Fights and Deaths on a Leaf

Walk into the lab of Yutaka Saito in Hokkaido University, Japan, and you would likely find him glued to a microscope. A piece of bamboo leaf, green, square and smaller than a bottle cap, sits under the lens. On the leaf is a white silk patch under which specks appear to move. Squint your eyes and…there! The specks are moving.

Saito adjusts his lens. As the specks come into focus, they grow into eight-legged and hairy animals with the curves of a sesame seed. These are spider mites, squishy invertebrates in the class Arachnida. Other members of the crowded class include ticks, scorpions and (actual) spiders.

Saito gaze at each mite as it mates, feeds and defecates on the leaf. These are spider mites of the genus Stigmaeopsis. All eleven species of this genus build and live in silk nests on leaves.

Nearby, neat rows of Petri dishes cover a desk. In every dish are about a dozen leaflets just like the one under Saito’s lens. Every leaflet has a silk nest spun by spider mite females that Saito had gingerly placed there.

Maneuvering spider mites—each smaller than 1mm—with a paint brush isn’t quite the same as picking peanuts with chopsticks. Peanuts don’t run. Peanuts don’t get accidentally squashed by chopsticks, either.

Saito said the microscope makes it easy to pick up spider mites. But he also told me that he has been studying mites for 45 years.

TbambusaeGrpFeed

Typhlodromus bambusae males attacking a Stigmaeopsis female. [Credit: Yutaka Saito]

You may question how anybody could dedicate decades to such tiny and lowly creatures like mites. Why not lions, whales or even flies (“Cats!”, shouts the Internet)? Unknowingly, your question has just exposed a sad vacuum in your life—you have never watched spider mites live.

A leaf is a stage; a leaf with mites is drama; a leaf with mites and their enemies is epic. I had spent four summers in a lab with spider mite colonies. Whenever I sat and watched those mites, the clock does a Usain Bolt.

Spider mites never seem to rest. Spider mite females lay eggs daily. Soon, every mite mother is surrounded by tens of her children too weak to defend themselves. Earlier studies by Saito and other scientists found that Stigmaeopsis spider mites will fight and even kill predators that attack their children. Scientists suspect that Stigmaeopsis mites use silk to protect themselves, but nobody has shown it yet.

In a new study, Saito and YanXuan Zhang from Fujian Academy of Agricultural Sciences, China, found that Stigmaeopsis nanjingensis females use silk to keep other predatory mites out of their nests. Sadly for S. nanjingensis mites, silk isn’t always enough.

S. nanjingensis mites live on the undersides of bamboo leaves. A female starts her family life by weaving a silk cover over natural bents on the leaf to form a nest. She seals the nest and lays eggs inside. Over two weeks, each egg hatches into a larva that develops into nymph and then adult. S. nanjingensis sucks nutritious liquids from leaves. All the food they need flows through the floor of their nests.

Outside the nest however, danger lurks. Spider mite nymphs are like juicy grapes on vines for many predators, not least the predatory mite Typhlodromus bambusae. Unlike the plant-loving S. nanjingensis, T. bambusae prefers mite juice, particularly that of Stigmaeopsis mites.

When T. bambusae nymphs find a S. nanjingensis nest, they immediately force their way in. Most often they succeed. The standard silk nest itself cannot fend off T. bambusae. What happens after the T. bambusae nymph intrusion however, depends on the S. nanjingensis adults, according to Saito’s study.

Using each bamboo leaflet as a test arena, Saito set up scenarios between S. nanjingensis and their predator T. bambusae. Some scenarios had only S. nanjingensis males and eggs in the nests, while others had only females and eggs. Saito observed how prey and predator fought for their lives.

S. nanjingensis adults do not tolerate T. bambusae. Resident males in the nest attack the invading T. bambusae nymphs. Within four days, half of all T. bambusae nymphs were found crushed inside the nest, apparently killed by S. nanjingensis males.

SnanjingensisMaleKillsTbam

“Not on my watch!” A Stigmaeopsis nanjingensis male kills an invading Typhlodromus bambusae male. [Credit: Yutaka Saito]

If S. nanjingensis females were the only defenders, the invading T. bambusae nymphs lasted longer. But by the fifth day, more than two-thirds of T. bambusae nymphs were dead, their bodies scattered outside the nest. Apparently, S. nanjingensis females chased T. bambusae nymphs out of the nest, then sealed and thickened the nest wall with more silk. Unable to penetrate the reinforced nest, T. bambusae nymphs starved to death.

Females of T. bambusae are however, larger and stronger than their nymphs. When a T. bambusae female finds a S. nanjingensis nest, she rips it open—even reinforced ones—walks in and unleashes a feeding carnage. T. bambusae nymphs stream in through the punctured wall and join the feast. The invaders might evacuate a few defending S. nanjingensis females, but they would kill and suck dry all eggs and males.

In Saito’s study, each S. nanjingensis nest stood alone on a leaflet. In nature however, S. nanjingensis mites live in continuous nests that form a large colony. Ganging up in numbers, S. nanjingensis mites can mount a solid fight. If a T. bambusae female barges into a nest with several S. nanjingensis adults, she faces dogged resistance. The larger predator is often ousted, even killed.

By ganging up and counter-attacking T. bambusae, S. nanjingensis flips from prey to predator. Such role-reversal, exceedingly rare in nature, is also found in S. longus, another spider mite that Saito studies.

The fights between the mites matter not just to Saito, but also to farmers whose Moso bamboo forests in Fujian, China suffer outbreaks of S. nanjingensis. In Spring when S. nanjingensis numbers are low, they can be controlled by T. bambusae. But if S. nanjingensis levels manage to climb beyond a certain threshold, they can better cooperate and disrupt T. bambusae attacks. A starving T. bambusae population grows slowly, releasing S. nanjingensis numbers to explode into summer.

So, rare discoveries of life-and-death action with economic applications, all under a leaf—good enough reasons for you to start looking?

 

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