These tiny soft pear shaped insects are fast becoming the pest of the year. Apart from the odd black fly infestation on the globe artichokes, aphids haven’t really posed much of problem for us in the past. This year, however, aphids are out in force , there are colonies of them spread throughout all the plots on the entire allotment and they are on everything including nettles.
Indications of their arrival started a few of weeks ago, we saw ants marching up and down legume stems, leaves and pea pods started to curl and contort, yellow flecks emerged on the parsnips foliage and we noticed shiny, sticky leaves on brambles and nettles. Aaarrrggghhh!
Aphids on the the underside of a bean leaf.
White Fly on nettles
Aphids can arrive as early as the spring, but typically reach peak numbers in the summer, just as most crops are growing and flourishing. Aphids come in a variety of colours, green, black, brown and white, some winged, some without wings. There are a staggering 4,000 + species of aphids, many of which are monophagous, they feed only from one plant species. But there are also some species of aphids, such as the green peach aphid, that feed on various different types of plant species. Each species has it’s own life cycle, too many to mention, but the lifespan of most aphid species is reasonably short, just 20 to 40 days. Aphids certainly make up for their short lifespan by something they all definitely have in common, they are all prolific breeders. Aphid numbers can double every three to four days. This poses a major problem for gardeners and farmers alike
Aphids are a destructive pest, piercing and then sucking sap from a plant using two sharp long stylets which are enclosed in a sheath called a labium. Aphids extract sap from the phloem vessels of the plant. This is the part of the plant that transports soluble organic material, in particular sucrose, made during photosynthesis. Essentially the aphids are tapping directly into the plants energy source. Aphids need nitrogen, but the phloem juices have a high sugar content. The aphid therefore has to consume a huge amount of phloem juice to get sufficient nutrition. Excess sugar is excreted by the aphid on the surface of the plant, leaving a sweet sticky residue called honeydew.
There are a number of issues with aphids feeding on plants like this, aphids transmit plant viruses and the sticky honeydew they excrete can attract sooty moulds. Additionally, large numbers of aphids can weaken the plant by depleting the plants food supply and can stunt growth when feeding on the plants tender, new growing shoots and leaves.
Given the large number of aphid species, there are many variations of the reproduction cycle but the following diagram, which I’ve shamefully stolen from Biomedcentral’s website, sums up the general reproduction cycle pretty well.
Aphids breed both sexually and asexually. In the autumn the female aphid lays eggs on a tree or a shrub. These eggs have a very thick shell that can withstand extreme temperatures. Aphids overwinter in egg form. In March, wingless female nymphs hatch from these eggs. These nymphs feed on the host plant that they’ve hatched on. When they reach adulthood they start to reproduce without the need for mating. This process is called parthenogenesis. The aphid produces live young, all of which are females, smaller but identical replicas of the mother aphid and astonishingly these young aphids are already pregnant at birth. After a week or so the young female aphids are mature enough to start reproducing parthenogenetically themselves. Aphids can produce 5 to 10 live young a day over a period of 30 days. If you start doing the maths you can see why plants get rapidly covered in aphids.
As the numbers of aphids increase over the summer the plant may become over populated. Smothered in aphids all competing for food, the plant may become weakened as the aphids drain it of sap, or give it some hideous virus. In order to survive it’s time for the aphids to move on and find a new food source. Hormones trigger the production of winged female aphids. These winged aphids take flight but they are not strong flyers. They get carried by the wind and drift to a new plant and form a new colony. The winged aphids start to reproduce parthenogenetically, once again producing wingless females, and so the cycle goes on.
In the autumn, as temperatures begin to drop, the light levels start to diminish and food sources becomes more scarce changes start to happen to the reproduction cycle. The female aphids start to produce both female and male young parthenogenetically. The male aphids are identical to the mother but have one less chromosome. These aphids can be winged or wingless depending on the species. The male and female aphids mate and the female lay eggs that overwinter, completing the life cycle.
Aphids have a close relationship with black ants. Aphids are soft and rather static as they typically have their snout stuck in a plant for a majority of their life. This makes aphids an easy target for a number of predators including ladybirds, birds, crab spiders ( and we had one of those on the sweet peas, dammit!) and lacewings.
Ants and aphids have developed a ‘mutual’ relationship. Ants provide the aphids with protection against most of these predators, enhancing the aphids chances of survival. In return, the sugar hungry ants harvest the honeydew excreted by the aphids, sugar is a major food source for ants.
Research from The Imperial College London, Royal Holloway University of London and the University of Reading, shows that there are chemicals on ants tiny feet that tranquillise and subdue colonies of aphids. The research showed that chemicals secreted by the ants makes aphids slow to disperse, even from dead leaves. Aphids dispersed more quickly from areas not touched by ants. So essentially the ants are ‘drugging’ the aphids in order to keep them herded and to ensure a constant food supply.
It’s also thought that chemicals produced in glands of ants can sabotage the growth of aphid wings. But in addition to this it’s been known for ants to bite the wings off aphids so they can’t get away. This all seems so wrong…. ants are bruts.
So once the ants have their ‘herd’ of aphids in place they go to great lengths to protect them. Ant attended colonies are definitely bigger than ant free colonies. When it rains the ants may move the aphids to a more sheltered spot! Ants have been known to carry aphids to new host plants. In the autumn, some honey ants even carry the aphid eggs back to the ant nest to overwinter underground. In the spring the ants place the eggs back to hatch. Quite extraordinary.
Well despite all this, we still have an aphid epidemic on our hands and as we garden organically tackling this pest is proving rather challenging. So far, I have been inspecting all the legumes daily, squashing any aphids I see. I’m rather like a monkey grooming its mate. I’ve become obsessed.
The aphids tend to congregate underneath leaves or on stems close to growing tips.
Whilst this method is time consuming, it has proved fairly successful, we are definitely keeping the numbers at bay and the legumes are growing.
The peas on plot one have sadly got Pea Enation Mosaic Virus. The leaves have started to turn yellow and the pea pods are curling. Not pretty. So far the peas on plot two are okay.. fingers crossed it stays that way.
There are other ways to try and control aphids without using chemical insecticides. Firstly you could try to encourage predators such as ladybirds to visit. This can be done by planting any of the following close to the plants you are tying to keep aphid free, dill, mint, fennel, yarrow and dandelions ( By the way, the Italian red rib dandelion is a delicious addition to salads, seeds are available from the fabulous Nicky’s Nursery).
Grow beautiful flowers such as cosmos, dahlia’s, and zinnias. The aphids may be attracted to these plants opposed to the vegetables! Then again.. it is an epidemic.
Spray the plants with water to knock the aphids off the plants.
Plant garlic or onions, aphids are not keen on the potent smell. You could even spray the vegetable plants with homemade garlic spray, providing you don’t mind the smell yourself!
Try and lure ants away from the aphid infested plant by placing a lovely pot of honey at the base of the plant. This should expose the aphids to more natural predators such as the ladybirds… and keep the ants happy!