Host location is an important part of the feeding behavior of herbivorous insects. This process can be divided into several steps that start with habitat location. Once the habitat or patch is located, the insect needs to find an appropriate host plant within the patch and assess its quality before accepting the plant as a host. Host location and acceptance are achieved by the insects response to a variety of cues. Habitat and host location rely on response to cues that can be perceived before making contact with the plant. These include visual cues like color or chemical cues such as volatile floral sense or chemicals released from the plants foliage. The insect can also respond to cues when in contact with the plant, such as mechanical detection of the planet's surface or the taste of the plant. Humans can exploit insect host finding cues to control and monitor herbivorous insect pest populations. Many insect traps are designed to mimic specific plants structures and colors that are attractive to the target insects. Synthetic copies of chemicals derived from plants can be used to bait these traps. We will discuss the production and use of plant-based chemical insecticides and baits when we explore Integrated Pest Management, IPM, in a future module. But first, let's find out a bit more about insect herbivore feeding strategies. Unlike vertebrate herbivores, most insect herbivores have some degree of specialization in their diet. Feeding activity can be specialized on one or a few closely-related plants or extremely general if plants in many different families are used by a single species of insect herbivore. These feeding strategies are commonly termed monophagy, oligophagy and polyphagy. In herbivorous insects, monophagy is the most specific type of feeding strategy, in which only one or a few closely-related plant species generally within a single genus are fed on by the insect. Insect herbivores can specialize on one or a few plant species and even just a specific part of the plant because of the low growth requirements associated with their small body size. A single plant can provide ample food and habitat for many insects which enables them to complete their entire life cycle on one or a few host plants. Can you identify which of these vertebrate herbivores are specialists? Some species like pandas and koalas are able to specialize on bamboo and eucalyptus trees respectively because there are vast stands of the host plants available to them. Most other vertebrate herbivores feed on lots of different plant species to obtain enough nutrients to fuel their activities. The small body size and therefore low resource requirements of insects may be one of the main reasons why so many herbivorous insect species have specialized diets compared to vertebrates. A good example of a plant specialist is the monarch butterfly which feeds exclusively on milkweed plants in the larval stage. Host plant location and acceptance is crucial to the survival of specialist insect herbivores. As such, these insects often have elaborate host finding behaviors. Specialist insects face additional challenges in habitats with inconsistent conditions because their continued survival depends on the reliability of a single food source. Should a resource becomes scarce or absent, the herbivore must be able to survive on stored energy reserves throughout this difficult period. Another challenge faced by these insects is that predators can learn to find them based on their food source. Why then would diets specialization have evolved in the first place? The answer lies in the insect's biology. If an insect feeds on only a single food source, its physiology can be adopted for efficient location and digestion of a single type of food. The insects also invest less energy into the digestive process compared to insects that feed on a wider range of host plants because specialist insects will only need to produce enzymes for digesting a single food source. You can think of it as a type of minimalism. Because it is restricted to this single food source, its physiology is focused on finding, consuming, and digesting this single type of food. Another benefit associated with diet specialization has to do with defensive adaptations. An insect that has a long evolutionary history with its host plant can usually tolerate the defenses produced by that plant. Insects that overcome the effects of such defenses, then will be able to feed on the host plant. These insects specialists often use chemicals produced by plants as cues for host location and acceptance. Sometimes the insects can actually use the plants toxic chemicals to make themselves distasteful or even toxic to predators. We see this in the monarch butterfly caterpillars, which sequester toxins from milkweed host plants and retain the chemicals into adulthood. The striking coloration of monarch butterfly larvae and adults serves to warn predators of their distastefulness. This combination of aposematic coloration and distastefulness is so effective that some nontoxic insects mimic the coloration of distasteful insects to ward off predators without investing in chemical defense. This is known as mimicry. Mimicry can also evolve among several distasteful species which serves to reinforce the message to predators as we can see in the Heliconius butterflies. Plant chemicals can also be used by insects to synthesize compounds necessary for survival. Bark beetles for example, consume and use a tree's defensive chemicals to synthesize their own pheromones, which then attract other beetles for host colonization and mating. Insects that employ oligophagy feed on a broader range of plant hosts than monophagous species. Oligophagous insects feed on plants that are related. Usually, those classified within the same taxonomic family. The insects are able to do this because closely related plants often produce a similar range of defensive chemicals. Therefore, oligophagous insects only need to maintain a limited number of digestive enzymes to break down a limited array of host plant defenses produced by several related plant species. They can also use plant chemicals produced by host plants for host location and acceptance. A good example is the cabbage white butterfly, which feeds on a variety of plants from the family Brassicaceae, including cabbage, broccoli, and Brussels sprouts. Plants in this family rely on chemicals called glucosinolates for defense against generalist insect herbivores. The adult cabbage white butterflies however use the same defensive chemicals to identify and assess host plants. By gently drumming on the leaves, receptors on the butterflies legs detect minute amounts of glucosinolates. Female butterflies use this information to determine if the host plant is a suitable oviposition substrate on which its offspring can feed. The larvae of the cabbage white butterflies have adopted to detoxify the glucosinolates in the foliage of these host plants. Like monophagous insects, the survival of oligophagous insects depends on the availability and reliability of the appropriate resources in the environment. Although there are both advantages and disadvantages to specialize plant feeding, around 75 percent of herbivorous insects have evolved to be a monophagous or oligophagous. This suggests that the benefits associated with specialized diets outweigh the limitations. The final type of feeding strategy employed by insect herbivores is known as polyphagy. Polyphagous insect herbivores are generalists that feed on multiple plant taxa across different families. Locusts are good examples of polyphagous insects as they feed on a wide variety of vegetation. Generalist insect herbivores often maintain a wide geographic distribution because their survival does not depend on the presence of a single type of plant. The wide diet breadth of these insects allows them to exploit a home range more thoroughly and for greater periods of time than specialist insects. Polyphagous herbivorous tend to be more susceptible to plant defenses as they have not evolved detoxification mechanisms for every plant defense encountered in their polyphagous diet. As a result, only a minority of insect herbivores exhibit polyphagous tendencies. Nonetheless, the evolution of polyphagy may be favored if there is a large spacial or temporal variation in the distribution of host plants or if preferred host plants are rare. With so many insects feeding on plants, it is unsurprising that many plants have evolved broad spectrum defenses against these herbivores. These defenses can be physical or chemical and we will explore them in more detail in the next video.
