Insects and plants have co-evolved over millions of years and now need each other for food, seed dispersal, pollination, defence- the list goes on. But Insects and plants are also part of an ‘evolutionary arms race‘. As one species diversifies to take advantage of another or escape a harmful effect, the other species develops counter-adaptations. Insects have adapted to their environment by co-evolution and mutualisms, but which ones are benefiting overall and how are they exploiting their ecosystem?
Sequestering Plant Toxins
Plant allelochemicals can reduce food intake by insects (as a repellent or deterrent), reduce the food utilization efficiency or simply interfere with an insect’s metabolism (e.g. poisoning). Insects have not only evolved the ability to detoxify, conjugate, rapidly excrete or avoid toxins, some can even sequester the chemicals themselves after consumption. For instance, monarch butterflies sequester the cardiac glycosides from their host plant (milkweed) in their tissues as a means of defending themselves from attack by predators. The cinnabar moth too has become sepcialized to feed on ragwort, which contains pyrrolizidine alkaloids which the adult moth can sequester from toxins they obtained as a larvae to use as a defence. Depending on their metabolism, predators find their meal of monarch butterflies or cinnabar moths toxic or distasteful, and avoid eating that prey item again, thus protecting that phenotype.
Using Plant Signals
Plant allelochemicals can also be a volatile signal specific to the plant that makes it easier for an insect to find it’s host plant (for food, oviposition, etc.). The pine beetle (Ips pini) orients towards its host pine trees using volatile terpene compounds, which the plant intended for use in defence against herbivores. Indirect induced defences of a plant can also send signals to insects that their prey are present, like wild tobacco which emits methyl jasmonate from its leaf tissues when attacked. These compounds act as a distress signal: predators and parasitoids of the herbivores of tobacco use the volatile chemicals to locate their prey. The pest itself, the tobacco hornworm (Manduca sexta), can also use this to avoid ovipositing on their host plants which are emitting this signal. Insects also take advantage of the visual signals that plants emit by blending in with foliage (crypsis) or by resembling an inanimate object (such as a twig, as in the stick insect) or mimicking a plant (e.g. the orchid mantis).
Nectivarous insects have taken advantage of nectar-producing plants for millions of years, and this mutualism has resulted in dual benefits- plants want efficient pollinators, and pollinators want competition-free access to flowers with abundant and rich resources. Plants also attract pollinators with rewards such as pollen, oils (for nest building and feeding to larvae), scent compounds (pheromones for mate attraction) and floral tissues (beetles consume them). Some ‘cheating’ pollinators (that maybe haven’t evolved the length of tongue required) take advantage of this mutualism by biting a hole through the flower’s corolla at the nectary, without contacting the anthers/stigma to pollinate the flower- these are called ‘nectar robbers’. Some pollinators might even use the fruit or plant as a nest- the fig wasps for instance lay their eggs into the fruiting bodies of the fig, and it’s long ovipositor contacts the pollen on the way.
Ants and Plants
Ants have a long evolutionary relationship with plants, and perhaps the most notable is in the Atta genus, where ‘leafcutter ants’ bring leaf material back to their colony to feed vast gardens of fungus. These fungi in return produce hyphae called gongolydia which the ants feed to their larvae, giving them a diet rich in carbohydrates, sterols and amino acids. In another mutualism ants provide protection in the form of domantia -ant houses- which are often modified plant structures such as acacia thorns. The ants keep and rear their larvae inside the domatia, and the acacia tree is rewarded with fewer herbivores on it and improved seed sets. Ants can also be bodyguards for butterfly caterpillars, which provide the ants with sugary rewards from specialized nectaries. Ant often ‘herd’ homopterans such as aphids, and the ants protect aphids from attack by other ants or parasitoids and the aphids in return provide the ant with sugary frass produced by aphids as they feed on plant sap.