Insect adaptations to herbivory and plant defence mechanisms are driven by the coevolutionary arms race between the evolutions of offence (herbivore) and the evolution of defence (plants). A plant’s succession in defending against a herbivore will drive the evolution of a successful herbivore, which in turn will drive the evolution of new defences from the plant. This results in a broad spectrum of adaptations seen in insects (and plants). The incredibly diversity seen in insects illustrates the range of these adaptions, Insecta is in fact the most speciose class in the Animal KingdoM and the relationship between plants and insects is the most dominant biotic interaction (Samways, 1993).
Plants have many methods of defence. They can encourage herbivores to avoid them by producing chemical repellents, or through morphological changes to utilise mimicry. For example, Passiflora species have developed yellow spots on their leaves which causes the butterfly Helocunius to avoid laying eggs on the leaves as they appear to already be occupied by eggs (Williams & Gilbert, 1981). Plants have also developed morphological resistance, such as spines, hairs and sticky glands which immobilise or puncture insects, and chemical resistance to insects in producing toxins. The existence of these defence mechanisms and the abundance and success of herbivorous insects demonstrates the extent of the adaptations these insects need to thrive on plants, and therefore the incredible diversity of insect herbivores quite apparent (Bernays, 1998).
Aside from chemicals essential to plant function, plants also contain whats is known as secondary plant compounds and allelochemicals which are defensive in nature but originally may have simply been metabolic waste or had other metabolic functions (Gullan & Cranston, 2014). These chemicals are inevitably variable between plant species dependent on their environment or genetic differences. As mentioned previously these chemicals act an either of two ways, at a behavioural level or at a physiological level. The behavioural defence repulses insects, or impede oviposition and feeding preventing the insect from exploiting the plants resources. Physiologically, these chemicals may poison or reduce the nutritional contents of the food the insect is eating (Gullan & Cranston, 2014). Complications arise when the chemicals a plant produces repel one insect but attract another, such an interaction is caused by chemicals known an kairomones (Gullan & Cranston, 2014), and such insects that are attracted have developed adaptions to these chemicals either in the form of detoxification, toleration or even sequestrating them. A good example of this is the monarch butterfly, Danaus plexippus, which can and usually does oviposit on milkweed plants which are known to contain cardenolides, toxic cardiac glycosides (Gullan & Cranston, 2014). The monarch butterfly larvae can sequester these toxins in predator defence.
Insects must be able to select its host plant from an array of available species, and also ensure that its feeding activity coincides with the availability of its host plant. Phytophagous insects have adapted to identify their specific or generic host plant. Location can be achieved through olfactory cues which would be plant volatiles such as terpense in conifers, Pinophyta sp., or mustard oils in crucifers, Brassicaceae sp. (Meyer, 2016). These attractions can be primary plant compounds that a plant needs for growth and survival such as glucose, nucleotides and amino acids. Visual cues are slightly simpler, mostly consisting of shapes and colours that insects associate with food. For example, white water dishes attract aphids, Aphidoidea sp., and the adult apple maggots, Rhagoletis pomonella, are attracted to red spheres (Meyer, 2016).
As plants use many chemical and physical methods of deterring herbivores, many phytophagous insects have become very specialised to their choice of host plant. Very few insects are generalists that feed on many plant species (polyphagous), that produce broad spectrum enzymes capable of detoxifying and overcoming plant defences, because it is metabolically expensive to produce such enzymes (Meyer, 2016). However, such broad feeding capabilities would also mean there would be no shortage of food. Comparatively, monophagous insects are restricted to the choice of one host plant and therefore must adapt its physiology and behaviour to a single resource. Occupying the middle ground are oligophagous insects who will feed on closely related genera of plants, or members of a single taxonomic family.
Photophagous insects are also grouped by their specialised interactions for plant feeding, where adaptations have led to specialisations in feeding behaviour such as what part of the plant is eaten (leaves, fruit, flowers, stem, root, xylem etc.), where the insects feed (internally, externally), mode of feeding (biting, sucking), and the developmental stage at which the insect is found feeding on a plant (larvae, adult). These modes of feeding are described as feeding guilds, and of combinations of the four factors described above one paper found that there were, or should be, 24 guilds (Novotny, et al., 2010). However, for the purpose of simplicity I will discuss the five main feeding modes and the adaptations insects have developed to specialise in this way.
Defoliators, or leaf chewing insects, have a very visible effect on plant material possess biting and chewing mouthparts. The most common and diverse groups of insects of this feeding guild are the Lepidoptera, whose caterpillars feed on leaves, and Coleoptera (the largest group of insects) where both the adult and larvae eat leaves (Gullan & Cranston, 2014). However, other parts of the plants such as stems and roots are also eaten. Among the Acridae, grasshoppers, there are species who feed only on grasses and have highly specialised, chisel-like incisor regions of their mandibles that enable them to snip through tough parallel veins, and flattened, grooved molar regions for grinding tissue (Bernays, 1986). As leaf chewing insects, compared to gall inducing and plant mining and boring insects, are fairly exposed on the outside of the plant, many defoliators have adapted their behaviour to feed at night when fewer predatory birds and other visual predators are present. Further to this, many defoliators will feed on the underside of the leaf in attempt of predator avoidance. Being that the most common groups are beetles and caterpillars, these are also notoriously brightly coloured insects. This bright colouration communicates to potential predators that the insect is poisonous, known as aposematic colouration, which is an example of an insect sequestering the toxins of the plant into its own defences rather than synthesizing them independently. This is such an effect adaptation in predatory defence that non-poisonous insects have developed mimicry to appear poisonous. Alternatively, insects have evolved to take on the colour of their host plant or match their background, known as crypsis (Cloudsley-Thompson, 1981). A special example of this is seen in stick-insects, Phasmatodea, whose colouration and structure of the exoskeleton perfectly camouflage them as sticks and even leaves.
Plant mining and boring insects are a feeding guild that feed on the internal materials of living plants (Gullan & Cranston, 2014). They also possess biting and chewing mouthparts to process the solid plant material. The most common insects in this feeding guild tend to be insect larvae that reside inside the plant, living just below the protective layer (epidermis). Evidence of leaf mining activity is shown through damaged, blotched and blistered leaves, and different species will produce different, identifiable patterns of damage. Species such as the apple leaf miner, Lyonetia prunifoliella, produces blotch mines in the leaves of tree species such as apple, Malus pumila, and blackthorn, Prunus spinosa (Gullan & Cranston, 2014). The Dipetera species Chromatomyia primulae creates long, narrow, linear mines (Gullan & Cranston, 2014) which are white in colour on primrose (Primulaceae) and cowslip leaves, Primula veris.
Mining is not only restricted to leaves, and mining insects thrive in many parts of a plant including stems and fruit. Marmara species belonging to the order Lepidoptera have a range of mining habits, some mine beneath the skin of fruit, others mine under the surface of stems and even in the joints of cacti (Gullan & Cranston, 2014). This stem mining however is distinguishable from the deeper burrowing of stem boring, where the insect feeds on the deeper plant tissues. Boring feeding habits are separated into groups defined by the part of plant eaten, for example larvae that feed in wood, stalks, fruits, nuts, seeds and roots
Sap-sucking insects primarily consist of the true bugs, Hemiptera, who are characterised by their piercing, long, thread-like mouthparts. Sap-sucking insects withdraw cell contents, such as xylem or phloem contents from plant tissues (Gullan & Cranston, 2014). Different feeding sites are targeted by different species: the xylem by cicadas and spittle bugs; the phloem by leafhoppers, aphids, soft scales and mealybugs; and the parenchyma by many Heteroptera and some scale insects (Gullan & Cranston, 2014). Of the mouthparts only the mandibular and maxillary mouthparts containing the salivary canal and a food canal enter the tissues of the plant. To deal with this watery diet hemipterans have gut specialisations in that the anterior and posterior sections of the midgut form a filter chamber through having close contact, allowing water and small molecules to short circuit the main absorptive portion of the midgut, preventing dilution in this region (Gullan & Cranston, 2014) ….
Gall inducing insect species are mostly confined to a specific host plant. Galls are abnormal plant growths that in this case are produced in reaction to insect activity, though they can also be cause by other organisms such as bacteria, fungi, nematodes and mites. Galls are produced in reaction to salivary secretions or mechanical damage from feeding or egg laying activity which induces increased production of plan growth hormones (Wawrzynski, et al., 2005) which cause localised plant growth which can result in hypertrophy or hyperplasia. The insect that makes the gall develops inside the structure and as it grows the gall also increases in size. Aphids are probably the most recognisable of UK gall inducing insects, however adelgids are insects similar to aphids who have specialised to take full advantage of the plant gall. The eastern spruce gall adelgid is a key pest of Norway spruce and white spruce, completes its full life-cycle on one single host (Wawrzynski, et al., 2005). These adelgids can cause major damage to trees when their numbers are abundant as they form galls at the base of new shoots which can weaken stems, making more likely to breake under heavy snow (Wawrzynski, et al., 2005). Another interesting species of adelgid is the Cooley spruce gall adelgid, overwintering woolly aphid females lay eggs near the developing buds on spruce trees, who then hatch and eat the new growth thus inducing galls where they stay and develop from nymphs into winged insects (Cranshaw, 2013). These winged forms migrate to Douglas-fir trees where eggs are then laid on the needles, and some of these aphids develop wings and fly back to spruce to lay eggs to produce the overwintering forms of the species (Cranshaw, 2013). This is just one example of insects synchronising their life cycles to environmental cues in the same way as their host plant.
As plant seeds can contain much higher levels of nutrition that other plant material it is unsurprising that some insects have specialised to take advantage of this resource. Harvester ants are one such specialised species who have adapted their behaviour to store seeds in underground in what is essentially a granary (Gullan & Cranston, 2014). This adaptation overcomes the limitation of specialising on seed predation in that seeds are only produced seasonally, therefore not a reliable constant resource. This method of storing seeds is not completely detrimental to plants, as naturally many of the seeds that are stored will germinate and so the ants assist in seed dispersal (Gullan & Cranston, 2014).
Beetles have also adapted to utilise this valuable resource by ovipositing onto seeds, fruits and developing ovaries. The larvae can then burrow into the fruit or seed and develop inside, species such as the granary weevil Sitophilus granarius and rice weevil S. oryzae do this and their larvae develop inside the dry grains of rice, corn and wheat for example. Another species of weevil, Anliarhinus zamiae, has a very rare specialisation in feeding. The adult weevil attacks their cycad hosts’ gametophytes, which are defended by several layers of protective tissues and also by chemical defences (Gullan & Cranston, 2014). These weevils overcome such an obstacle by using the females rostrum to access the gametophytes by drilling a deep hole, the female then turns around to insert the ovipositor and lays eggs inside (Gullan & Cranston, 2014). This is an incredible adaptation as the females rostrum is believed to be the longest in relation to body size of any beetle (Gullan & Cranston, 2014).
Not all relationships between photophagous are negative, or at least negative for one party. Many plants have utilised the inevitability of an intimate relationship with insects for their own advantage, creating symbiotic or at least mutualistic relationships which benefit both parties involved. One example of this is pollination, whereby insects that collect nectar or pollen from a flower, which are both energy rich resources. Bees collect these for survival and energy requirements, but while acquiring pollen and nectar they aid in the reproduction of plants by transferring pollen, which contain male germ cells, from the anthers of the flower to the stigma, also in the flower. A pollen tube runs through the stigma to an ovule in the ovary, thus fertilising the egg. Insects pollinate most flowering plants and insect pollination is known as entomophily, which is more effective and efficient than anemophily, wind pollination. In this case it is beneficial for plants to have insect pollinators that are specialist of one or a few plant species (Gullan & Cranston, 2014). Bees have hairs which the pollen sticks to as they enter the flower so when a bee visits another flower the pollen is transferred onto the stigma (Canada Agriculture and Food Museum, 2015).
As mentioned above many ants are seed predators which on the face of it is very detrimental to plants as they invest so much through reproduction to produce seeds. However, some plants benefit from this behaviour in ants. Many plants have seeds that are inedible to ants because they are too hard, but possess elaiosomes, which are food bodies with chemical attractants that stimulate many ant species collect and disperse them – known as myrmecochory (Gullan & Cranston, 2014) (Vander Wall, et al., 2005).
In conclusion, insects have developed many adaptations to manage the ingestion and digestion of plant matter. This can be seen through adaptations in their morphology, such as the mandible specialisations in grasshopper species for cutting leaves; behaviour, seen in the synchronisation in the life cycle of adelgids to that of their host plants; physiology, such as that in the filter chamber in hemipterans to cope with such a watery diet. Feeding adaptations themselves manifest in other ways. By insects adapting to be specialists in certain host plant species, they occupy different ecological niches and therefore reducing the interspecific competition that generalists may experience.
These adaptations in insects are not limited to the act of feeding. In the arms race and coevolution between plants and photophagous insects, plants have developed methods of defence that insects must overcome to get to be able to feed on the plants. As of the nature of an arms race insects have responded to the plants’ defence mechanisms with a variety of adaptations. Plants produce many primary and secondary toxic compounds that function as repellents or poisons to insects. As an adaptative response to this, some insects are able sequester these toxins and use them as predator defence, or detoxify them so they are no longer harmful, or at the very least tolerate them so they can still feed on the plant despite the presence of these toxins.
Mutualism is another adaptive response to the conflict between phytophagous insects and plants, the lack of conflict arguably an adaptation by both the plants and insects to gain the resources and carry out necessary functions for survival. Such relationships are important in providing ecosystem services, such as pollination and seed dispersal, and therefore extremely beneficial to the health of the overall ecology of an ecosystem which obviously encompasses involved mutualists.