A Harmful Symbiosis: Parasitism in the Insect World

Parasites disturb and terrify many people. The idea of something living inside of or biting into us, in order to (sometimes unknowingly) steal resources like blood or nutrients, can send shivers down the spine. It doesn’t help that parasites are some of the best known vectors of disease, whether it be mosquitoes spreading malaria or ticks spreading lyme disease. Suffice it to say, it’s very likely that most people would say they don’t want to be parasitized, and some may even go so far as to support attempts to drive human parasites to extinction. Unfortunately for both us and insects, however, the insect world is full of parasitic organisms, some of which parasitize humans, and others of which parasitize other insects. Insect parasitism is simply widespread and diverse, so much so that understanding it is critical to understanding insect relations as a whole.

The ever-famous mosquito parasitizing human skin. (Source: Bryan Reynolds 2015)

There are actually two distinct types of parasitism exhibited by insects: standard parasitism, and parasitoidism. The general distinction is that standard parasites do not directly kill their hosts (although they can indirectly via spread of pathogens), whereas the end result for parasitoids is pretty much always host death. Standard parasites are the type that people are most intimately familiar with, thanks to the widespread range of mosquitoes and the high death toll of the diseases they transmit, but they’re not even close to the only standard parasite within class insecta. In fact, both humans and insects can be parasitized by insects, and there’s three major methods different parasites can use to accomplish their goals. The first is through occasional outward feeding, which is the method employed by the aforementioned mosquito (order diptera, family culicidae). Second is through permanent residence on the host, which is enacted by the fleas (order siphonaptera) that can commonly bother our pets, or lice (order psocodea) that can commonly bother us. Third is through permanent (or semi-permanent) residence within the host, which is carried out by the human botfly (Dermatobia hominis, order diptera, family oestridae), which lives inside human tissue in its larval (maggot) phase, and stylops (order strepsiptera, family stylopidae), an insect whose female spends her entire life cycle inside other insects. They’re usually smaller than their host, and they have a variety of tools for accomplishing their parasitic tasks, ranging from hooks to claws to piercing suckers, and a sucker (with or without piercing) mouthpart is the most common tool used in feeding, however, this is not always the case (Burkett-Cadena, 2019). Any life stage can be parasitic, with some cases of the entire life cycle being parasitic (female stylops), but others only part of it (human botfly larvae) (Australian Museum, 2022).

The second form is parasitoidism, and thankfully for us, parasitoids do not target humans. They almost exclusively target other invertebrates, and like standard parasites, these are usually smaller than their hosts. Unlike standard parasites, however, only the young actually do the parasitizing, with the adults usually acting as nectar feeders or outright predators, and only females are involved in actually finding and using the host. They’re also highly selective, usually only targeting one or several closely related species, and sometimes even limiting to a specific life stage. As was mentioned before, parasitoids directly kill their hosts, but this usually doesn’t happen quickly. Rather, the young parasitoid grows by feeding on the host’s fluids or organs until it’s fully grown, oftentimes preventing their host from developing so they can continue eating, and the host doesn’t get to die until the parasitoid is fully developed and/or its lost all its fluid or organ function. In order to reach this point, the parasitoid must get a host, and similar to standard parasites, it has three methods of doing this, all via egg laying. Eggs can either be laid within, on, or near host eggs or young, or the same for an adult host that’s been stung and paralyzed, or on a host’s preferred plant species (Australian Museum, 2022), with an ovipositor often being used if the egg in these methods needs to be inserted. The most famous groups of parasitoids are wasps (order hymenoptera), which parasitize a number of prey including aphids, scale insects, whitefly, caterpillars, and eggs (Parasites of Insect Pests, 2020), but there’s also species of parasitoid flies (order diptera, family tachinidae) and beetles (order coleoptera). (Australian Museum, 2022).

A parasitoid wasp attacking a caterpillar. (Source: Scott Bauer/USDA 2006)

It can be hard to see a bright side to all the terrifying information in the previous paragraphs, but nearly everything inherent in a natural system serves a purpose, and parasitism is no different. Parasitoids are incredibly effective at keeping insect herbivore and pest populations in check, so much so that some plants actively attract parasitoids when under attack and that humans use parasitoids to control pests of economically valuable crops. For their part, parasites can help control pest animal populations via disease spread, with one example being mosquitoes and fleas reducing introduced European rabbit numbers in Australia. Finally, both parasitoids and parasites are part of the food webs of their ecosystems, and contribute to nutrient and energy cycling by being eaten, killing, and defecating (Australian Museum, 2022). Therefore, even though this small facet of nature can be terrifying to think about, and may harm the unfortunate victim, it nonetheless offers important, often critical contributions to both natural systems and to our human economy.

Works Cited

Burkett-Cadena, N. D. (2019). Morphological Adaptations of Parasitic Arthropods. Medical and Veterinary Entomology, 17–22. https://doi.org/10.1016/b978-0-12-814043-7.00002-9

New South Wales Government. (2022, April 6). Predators, parasites and parasitoids. The Australian Museum. Retrieved May 27, 2022, from https://australian.museum/learn/animals/insects/predators-parasites-and-parasitoids/#:~:text=Some%20well%20known%20parasites%20are,entire%20lifecycle%20within%20the%20host. 

University of California Agriculture & Natural Resources. (2020, June 8). Parasites of Insect Pests. Statewide Integrated Pest Management Program. Retrieved May 27, 2022, from http://ipm.ucanr.edu/QT/parasitesinsectcard.html#:~:text=A%20parasite%20is%20an%20organism,just%20as%20they%20reach%20maturity.