, and lifestyle. They are the major contributor to biodiversity in most habitats, except in the sea, they play a variety of important ecological roles in the many functions of an ecosystem. In the case of nutrient recycling, insects contribute to this vital function by degrading or consuming leaf litter, wood, carrion and dung, and by dispersal of fungi. Insects form an important part of the food chain, especially for entomophagous vertebrates such as many mammals, birds, amphibians, and reptiles. Insects play a critical role in maintaining community structure and composition; in the case of animals through diseases transmission, predation and parasitism, and in plants through phytophagy and plant propagation through pollination and seed dispersal. From an anthropocentric point of view, insects compete with humans; they consume as much as 10% of the food produced by man and infect one in six humans with a pathogen.
Community ecology
is the process by which a group of organisms which live in the same location interact. There are indirect interactions, such as reproduction, foraging patterns, and decaying. There are also direct interactions, which take the form of symbiosis, competition, and predation, which are the most easily notable. Every organism at its most basic state could be a consumer in some situations, and a producer in others. The culmination of all these interactions is what defines a community and what differentiates one from another. Insects often play numerous roles in these communities, although these roles vary widely based on what species is present. Insects recognize their host by means of their visual, olfactory, gustatory, and tactile cues.Decomposers
Decomposer insects are those that feed on dead or rotten bodies of plants or animals. These insects are called saprophages and fall into three main categories: those that feed on dead or dying plant matter, those that feed on dead animals, and those that feed on excrement of other animals. As dead plants are eaten away, more surface area is exposed, allowing the plants to decay faster due to an increase in microorganisms that eat the plant. These insects are largely responsible for helping to create a layer of humus on the soil that provides an ideal environment for various fungi and microorganisms. These organisms produce much of the nitrogen, carbon, and minerals that plants require for their growth. Carrion feeders include several beetles, ants, mites, wasps, fly larvae, and others. These insects occupy the dead body for a short time but rapidly consume and/or bury the carcass. Typically, some species of fly are the first to feed on the dead body, but the order of insects that follow is predictable and is known as the faunal succession. Many dung beetles and dung flies are attracted to the smell of animal feces. The adults often lay egg on fresh excrement and the larvae will feed on the organic matter. Many species of dung-feeders have evolved and only feed on feces from a specific species. There is even a species of dung-beetle that will roll feces into a ball, push it into a pre-dug hole, lays egg in the dung, and then covers it with fresh dirt to provide a perfect nursery for its larvae.Carnivores
Carnivorous insects survive by eating other living animals, be it through hunting, blood sucking, or as an internal parasite. These insects fall into three basic categories: predators, parasites, and parasitoids.Predatory insects are typically larger as their survival is dependent upon their ability to hunt, kill or immobilize, and eat their prey. However, there are several exceptions, with ants being the most notable. Ants, and other colony insects, can use their sheer numbers to overwhelm their prey even if the ants are significantly smaller. They often have specialized mandibles for this task, some causing excruciating pain, paralysis, or simply having a high bite force. Conversely, insects that live on their own must be able to reliably bring down their prey and as such have developed a myriad of unique hunting methods. Some actively travel, in search of prey, while others wait in ambush. Others may release chemicals to attract certain creatures, and others will eat anything they can.
Parasitic insects live on or within their hosts. The parasite causes the host some harm, but not enough to kill it. The presence of the parasite is often not noticed by the host, as the size discrepancy is typically so vast. Parasites vary widely in how they survive in or around their hosts; some complete their full life cycle within the body, such as the females of most Strepsiptera species, while others may only stay in for the duration of their larval stage. Kleptopasrasites obtain food by stealing it from their hosts. A kleptoparasite may opportunistically feed on prey that has been recently killed by a predator, such as many adult freeloader flies, or it may deceptively live in the host's nest, such as the majority of the ant crickets. There is as great of variation in methodology and species in parasites as in any other type of insect. The most threatening parasites to humans are ones that live outside the host and consume the host's blood. These species transmit viruses, disease, and even other, smaller parasites to the host, spreading these throughout the populations of many third world countries with poor health care.
A subcategory of parasites known as parasitoids. A parasitoid is an organism which develop on or in another organism, derives its nourishment from the host and eventually kills the host. In insects, a parasitoid is an insects whose larvae grows by feeding in or on another arthropod and eventually killing the host. The majority of parasitoids insects consume their victims as larvae, while the adults often feed on nectar or other organic material. One family of wasps, the spider wasps, will paralyze spiders before bringing them back to their nest and laying an egg on the spider's abdomen. Other parasitoid wasps, such as ichneumon and braconid wasps, lay their eggs on or directly inside of their hosts. Many of the adult female wasps have long ovipositors, which may be longer than the entire body length of the adult. Parasitoid beetles in the family Ripiphoridae attack various types of insects, as do most members of the large family of tachinid flies.
Herbivores
Out of all described eukaryotes almost one third are herbivorous insects, about 500,000. They feed on living plant matter or the products of a plant. They are also called phytophagous insects. These insects may eat essential parts of the plant, such as the leaves or sap, or they may survive on the pollen and nectar produced by the plant. These insects will compete with other organisms for limited plant host in an environment where there is constant change in plant availability and quality. Herbivorous insects often use olfactory or visual cues to determine a potential host plant. A visual cue could simply be the outline of a certain type of leaf, or the high contrast between the petals of a flower and the leaves surrounding it. These are typically associated with the olfactory signal an insect may receive from their intended meal. The olfactory cue could be the scent of the nectar produced by a flower, a certain chemical excreted to repel unwanted predators, or the exposed sap of a cherry tree. Either of these two senses could be the driving force behind an insect choosing to consume a certain plant, but it is only after it takes the first bite, and the confirmation of this food is made by its sense of taste, that it truly feeds. Since most insects depend on plants as their source of food, plants have developed some defensive mechanisms to protect themselves from insects. These mechanisms are largely grouped in two; antibiosis and antixenosis resistance. With antibiosis, the defensive trait of the plants affects the growth, survival and development of the insect but with the antixenosis defense, chemical and morphological factors affect the feeding behavior of plant. Plants have evolved to produce several secondary metabolic substance to protect themselves from herbivores insects. These chemicals are grouped into alkaloids, terpenoids and phenolics. Insects also have developed mechanism to detoxify these chemicals produced by their host plants. After a herbivorous insect is finished feeding on a plant, it will either wait there until hungry again, or move on to another task, be it finding more food, a mate, or shelter. Herbivorous insects bring significantly more danger to a plant than that of consumption; they are among the most prominent disease-carrying creatures in the insect world. There are numerous diseases, fungi, and parasites that can be carried by nearly any herbivorous insect, many of which fatal to the plant infected. Some diseases even produce a sweet smelling, sticky secretion from the infected plant to attract more insects and spread farther. In return plants have their own defenses. Some of these defenses are toxic secondary metabolites to deter insects. These toxins limit the diet breadth of herbivores, and evolving mechanisms to nonetheless continue herbivory is an important part of maintaining diet breadth in insects, and so in their evolutionary history as a whole. Both pleiotropy and epistasis have complex effects in this regard, with the simulations of Griswold 2006 showing that more genes provide the benefit of more targets for adaptive mutations, while Fisher 1930 showed that a mutation can improve one trait while epistasis causes it to also trigger negative effects - slowing down adaptation.Schoonhoven and associates, from Blaney et al 1985 to Schoonhoven et al 1992, illuminate the interplay between chemoreceptor stimuli in Lepidoptera and Orthoptera. They used Helicoverpa armigera, Spodoptera littoralis, S. frugiperda, Chloridea virescens, and grasshoppers. They find that most insects respond immediately and roughly equally to phagostimulant indicating good food and phagodeterrent indicating a food to be avoided, or a material which is not food substances. They also present some divergent examples, both delayed response suggesting that food decisions were mediated by cognition and not just simple chemoreception and unequal chemoreceptor stimulation with gustatory cells firing equally when presented with any material, but deterrent cells firing to a greater degree for undesirable materials. Both salicin and caffeine are antifeedants, and some of the Schoonhoven group's investigations test both the deterrence they produce and habituation to them. The Glendinning group has done some similar work. They find Manduca sextas habituation to salicin to be cognitively mediated because deterrent sensory cell stimulation barely decreases even when avoidance ceases. On the other hand Glendinning et al 1999 finds M. sexta habituation to caffeine to be due to change in chemoreceptor activation because it decreases significantly, and at the same time as cessation of feeding avoidance. The same work tests the cross-effects of habituation between the two chemicals, finding that they probably share a second messenger. For both phagostimulus and deterrence stimuli they find that the effects of multiple stimulations by multiple substances upon the same cells, simultaneously produce additive effects, up to the cell's firing rate ceiling.
Climate change is expected to change herbivory relationships. Liu et. al 2011 finds no change in distribution in one example, but instead the same herbivore switched primary hosts due to altered flowering time. Gillespie et al 2012 found host mismatch due to temperature shift.