Gardening: What’s eating your aphids? Part 2 of 2

Aphids. The mere mention of these plant-sucking insects strikes fear into the hearts of gardeners and farmers alike.

Why are aphids so fear-inducing? During most of their population life cycle, aphids are all female, born pregnant and give birth to live offspring (clones of the mother aphid) that immediately start feeding, literally sucking the life out of plants.

I’ve personally lost garden and greenhouse plants to aphid attack. Aphids, though, are like the cows of the insect world, where some ants farm certain aphid species, and all predatory insects will eat aphids if they are available. These predatory insects can be termed “beneficial” because of their benefit to humans (follow @FieldHeroes on Twitter or visit fieldheroes.ca to find out more about beneficial insects).

In Saskatoon, the key aphid predators are ladybeetle adults and larvae, each of which can eat 50 to 120 aphids per day depending upon species, sex, size and larval stage. You might see several species of ladybeetle, but in the last few years the introduced seven-spotted ladybeetle has dominated. This species overwinters as adults, which is why you see them emerging en masse in the springtime.

Other predators you might find are the slug-like hoverfly larvae, whose adults are beneficial pollinators and try to make you think that they are bees or wasps with their wasp-like mimicry of their warning signals (yellow/black striping).

Also feasting on aphids are minute pirate bugs, so named because they are small (minute) and coloured black-and-white like a pirate’s Jolly Roger flag. These can pierce and suck the life out of 12 aphids per day.

The damsel bugs are like little praying mantises with their raptorial front legs, useful for grabbing aphids; they then pierce and sucks their aphid prey dry.

The most ferocious aphid predator, green lacewing larvae, are sometimes called “aphid lions” for their habit of running down aphids, throwing them above their heads and shaking them. Look for my video of this phenomenon on the @FieldHeroes website.

One special class of predator, the parasitic wasp, is rarely seen, but the aphid carnage they leave behind can be observed about 15 days later and for the rest of the season. The female parasitic wasps, or parasitoids, sting aphids and lay an egg inside them. The egg hatches into a voracious little larva that eats the insides of its aphid host.

When the wasp larva is ready to pupate (about eight days post sting) all that is left of the aphid is a swollen, often golden brown, husk called a ‘mummy’. Five days after the mummy forms, a fully formed, adult parasitic wasp chews a trap door in the back of the aphid mummy and crawls out (see video at Dr. TWist’s YouTube channel). Sometimes, depending on the time of year, the wasp might overwinter protected in the aphid mummy.

There are several species of parasitic wasps in our neighbourhood. One species, in a different genus from the wasps that make the golden mummies, makes white aphid mummies by having their larvae chew their way through the “belly” of the aphid and pulsate underneath the hollowed out aphid. If this reminds you of the Alien movie franchise, you’ll realize that often the best science fiction is not actually so fictional.

If all of these predators work in concert early enough in the aphid invasion, they can halt the population increase of aphids. The ladybeetle numbers this spring seemed to be lower that usual and many aphid populations escaped the controlling pressure of the predators and grew quite large. I suspect there were probably some bad overwintering days last winter for our local predators.

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