I observed some sort of asteraceous plant, went back later to measure it, and noticed aphids on the flower stem. The field “Is observation one of these special types of feeding?” can be set to no, pollination, scavenging, or parasitism. Parasitism is defined in the tooltip as “eating parts of an animal (e.g. blood)”. But there are plants that are parasitic on other plants, and aphids suck sap, which is the plant equivalent of blood. So is an aphid a parasite?
Yes, in general definitions, aphids are parasites on plants (i.e., they eat only part of the plant, and usually don’t kill their victim). However, it seems whoever set up that observation field wanted to restrict the use of the field to animal parasites. I’m also not sure in what way “pollination” is a type of feeding. You might search and find another field that is more useful for parasite observations – there are lots of them: https://www.inaturalist.org/observation_fields?q=parasite&commit=Search.
Yes. Aphids and all Hemiptera, including my speciality, true hoppers, are all plant parasites.
Generally there is no clear and simple definition of parasitism that includes everything that we call “parasite” and excludes everything that we don’t. And parasites on plants are treated differently to parasites on animals. It’s one of those “nature doesn’t fit our boxes” things.
An aphid is considered a parasite because it lives on its host; feeding weakens, but doesn’t kill the host (usually); its fitness depends on the host’s fitness.
I agree that the field is probably not suited to the aphid-observation, though. And it seems somewhat arbitrary to differentiate between pollination, scavenging, and parasitism.
Pollination is feeding if the pollinator gathers pollen or nectar for food, which bees do.
It seems artificial to treat sap-feeding as parasitism but leaf-chewing as predation—not that the latter is one of the options provided.
I would think virtually all plant feeding by insects is parasitism. (Unless there’s pollination happening or the plant is killed) We use the term “host plant” to refer to what they eat, indicating a parasite-host relationship. what defines parasitism (vs commensalism or mutualism) is that the host’s fitness is decreased by the parasite. So the question should be whether the insect’s feeding has an overall detrimental impact on the plant, without killing it. If the plant is killed, then it’s a case of parasitoidism or predation. If the plant isn’t impacted at all, then it’s commensalism. If the plant gets an overall benefit (as in pollination), then it’s mutualism. The manner of feeding (eating pollen, sucking sap, chewing on leaves, boring in stems, etc.) is not relevant to the classification of something as a parasite.
I agree with a lot of this including thinking about positive/negative effects and how that distinguishes parasitism from mutualism and commensalism (and ammensalism). However, if this
were the case, how would it be different to deer eating plant leaves or any other type of herbivory that the plant survives? Most people wouldn’t call this parasitism but predation.
I think the general key to differentiating parasitism vs. predation is that, while both have positive effects for the predator/parasite and negative effects for the prey/host, parasitism is generally considered to be a longer-term and somewhat specific relationship (often a type of symbiosis, though there’s debate about that too).
So a Japanese beetle that eats a variety of plants in my garden and moves around between them opportunistically would just be a predator, like the deer and rabbits. On the other hand, an aphid that generally spends a larger portion of its life cycle on one plant (even if it is capable of using multiple plants as hosts) is a parasite due to this longer term (from the perspective of the presumptive parasite) association.
From a professional, biological perspective, it’s this: Aphids are not parasites, but pests, causing significant damage to crops. They suck plant sap, partially digesting it, and excrete the remainder as honeydew. The amount of sap they consume is very large, reaching up to one-third of the insect’s weight per hour.
I don’t recall ever hearing herbivory being called predation or herbivores being considered to be predators. Isn’t predation very much an animal on animal interaction with a few exceptions such as fungi that prey on nematodes?
In my experience, there are two approaches to defining predation: a more narrow one in which the prey must die in the interaction (even in this case some types of herbivory are still included, like seed predation, eating a whole plant, etc.) and a broader one which is a negative consumptive interaction (in which case parasitism is a type of predation, but not all predation is parasitism).
I don’t think whether the difference between herbivory and predation is critical to the issue above though. One could swap in “herbivory” for “predation” as in
“So a Japanese beetle that eats a variety of plants in my garden and moves around between them opportunistically would just be an herbivore predator, like the deer and rabbits. On the other hand, an aphid that generally spends a larger portion of its life cycle on one plant (even if it is capable of using multiple plants as hosts) is a parasite due to this longer term (from the perspective of the presumptive parasite) association.”
and the distinction of parasitism still works.
Surely “pest” and “crop” are human-centred economic labels, rather than biological terms. Aphids are plant-parasites that are only classified as pests if they cause damage to plants that have economic value to humans.
This is very far from being true. Many hemipterans prey exclusively on other animals. This of course includes the Cimicidae (Bed Bugs), some of which prey mainly on humans. Also, a great many hempiterans are merely plant-feeders (or omnivorous), rather than being true plant parasites.
Fascinating topic. Got me thinking of lots of scenarios. Is the human practice of obtaining milk from cows, goats and other animals a form of parasitism?
No, it’s mutualism
Livestock numbers benefit from human management:
“Just the United States’ share of cattle matches the biomass of all wild mammals on Earth”
https://www.visualcapitalist.com/biomass-of-mammals/
Cape buffalo must be the biggest parasites on earth then, since they graze grass without killing the plants outright
Sorry. I were in a hurry. I forgot about assassin bugs etc. because I were busy with a guide.
@bradleyallf what do you think?
Crop, cultivar, and pest in the general sense are also biological and scientific terms, at least in Europe. Aphids can be considered parasites, but only in the broader sense of the word; in parasitological terminology, however, they are not.
I’m presuming from the discussion that eating leaves may not have been included because it’s a baseline norm, with the field put there with a view to capturing less frequent scenarios.