Of ants and aphids (and some hoverflies too)

blog picby Aditi Mishra – I picked up a bad habit this year. I steal from the ants.

Yes, the ants. It’s not like I have fun stealing, but unfortunately I have to.

You see, I need aphids, a.k.a. ‘ant-cows’, to rear hoverflies, i.e. insects of the family Syrphidae. Hoverfly larvae are aphid-guzzling machines. And who can give us better aphids than aphid-herding ants?

In case you are wondering what aphids are – they are many species of sap-sucking arthropods with amazing reproductive capabilities. A single aphid can give rise to 600 billion descendants in one season.

That is 6 followed by 11 zeros.

Going forth and multiplying

So how do they do it? Aphids owe their astonishing ability to increase in numbers to three things.

First, asexual reproduction – aphids need no man. Aphid moms are strong, independent mothers indulging in parthenogenesis, i.e they make babies without mates.

Second, viviparity – aphids give birth to their young ones. This reduces a lot of risk since aphids don’t have to lay eggs in unpredictable environments just to see them become some other insect’s lunch. Plus there is no waiting. Aphid kids pop out ready to reproduce.

Third, paedogenesis – since aphids can reproduce asexually, the young start reproducing very early – sometimes as early as in her mother’s womb! So an aphid female can be pregnant with her daughter and granddaughters that her daughter made.

So aphids are the Matryoshka dolls of the insect world. Simple regular arthropods – that are born pregnant and reproduce asexually.

Aphids often suck up too much plant sap and swell up like balloons, oozing sugary secretions from their pores. This is exactly what makes them attractive to ants. Ants have domesticated aphids to harvest these sugary secretions.

Ant is my shepherd

The aphid husbandry skills of any ant would put humans to shame.

Ants release semiochemicals which impede aphids’ ability to develop wings and make them sluggish. This domesticates aphids, making them docile and devoid of anti-predatory instincts. If an aphid develops wings even after all that, then the ants bite their wings off.

But that’s not all.

Like any good farmer would, ants protect aphid eggs from harsh winters. They do this to start a new aphid colony next spring.  Additionally, ants actively manage the resource allocation for their herds. Like shepherds herd sheep to greener pastures, ants take the aphids to a new plants if the old site becomes overcrowded.

What do ants get out of all this? Honeydew. Honeydew is the extra sugar ingested by aphids that they secrete out. Ants milk aphids by stroking them with their antennae which forces the liquid out. In some cases, the aphids have lost all ability to excrete so they must depend on their ant masters to be milked.

But ants are not the only ones eyeing this honey pot.

A plethora of insects from flies, bugs and beetles like to lay eggs in aphid colonies and munch on them. After all, who doesn’t like insects tasting like jelly beans?

Syrphids’ choice*

Hunting sugary, sluggish aphids must be one of the easiest hunts in the entire animal kingdom. Additionally, some larvae (for example the hoverflies of the Syrphinae family) accumulate aphid carcasses in their hindgut to serve as cryptic color patterning.

Hence aphids are one of the best bounty a hoverfly mother could guarantee for her unborn larvae. However, it comes at a great risk to her.

Laying eggs next to an aphid colony patrolled by ants is dangerous. Ants are really protective of their herd, frequently attacking predators like ladybugs. They even locate and destroy ladybird larvae and eggs. Certain species of ants attack hoverflies as well.

However, the good news is that there are aphid colonies without ants as well. The bad news is that these colonies have their own issues.

Without ants, the dispersal abilities of aphids gets pretty limited and overcrowding a huge concern. While aphids can develop wings, these wings are not of much use anyway. At best all the aphids can hope for is to catch a breeze and drift away.

This opens new complications for hoverfly mothers everywhere. As colony collapses become more common, gravid females must know how to predict the future of an aphid colony.

Matching up to the challenge, syrphid mothers plan well.

Researchers have found that a hoverfly looking for a site to lay eggs would visually inspect the vegetation on which the aphid colony is before she even goes anywhere near them. She favors healthy green foliage in her survey.

Subsequently, she looks and smells the aphids’ colony to determine if they are worthy of her eggs. Finally, she tastes the honeydew, savoring its amino acids and sugar to decide whether to stop or move on.

The hoverfly’s judgment is more nuanced than one would expect. Females prefer younger aphid colonies over denser ones. In fact they often avoid plants heavily infested with aphids. Scientists speculate that this behavior protects them from choosing overcrowded aphid colonies with imminent crashes.

Avoiding extremely dense colonies has other benefits too. Scientists have found that aphid colonies with the lowest number of founding members grow the fastest.

Smart Syrphid mothers save the day.

blog pic 2

Aphid freedom comes with responsibility

While lack of ants sort of inconveniences hoverflies, it’s a much harsher reality for the aphids themselves.

Domesticity has to become a thing of the past, now that defense and dispersal can no longer be outsourced to the ants. Some aphids adapt to this by producing soldier aphids – aphids whose sole job is to defend.

Individuals become more aggressive as well.

Aphids are kickboxers by nature – they dig their hind legs into their aggressors. Now that the ants are gone, aphids must take up this old hobby once again. On a less aggressive note, the aphids grow spines making them unpalatable, and they can exude waxy fluids that literally seal their predators’ mouths shut.

Cool defense.

So it turns out aphids do fight back and ants might be guilty of slavery.

I don’t feel so bad about stealing from the ants after all.


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