Use of the Parasitic Wasp for Greenhouse Aphid Control
The parasitic wasp is a natural method of aphid control. Green, black and white fly all cause tremendous damage to plants in our gardens and greenhouses
As more and more of us shy away from insecticides due to the possible health risks involved in working with such chemical substances, natural means have had to be found.
The parasitic wasp lays her egg inside an aphid, which as it grows and matures, kills the host aphid.
When fully adult, the parasitic wasps eats its way out of the abdomen of the host aphid, leaving a gaping hole behind.
The aphid remains are known as an aphid 'mummy'.
Parasitic wasps are harmless to humans and animals, and unlike aphids, do not damage greenhouse plants and fruit.
There are over 60,000 species of parasitic wasps, and all belong to the order Hymenoptera which also includes bees and ants.
They are generally tiny little flies 1/1000th to 3/4th of an inch long.
Their color, markings, wing and body shape relate to their species type, but all tend to have long filiform type antennae and the females have a pointed egg-laying structure (called an ovipositor) at the tip of the abdomen.
A really fascinating fact about Hymenoptera wasps is their ability to determine the sex of their offspring.
An unfertilized egg always produces a female, while the male comes from a fertilised egg. In this way, female parasitic wasps can completely control their rate of growth.
In a situation where there are huge numbers of aphids, more female eggs will be laid as their food supply is ensured.
Female parasitic wasps seek out aphids in which to lay eggs.
She uses her ovipositor to pierce the outer shell of the aphid to lay her egg or eggs (again depending on type).
Within a few days the aphid turns a yellowy-brownish color and bloats due to the developing wasp inside, and dies.
You can easily spot a parasitized aphid because of its color and shape.
You may not even be aware that parasitic wasps are working among your infested plants until you see this.
This is another good reason not to use insecticides on your plants which will kill these beneficial predators too.
Greenfly, meanwhile, reproduce at an alarming rate.
The female greenfly can clone herself, producing 3 – 5 offspring per day for about 25-30 days.
She does not mate with the male of the species at all until autumn when the temperature drops to below 40 C.
Then she will produce male offspring from which to mate from, before hibernating for the winter only to start the whole reproductive cycle off again in the spring.
Parasitized greenfly stop reproducing within 1 – 5 days, so you can see just how quickly a mating pair of parasitic wasps can decimate a colony on green fly, black fly or white fly.
The egg hatches in about 2 days, and approximately 10 to 14 days after the egg was laid, the fully adult parasitic wasp eats a hole through its hosts belly and emerges.
Female wasps will immediately mate and then seek a host in which to lay eggs and the whole cycle starts over.
Parasitic wasps only live for perhaps 4 or 5 days, but an adult female can parasitize 100 aphids in that time.
They are not true parasites in that parasites live off their victim in a symbiotic relationship.
They are parasitoids which kill their victim.
The only downside of using parasitic wasps for natural aphid control is that the green fly continues breeding in temperatures down to 40F, while the parasitic wasp stops when the temperature falls below 56F.
This is quite a variation in temperature and can allow an aphid population to exert a lot of damage to crops before the temperature falls too low for them to operate.
70F is considered to be the best operating temperature for parasitic wasps.
Each species of parasitic wasp is predatory to only one type of aphid.
They have been likened smart bombs because of they way they home in on a particular type of aphid and annihilate them.
If you are planning on buying some parasitic wasps for use as aphid control, you will need to know the type of insect that is troublesome to you.
Greenfly, whitefly and black fly themselves have multiple sub-species. For example, the cabbage greenfly is not the same insect as the rose greenfly. Each have a preference for a specific type of plant.
You can now buy parasitic wasps for your greenhouse aphid control.
Introduce them when you have signs of an aphid attack and watch your parasitic wasps take care of the problem before it gets out of hand and kills your plants.
In summer, leave the doors and windows of your greenhouse open for ventilation.
Green flies can travel quite a distance, and should any choose to escape during the 2 days after the egg has been implanted, you can relax in the knowledge that your garden plants will now be offered a level of protection against decimation by aphids thanks to the emerging adult parasitic wasp that is inside the greenfly.
For example, the white fly's natural predator is a parasitic wasp called Encarsia formosa.
Encarsia formosa Gahan is used commercially world-wide specifically for the control of whitefly.
The females are 0.6mm is length, with a bright yellow or orange abdomen and black head and thorax.
Males are dark in color and rarely seen.
Encarsia formosa attacks at least 8 genera of whitefly, and at least 15 different species, making it an ideal insect to introduce in greenhouses where repeated whitefly infestations are a problem.
The female encarsia lays her egg directly onto the whitefly which stays white and continues developing until the egg matures to the pupae stage, where the white fly will turn black.
10 days later the adult encarsia emerges, and the presence of the black scales among your whitefly scales denotes a successful introduction of the parasitic wasp.
Adult encarsia females lay between 10 and 15 eggs per day, providing she is given enough heat and light. 60F- 70F is considered an optimum temperature. As the wasp ages, she lays less eggs.
Encarsia formosa not only lay eggs on live white fly, they also consume them in huge quantities too.
However, a massive 50% of white fly will not be harmed by the encarsia wasp. Scientists do not know whether that is because those whitefly managed to avoid attack by defensive movements, or whether it is because their wasp predator chooses to keep the population alive as a form of self-preservation for themselves. Food for the future, if you like.
It can be safely said that the introduction of encarsia formosa will keep you aphid population under control, even if it doesn't eradicate them.
Parasitic wasp in dad's greenhouse
My father introduced encarsia formosa into his greenhouse 5 year ago, after a particularly severe attack of whitefly.
At the end of the season, the parasitic wasps seemed to have failed, and many plants died completely covered in a whitefly blanket.
The following year, the whitefly population was still high, so he chose to 'rest' the greenhouse for a season. All plants were removed and nothing was grown at all.
The next year again, there was no sign of infestation and his greenhouse was completely clear of aphids.
The following year, at the first sign of aphids (greenfly this time) he planted French marigold as companion plants to help protect his tomato plants, and this worked to a degree as the aphids attacked the marigolds with fervour, and mostly left the tomato plants alone.
He overwintered the infested marigolds. Still the aphids did not spread to surrounding plants. However he did notice some aphid mummies, and suspected a parasitic wasp was at work.
He cut the top off one of the heavily invested marigold plants and encased it with an old plastic food cover. A few days later the casing full of tiny wasps.
All last year his greenhouse remained largely aphid free, except for those attacking the marigolds. Even when the infested marigold plants were in physical contact with the lower leaves of the tomato plants, the tomato plants remained clear of greenfly, except for one of two on the lower leaves only.
Another view of the parasitic wasp (click to enlarge)
His beautiful Iresine plant in the house suffered a severe attack of green fly, so he moved the whole pot into the greenhouse for a few days. ALL the aphids were killed in this short period.
If anyone can recognise this variety of parasitic wasp, please let me know its name.
We do not know where it came from. There are no other greenhouses in the immediate area for them to have escaped from.
Are they growing wild here locally? Are they indigenous to the West Coast of Scotland? We would like to know. We can safely say that they are very productive and hard working, and effective.
On yellow flypaper, several varieties of flies were caught in the greenhouse. Among them, in large numbers, is a fly my father called "the grey fly". He does not know what it is, nor whether it is also another friendly predator.
Considering that the parasitic wasp only tends to kill off 50% of a greenfly infestation, it is interesting to note that the greenhouse is largely aphid-free except for those on the marigold (calendula) plants.
Could this grey fly also be devouring greenfly?
Does anyone recognise it from this not-so-clear heavily magnified photo?
Grey fly - note the Starship Enterprise shaped bow?
On a final note, the decrease of aphids and the presence of both the parasitic wasp and the 'grey fly' has coincided with a new and continued leaf miner attack.
If anyone knows why there should be any relation between the two, I'd be delighted to hear about it.