Parasitism

Living in a Seattle high rise, I’ve been delighted at how well my balcony strawberries are growing. But we’re moving again, so I brought all my plants inside as a form of quarantine. There’s no sense bringing pests with me to a new location, is there? Especially when there is no balcony at the new location and all of my plants will be indoors. Good thing, too, because aphids had found my strawberries. How they found us on the 7th floor, I’ll never know.

Aphids on strawberry stems (Kate Russell)

Aphids are insidious sapsuckers and extremely prolific. I’ve been squishing them as soon as I see them, but there always seems to be more. While out walking my dogs, I kept an eye out for ladybugs, but no luck. What surprised me was to see that something was helping me kill off the aphids. I’m still not sure what it is.

What I can see are white aphid husks, dried-up little mummies. These are aphids drained of all their bodily fluids by parasites.

Aphid mummy on strawberry stem (Kate Russell)

Parasitism refers to one-sided relationships in which one organism lives on or in a host, inflicting harm. In his book The Meaning of Human Existence, Edward Wilson describes parasitism as “predators that eat prey in units of less than one.” Parasitism isn’t the same thing as the benign hitchhikers of phoresy. Parasites are like vampires. They cause damage. In the case of my strawberries, the aphids are bad parasites. Whatever is killing the aphids are good parasites. Once again, it depends on how you look at things.

Types of parasitism

Ectoparasites occur on the outside of their hosts. Endoparasites are found inside. Parasites don’t kill their host outright, with parasitoids being the exception, but they sure can make your plants less productive. The difference between parasites and parasitoids is that parasitoids kill their hosts while parasites just keep draining theirs. Parasites can be insects, fungi, nematodes, or bacteria.

Insect parasites

Most sap-sucking insects qualify as parasites. This large group includes aphids, cicadas, leafhoppers, mealybugs, planthoppers, psyllids, scale insects, squash bugs, stink bugs, tarnished plant bugs, and whiteflies. As these pests suck sap from your beloved garden plants, they may also spread diseases, making them pathogens and well as parasites.

At the same time, several parasitoid insects kill our plant pests. Braconid wasps, chalcid wasps, Goniozus navel orangeworm wasps, ichneumon wasps, and tachinid flies are all happy to suck the life from more bothersome visitors.

Rust on rose leaves (Kate Russell)

Leaf spot on oak leaf (Ian Alexander) CC BY-SA 4.0

Fungal parasites

It is estimated that fungi cause 85% of all plant diseases. Parasitic fungi attack living organisms. Their more peaceable relatives, the saprotrophic fungi, feed on dead and decaying plant materials. Parasitic fungi are responsible for apple scab, black spot, blights, brown rot, downy mildews, powdery mildew, rusts, smuts, and wilts. These microscopic predators travel on the wind, landing on damp leaves and stems. Each spore produces a germ cell that burrows into the plant and starts feeding and reproducing. On the other hand, some fungal parasites prefer feeding on scale insects.

Soybean cyst nematode and egg (Agricultural Research Service) Public Domain

Nematodes as parasites

Nematodes cause an estimated $100 billion in economic losses each year. Some nematodes are root grazers, but others are parasites. Parasitic nematodes burrow into root systems to access a continuous food supply. These pests can also carry diseases. Or, they may be beneficial nematodes that feed on cutworms and corn earworm larvae. I know, it gets confusing.

Plant cell containing multiple Rhizobacteria (Louisa Howard, Dartmouth) Public Domain

Bacterial parasites

Splashing water from rain, sprinklers, and irrigation is how bacteria get around. Luckily, most bacterial parasites are beneficial to their plant hosts. These are the nitrogen-fixing Rhizobacteria of the legume world. Rhizobacteria live in and around the roots, converting atmospheric nitrogen into a form usable by plants, making them part of the Nitrogen Cycle. These beneficial parasites also help plants fight diseases.

​Then again, there are bacterial parasites that cause problems. These are known as deleterious rhizobacteria (DRB). DRBs can reduce beet crops by 48%.

Parasitic plants

The mistletoe of holiday fame is a partial parasite (hemiparasite). It gets part of its food from its host. Dodder gets all its nutrients from other plants, making it a holoparasite. Did you know that some parasitic plants track down their victims by scent? There are about 4,500 parasitic plants in the world. The worst culprits, broomrapes and witchweeds, may be lurking in your garden right now. These parasitic plants cost billions of dollars in crop losses each year. We’ll talk about them more another day.

​Parasites make up more than half of Earth’s biodiversity. Like them or not, parasites serve important functions. Without them, evolutionary change would be reduced, and we would all be more vulnerable.

​​And I’m grateful to whatever is parasitizing the aphids in my strawberry pots.

Dodder on an acacia tree (Khalid Mahmood) CC BY-SA 4.0