Monday, October 31, 2011

Mummies invade Montana - Again!

(Halloween edition)

Montana is crawling with countless mummies, but not for Halloween. No, our mummy incursion takes place each summer, with all the makings of a low-budget horror/slasher movie.

When melting snow retreats to the mountains in spring, our native plants rise from the decay and set the stage for this invasion. It plays out on dogwoods and wild roses and fireweed flowers. But the scariest seat in the house has a front row view of a common thistle, because thistle is a favorite summer food for aphids.

Small healthy aphids and plump parasitized aphids, inspected by an ant.
Aphids - our tiny (1/32" to 1/8" long), native insects that suck the life fluids out of various host plants. They look sort of ghoulish themselves, but it's the aphids who are fated to become the victims of this gruesome plot.

Aphid populations increase quickly when summer days finally turn warm. A female aphid gives birth to 3-4 live young every day, without input from any males. (In fact, there aren't any males around until some are born in late summer.) Aphid numbers spike, and life looks good for a young female. Of course, this is always when strange things start to appear.

Around the edges of the colony, some of the small aphids suddenly swell up like balloons and start to change color. A week or so later, the plump aphids are dead and all that remains is empty exoskeletons, or what is known as "aphid mummies."

Countless aphids mysteriously turn into living mummies right before our eyes, as their bodies are slowly taken over by a parasitic creature. But by this time, we've already missed half the action. An inconspicuous stranger has already slipped in and out of the scene - a tiny, female wasp (Lysiphlebus testaceipes).

While our clueless aphid is happily slurping up plant juices, the little wasp sneaks up and injects a single egg into the aphid's abdomen. There are seldom any witnesses. The wasp egg hatches two days later, and a tiny grub begins happily slurping up aphid juices - from the inside out. The grub grows larger as it feeds, stretching and distorting the still-living aphid.

Aphid mummies (c) John AshleyThe parasitized aphid plumps up, stops reproducing, and stops secreting honeydew (the liquid that ants feed on). It turns a light tan color, and eventually dies 6-8 days after the infection.

When the wasp grub runs out of aphid juice, it cuts a small hole in the aphid's belly. It reaches out and anchors the aphid to a leaf or stalk, using silken threads. Then the grub spins a cocoon and pupates inside the aphid. Five days later, a new wasp emerges by slashing through its temporary host and flying away, leaving behind the crusty husk.

When about 20% of the aphids have been turned into empty husks, the aphid population begins to crash. By this time most of the living aphids have been parasitized - they just haven't been eaten yet.

The plot takes another twist in fall, when snow starts creeping back down from the mountains. When the outside temperature dips below about 60F, the wasp eggs delay their development. But the aphids remain active until the temperature falls below 40F, with some of the aphids overwintering as adults (in diapause, the insect equivalent of hibernation).

During Halloween, a single wasp egg silently hides inside of countless healthy aphids, waiting for the warm days of summer, ready to resume its gruesome incursion.

The re-runs start next June.

An ant searches through the empty husks of parasitized aphids, called "aphid mummies."