Anyone living and driving in the vicinity of Hermanus will have noticed the proliferation of Black Wattle (Acacia mearnsii) trees, especially in the Onrus River catchment at the entrance to the Hemel en Aarde Valley. Despite being a beautiful evergreen tree, they are definitely not welcome. They are nasty invasive water-sucking aliens from Australia. We can ill afford losing the water that these thirsty trees extract from a river system that is already under severe stress from too much water extraction, leaving precious little for the ecological health of the Onrus River Estuary. In the face of lower rainfall predicted in the Western Cape as a consequence of climate change, the popular Onrus “lagoon” is under threat.
Wouldn’t it be wonderful if we could release a ‘secret weapon’ against the wattles? Release the old bag forthwith, I say – by which I mean the wattle bagworm. But unfortunately this helpful insect is not able to be the solution – as I’ll explain.
Wattle bagworm – Photo: Flickr
The wattle bagworm (Kotochalia junodi) is a species of moth in the family Psychidae, which is indigenous to southern Africa. It evolved eating indigenous plants, but with the arrival of black wattles from Australia, they switched their diet with relish – so much so that it is now regarded as a pest of the black wattle.
However, the black wattle is grown commercially, largely as a source of vegetable tannin, and. growers would not look kindly upon bagworms infesting their wattle trees.
Bagworms are insects and, like all insects, they undergo metamorphosis, developing through several life forms – from egg to larva to pupa and then the adult moth form.
Wattle Bagworm. Photo: JungleDragon
Like all members of the family Psychidae, once hatched the larva spins a silken bag covered with materials such as thorns and twigs. The bag, thus adorned, provides protection for the growing larva against predators and is also excellent camouflage that matches the tree. As it feeds and grows, the larva extends the size of the bag. While it forages, it pokes its head out of the bag to eat voraciously, dragging its bag with it. It munches away until barely a leaf is left. If the infestation is heavy, bagworms can totally defoliate the host tree, so killing it.
Bagworms infesting a Cedar tree in Texas. Photo: Neil Sperry on FB
By late summer the larva is fully grown. It stops feeding, fastens its bag to the tree, spins an inner lining and pupates inside it. In early spring the male transforms into a winged moth.
The male moth does not feed after emerging, and lives for only a few days. But its near-transparent wings are strong, enabling it to seek out a mature female to inseminate. In contrast to the male, the female remains in her bag after emerging from the pupa.
She does not resemble a moth at all, but is worm-like, without wings, legs or even eyes. She is no more than a bag of eggs. She lies helpless, but not too helpless to turn around inside the bag to present her posterior to the open end of the bag to allow the male to inseminate her. The male inserts the point of his abdomen through the neck of the bag to mate with her.
After insemination, the female immediately starts laying her eggs – on average around 1600 eggs. Having accomplished her mission in life, the female starts to shrivel up, but remains in the bag with the developing eggs, which hatch in about two months.
This large clutch size reflects the fact that only a few of the eggs will reach adulthood and survive to reproduce.
If the female insect cannot fly to distribute her eggs far and wide, how do bagworms disperse from tree to tree? Bagworms have a cunning dispersal strategy. Once the tiny larvae (first instar) hatch, they crawl out of the bag. They don’t eat immediately, but first spin a gossamer thread to which they attach themselves. Then they rely on the wind to waft them away – much like certain spiderlings do. With luck they will land on a suitable edible plant. That’s easily accomplished in a whole plantation of yummy black wattles. If they don’t manage to catch the wind, they will eat their birth tree until it’s completely defoliated – resulting in the death of both bagworm and tree.
Bagworm pupae inside their bags. Photo: Shutterstock
Dead trees do not make foresters happy.
So however much we would like to see a host of bagworms hanging out on our local black wattles and helping to save our watercourses, this will remain but a pipe dream.
The Onrus River is in trouble
If you would like to find out more about the ecological status of the Onrus River from catchment to coast, you are invited to attend the first Overstrand Environment Conference on Thursday 16 February (a public holiday) in the Municipal auditorium. It starts at 08h00 and is entirely free of charge. Experts in the field will talk about various aspects and field questions from the audience.
You can also join the WC Wetland Forum on World Wetland Day, Saturday 18 February, for a guided, informational walk along the Onrus River starting from the estuary mouth at 0830 (for 09h00). Bring a picnic lunch.
If you would like to join this interesting walk, please respond to the following link: https://forms.gle/zxfReo1KLjeH1x9G9I