Module 6 - Plant Feeding and Impact of Herbivory

Question 1

Compare and contract specific and diffuse co-evolution and resource tracking

Answer 1

Approximately half of all insect species exhibit PHYTOPHAGY, or plant-feeding.
- There is a great diversity of diet and feeding behaviours among these herbivorous insects; both insects and plants involved have evolved adaptions

Coevolution: The process that occurs when the interaction of 2 species exert selective pressures on each other over evolutionary time, resulting in reciprocal adaptations

Specific Coevolution: When coevolution occurs between only 2, closely interacting species
- the adaptation of one organism drives evolutionary change in the species with which it interacts, which in turn drivers further adaptations in each of the species
- continuous selective pressure often results in an “evolutionary arms race” in which each species continuously evolves traits to counter those of the other species; they may try to outcompete the other or may diverge traits to minimize competition
- MUTUALISM: the 2 species have become so highly coevolved that they seem to have evolved perfectly to each other; each species ha evolved adaptations to gain some benefit from the other, with the end result that both species benefit

DIFFUSE COEVOLUTION: Organisms generally experience pressures from many organisms in the enviro, coevolution occurs among a group of interacting species in a process known as diffuse coevolution
- occurs when reciprocal adaptations arise among multiple interacting species, which may be closely or distantly phylogenetically related
- perpetual evolutionary arms race is less common outcome
- can occur among host plants and herbivores, or among plants and the associated insect pollinators

Question 2

Ecological Fitting

Answer 2

Interactions among organisms in a novel habitat due to pre adaptations
- the insect has not truly coevolved with the organisms in the novel habitat, and does not have a shared evolutionary history with the novel hosts or enemies, but has adaptations from a prior enviro that allow it to exploit the new habitat or host
- ex. Host switching of insect herbivores, in which insects may consume a novel host if the insects are introduced to a novel enviro or if a novel plant species is introduced in the insects’ rang
- Once a species has expanded into a novel range or developed novel interactions with other organisms through ecological fitting, it may coevolve with those organisms. Coevolution can result in adaptations that lead to reproductive isolation from other populations of the same species = host race formation, and can lead to speciation

Question 3

Distinguish the costs and benefits of plant-feeding specificity in insects

Answer 3

1. Monophagy: the most specific feeding strategy, these insects consume only one or a few closely related host species, and generally use only a few host plants within the same genus

Benefit: the insects only need adaptations for a small set of host defences; reduces the energy requirements and complexity of physiology required for digestion
- Ex. Monarch caterpillars feed only on a few species of milkweeds. They have evolved to sequester the toxins produced by the plants. The orange and black colouration of adult monarchs is called APOSEMATIC COLOURATION and serves as a warning of their distastefulness to potential predators. Other butterflies have evolved similar colouration and patterns, known as MIMICRY.

Drawbacks: Constraint on host location in complex landscapes. Scarcity of the host can be detrimental to the specialist herbivores that depend on it, although specialist insects are generally highly attuned to locate hosts. Another drawback is that specialists may face greater pressure from predators or parasitoids who may use the same chemical cues provided by the plants to locate them.

2. Oligophagy: Insects that consume several related host species, generally within the same taxonomic family.
   - Plants within a given family produce similar defences, so the physiology of oligophagous insects can still be specialized for a limited set of defenses = they must maintain only a limited set of adaptations to plant defences such as digestive ezymes to break down the limited array of chemicals
   - another similarity to monophagous insects is that oligophagous insects can use the chemical defenses produced bythe host plants for host location and acceptance

Drawbacks
- insects much locate a particular type of host plant and may be located by predators or parasitoids using the defensive chemistry of the host plant

*advantages outweigh disadvantages as 75% of insects are monophagous or oligophagous

3. Polyphagy: the most broad diet breadth of herbivorous insects; they are generalists that consume plants across multiple taxonomic families
   - must maintain a large suite of enzymes to detoxify and digest a range of host plants, but still exhibit preferences for some plant species/families
   - expansion of particular sets of genes (ex. associated with detoxification) are required for these insects to maintain their broad diet
   - can detect and assess plants based on chemical cues like monophagous and oligophagous insects
   - less common for polyphages to use chemical cues as attractants to hosplants
   - will accept many plants as hosts but must still find hosts in the enviro so they have a suite of host-finding adaptations and behaviours; the adaptations required to digest this broad range of hosts may be a limitation that explains why only 25% of herbivorous insects are polyphagous

Question 4

Discuss plant defense mechanisms against insect herbivory

Answer 4

1. Physical Defences: morphological traits that prevent herbivore access or exploitation of the plant
   - defences may be CONSTITUTIVE or INDUCED

Constitutive defences: produced regardless of the presence or absence of threats like herbivores or pathogens. Often include defences like TRICHOMES or wax and some chemical defences.
- Trichomes: a common physical defence, small hairs on the leaves or stems; prevent small insects from accessing the plant surface = physical barrier to herbivory and oviposition. Both a physical and chemical defence bc there are associated glands to release chemical deterrents.
- Resin: sticky substance is a viscous liquid that can be used to physically push insects out as they attempt to bore in

Induced defences: produced as a result of the presence of herbivores or pathogens. Can include structural defences like silicia, but many are chemical.
- increased lignin and cellulose content make leaves tough and difficult to consume
- minerals like silica (found in grass) to provide a physical barrier; tough and abrasive and wears down the mandibles of insects that feed on them; it is also indigestible, damaging the gut

Non-Preference defences: traits that deter insect feeding by reducing the insect’s preference for the host without impacting digestion. Includes chemical defences that deter insect herbivores without impacting metabolism.
- tricomes
- waxy secretions
- spines, prickles, thorns

2. Chemical Defences: compounds which are not used for growth or reproduction, called SECONDARY PLANT METABOLITES
   - these chemicals may deter insect herbivores through multiple mechanisms, but may disrupt digestion and metabolic activities, which is called ANTIBIOSIS
   - numerous, diverse, abundant
   - employ a range of secondary metabolites to deter a variety of herbivores
   - Glucosinolates are a well-recognized class of secondary plant metabolites; released when tissues are disrupted
   - Phenolics are abundant and diverse chemicals; serve in defence against herbivores, pathogens, and competitors, and as phytohormones to induce other defences
   - Terpenes are another class of chemical defences produced by many plants, particularly conifers; toxic to herbivores and pathogens, and also serve as a deterrent; can be exploited by herbivores for host recognition
   - Humans can manipulate secondary plant metabolites- ex. developed as insecticides due to their toxicity to insects
   - some volatile plant metabolites fxn in indirect defence, signalling the presence of herbivores to the associated predators or parasitoids; plants detect the type of herbivore based on compounds in the insect’s saliva, which allows for signal specificity
3. Mutualistic Relationships: plants can form symbiotic relationships with insects, and these include mutualistic associations in which the plant is defended by its insect symbionts
   Occurs in MYRMECOPHYTES (ant-plants)
   - specialized to form mutualistic relationships with some species of ants
   - ants benefit from shelter produced by the plants as cavities called DOMATIA, as well as from food; plant benefits as well, the ants attack herbivores that attempt to consume it , waste of decomposed ants acts as a nutrient source
4. Behavioural Defences: behaviours exhibited by plants to reduce herbivory
   - “sensitive plants” which respond to pysical stimuli by folding leaflets together
   - burst of a seedpod
   - visual mimicry

Question 5

Identify different feeding guilds of herbivorous insects, their specialized adaptions and the associated plant damage

Answer 5

1. Defoliators: these insects feed on the foliage of plants with biting and chewing mouthparts, removing entire portions of the foliage
   - Damage: defoliation may occur in small, discrete patches however it is more common for the entire leaf to be consumed
   - most trees can tolerate even complete defoliation; tolerance responses include increased photosynthetic rate in the remaining leaves, or the rapid regrowth of new leaves within the growing season; plants can even produce chemicals that allow them to recover nutrients from defoliator waste, as FRASS and portions of leaves dropped can contain important nutrients (ex. N)
2. Leafminers: insects that live and feed within leaves
   - convergently evolved a similar, dorsoventrally flattened body plan and chewing mouthparts that are projected forward
   - the pattern of the mine produced as well as the trail of frass within the mine can be diagnostic of the insect that produced the mine
   - limited amount of tissue within the leaves for leafminers to feed on = insects that exclusively mine leaf tissue consume less biomass than other herbivorous insects, so tend to be small
   - other insects are instars as early instars
3. Seed and Fruit Feeders: plant seeds provide the nutrition for seedling growth, and so plant fruits and seeds are highly nutritious; some insects take advantage of this and feed on plant repro structures like fruits, seeds, nuts, and pollen
   - FRUGIVORES: fruit feeders; major pests of agricultural products; females will oviposit on the surface of developing fruti
   - GRANIVORES: seed feeders; often kills the seeds, so granivorous insects are sometimes called seed predators; plants have several strategies to reduce this risk such as thick seed coats embedded with toxic chemicals, abort immature seeds, sychronized seed production and release
4. Sap Feeders: insects that feed on fluids within a plant, or the sap
   - The long, thin SYTLETS that comprise hemipteran mouthparts are highly modified mandibles and maxillae; probe into and around plant cells to pierce the phloem and xylem
   - Salivary channel in the stylets allows hemipterans to inject salivary secretions which aid in sap-feeding by either breaking down plant cellular components or forming a protective sheath around the stylets
   - most sap feeding insects specialize on particular tissues; aphids target the phloem, cicadas target the xylem
   - FILTER CHAMBER: an adaptation of the gut of sap-feeders, allows the removal of the excess water and sugar, while simultaneously concentrating the less abundant nutrients for digestion
   - can weaken plants as they lost the nutritive fluids necessary for normal growth and fxn
   - important vectors of plant diseases, as they offer a direct route for pathogen entry into plant tissues
5. Root Feeders: can cause significant damage and result in deficiencies in aboveground plant growth; a life history strategy that is most common in larval coleopterans and dipterans
   - feeding may occur externally, or the larvae can burrow within the roots to feed internally
   - some root-feeders consume mycorrhizae, the fungal symbionts which live in a mutualistic association with many plants, which further reduces the nutrient acquisition of the host plant
   - Infection by opportunistic pathogens (ex. fungi) can also result
   - Larval beetles tend to use biting-chewing mouthparts to consume roots, while fly larvae generally have hook-like mouthparts to scrape away the tissues
6. Stem Feeders: insects that feed on stems, vascular tissues, and wood
   - can damage the structural integrity of the plant, and can limit the transportation of nutrients due to damage to the vascular tissue
   - chewing mouthparts
7. Galls: insects may also feed within a structure called a gall; galls are a type of plant growth that is stimulated by a foreign organism, like insect eggs or larvae or viruses, bacteria, fungi, nematodes, and mites
   - can be induced on any part of the plant including the leaves, stems, roots and fruit
   - exact morphology is highly variable, but always involves an increase in the number or size of cells
   - gall provides both a habitat and food for the insect
   - gall formation and maintenance controlled by the insect
   - mechanical damage and saliva from feeding can initiate gall formation in growing plant tissues, while continuous stimulation of the plant tissues by insect damage and secretions is required to maintain the gall and promote gall growth. Because of this, gall growth will cease when the insect leaves or dies.
   - different types of mouthparts used to feed within the galls; biting-chewing mouthparts, piercing-sucking mouthparts
   - some galling insects are pests of agriculture, forestry or horticulture; while others can be highly valuable as biological control agents of weeds
   - galling insects may have mutualistic relationships (ex. fig wasps with fig trees)

Question 6

Assess the impact of insect herbivores on agriculture, forestry, and horticulture

Answer 6

1. Herbivorous Pests
   - herbivorous insects are important in ecosystems but they can cause damage to plants that are of economic significance to humans = considered pests.
   - major economic consequences
   - billions spent yearly on controls to minimize crop losses
   - invasive herbivorous insects pose a particular challenge, as the spread of INVASIVE SPECIES is aided by many factors including modern technologies, global transportation, and climate change; not transported with their native enemies lacking the control of predators, parasitoids, or pathogens that native herbivores face; ecological fitting allowing them to rapidly colonize
   - Human activities can contribute to pestiferous nature of native and invasive insects. Ex. monocultures are common in managed ecosystems as they are easy to plant and harvest and provide large quantities. As there is low diversity they are vulnerable to insect herbivores which are often capable of rapidly evolving adaptations to circumvent human control measures and plant defences to utilize the large resource pool
2. Weed Control
   - some herbivorous insects can be used as biological control agents for weeds
   - ex. Klamathweed beetle used as a biological control agent to control the populations of Saint John’s wort