Lynda Hallinan: Where there's a willow, there's a wayward swarm of scavenging aphids

The giant willow aphid is wreaking havoc on New Zealand farms and riverbanks.
Anne Boswell/Stuff

The giant willow aphid is wreaking havoc on New Zealand farms and riverbanks.

OPINION: It has all the makings of a dystopian horror plot: a savage, silent army of lecherous nymphomaniacs has infiltrated Godzone and is spreading a black plague across our land.

You may not have noticed it yet but there's an ill wind in our willows. Riverbank and roadside willows increasingly look like wildfire victims, their branches sullied with soot. It's not an arsonist but an aphid that's got them into this sticky mess.

Tuberolachnus salignus is no ordinary aphid. Commonly known as the giant willow aphid, this sap-sucking leviathan of a louse grows up to 6mm long, about four times the size of the common greenfly that sucks the sap from spring rosebuds. It's a warrior princess in a Halloween costume, with a thorn-like dorsal tubercle and a pair of devil horns on its back. When squashed, it bleeds what looks like blood.

A sugar-addicted aphid is to blame for the black mood of our nation's weeping willows.
Lynda Hallinan

A sugar-addicted aphid is to blame for the black mood of our nation's weeping willows.

To say this little pest is on a sugar high is an understatement. It taps into the flow of willow sap, mainlining glucose while raining sticky droplets of honeydew – that's a euphemism for sugary  crap – on to everything below. This honeydew in turn attracts wasps, ants and the tell-tale charcoal fungus known as black sooty mould. (Scale insects inflict a similar indignity upon citrus trees.)

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These sweet and sooty byproducts cause headaches for fruit growers and farmers, contaminating the wool of sheep who seek willow shade and fruit crops grown in orchards surrounded by willow shelter belts. The aphids are also starting to jump ship, with breeding colonies sighted in apple and pear plantations.

The National Beekeepers Association is also concerned about the aphids' effect on honey quality and hive health. When bees harvest honeydew, their honey takes on sour flavours and is prone to crystallisation, while stunted, stressed willows are shy to flower, robbing bees of a key source of early spring fodder.

No one knows exactly when or how the giant willow aphid breached our border security, but these illegal immigrants were first identified by entomologists two days before Christmas in 2013, in willows at Auckland's Western Springs Park. Any hope of a quick containment was quickly quashed when spot checks revealed they'd already colonised trees from Northland to Central Otago.

How did they spread so quickly? Well, this is no country for old men. Lady aphids don't need lovers: they are parthenogenetic, reproducing asexually. After  a winter lull, they start actively breeding and feeding in spring and, because each adult female can spawn between 30-70 live young, their numbers increase exponentially, producing vast populations of overlapping generations by summer's end.

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It's now believed they've been building their sleeper cells in New Zealand for at least a decade, but they only made it to my neck of the woods last year, turning the weeping willows in our wetland from golden umbrellas to gothic hunchbacks.

The New Zealand Poplar and Willow Research Trust says the effect of the aphid has been "devastating". A fortnight ago, on a road trip up north where the aphid population isn't checked by frost, I was shocked to see so many blackened trees.

Good riddance, say native enthusiasts and puritanical environmentalists who view willows as invasive wetland weeds. But that's a simplistic view, for willows are also high-country heroes, stabilising eroded hillsides and holding river banks together in times of flood.

"The willow which bends to the tempest," wrote the German theologian Albert Schweitzer, "often escapes better than the oak which resists it. And so, in great calamities, it sometimes happens that light and frivolous spirits recover their elasticity and presence of mind sooner than those of a loftier character."

So what can be done for those sad trees whose frivolous spirit is being sucked dry? Not a great deal. Insecticide bands around tree trunks are said to slow the progress of wingless aphids legging it back up their hosts in spring, while coppiced willows can be drenched with systemic sprays. I also know of some Auckland gardeners trialling the use of drones to spray the tops of mature trees, but that's hardly practical for most people.

The willow aphid's only natural predator in New Zealand is the ladybird. However, in a happy (or suspicious) coincidence, another illegal alien, the Asian harlequin ladybird, Harmonia axyridis, found here for the first time in 2016, is proving to have quite an appetite for them.

Whose army will gain supremacy? Who can say? In the meantime, they've mobbed my willows, and they're coming for yours next.

 - Stuff

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