New pests, more rain and climate stats
We’ve had more than enough rain in September. My weather station records report that it rained on 19 days with almost 170 mm of precipitation. That’s more than 3 times the monthly average of around 50 mm and 6 rainy days for my area. And as this week’s newsletter lands in your inbox, I will be driving to Clevedon in Somerset to talk about ‘Coping with too much rain.’ How very appropriate!!
It’s been a wet week around the world too. Thanks to Hurricane Helene, a massive 45 inches (1143 mm!!!!!) of water were dumped on the mountains above Asheville, Mississippi causing the city to be flooded and covered in mud. I can’t even begin to imagine what that amount of water looks like. Ironically this city has been listed as a top climate haven city. Clearly, no city is safe from these extreme weather events.
But there’s drought elsewhere. Huge tracts of the Amazon are burning at the moment. Not because of deforestation but drought. And while our summer really didn’t get going, elsewhere people had to cope with record breaking temperatures. Globally, summer 2024 was the warmest on record, beating the previous year’s record, much of this driven by the current El Niño event in the Pacific.
New pests
This week my focus is on another aspect of climate change - the arrival of new pests and disease and specifically one new pest that arrived in my kitchen garden.
For the last couple of months, some of my perennial onions and chives have been under a massive attack from a small black aphid. First, the Welsh onions were completely wiped out when this small pest simply smothered them, sucking the very life out of the leaves. The onions have since died back and I just hope that they will survive underground and come back in the coming months.
Then it was the turn of the garlic chives in the same bed. Just as they were about to open their pretty white flowers, an army of these little black pests descended on them and literally covered them – all over the flowers, stems and leaves – virtually nothing above ground was spared. It was similar fate for my walking onions, although the leeks nearby seemed to be unaffected.
What were they and why were they so successful? And why were there no predators? So many questions.
What were they?
A bit of detective work and advice via Instagram, came up with the culprit - the onion aphid Neotoxoptera formosana.
OK – I have never heard of it! I know there are lots of different aphids and this is one that I know nothing about. For this particular aphid, their hosts are members of the allium family - onions and shallots, chives, leeks as well as ornamental alliums.
According to websites and books, it’s found around the world, mostly the tropics and subtropics and is said to be restricted in Europe. The pest was reported at Wisley in 1999 and 2000 and was said to have overwintered on garlic cloves in a cold frame. Well, from the comments I have received on my IG post – I think there are a fair few in the UK at the moment!
What do they do?
These aphids are found mostly on the leaves, where they suck sugar-rich sap from the phloem using their long stylets or mouthparts. [I find this ability amazing - the fact that an insect can insert its microscopic stylets into a stem or leaf and locate the phloem, the tissue that the plant uses to transport sugars around.]
Anyway, when they find a good food source, their numbers increase rapidly. This is because the females reproduce asexually in a process call parthenogenesis i.e. the females give birth to babies that are identical copies of themselves - all are cloned females. And this means their numbers can increase exponentially when conditions are favourable and they can often get ahead of the predator and reach plague numbers. Not surprisingly, plants don’t stand a chance. As they lose their sugar, the plant is weaken, the leaves wilt, turn yellow and die. If this wasn’t enough, the honeydew, the sugary waste product produced by the aphids, can fuel the growth of sooty mould on the leaves. After all of this, its not surprising that growth is stunted and often the plant dies.
During this exponential stage the aphids are wingless, so they don’t spread far – hence you get patches of problems. They move from plant to plant along the ground, although sometimes they are carried by wing [and, of course, we’ve had a lot of unseasonably strong winds in the last few months]. But at some stages in their life cycle they do produced winged individuals that spread further afield.
Unlike other aphid species, which will reproduce sexually at the end of the year, this species spends its entire life cycle on allium plants and always reproduces by parthenogenesis. No males or eggs have ever been reported.
They can spread on storage organs like onion sets and can be transported long distances in this way.
Why no predators?
Alliums have a distinctive smell and this is believed to enable the aphids to locate their host plants, but I have not seen any predators around these aphids. It is thought that the allium smell masks their presence and I think this must be the case in my garden. Despite having plenty of ladybirds in the garden, I’ve not seen any on them, and others report the same. Another possible predator is the hoverfly larva, but numbers have been low this year.
How can I control them?
I started to control them by wiping them off, but they just kept on coming back. I moved onto squirting them with jets of water, which seemed to help initially, but they reappeared. So, I have resorted to cutting back badly infested stems and plunging the stems in a barrel of water, so the aphids don’t survive. But I still have a problem….
So what are the control options?
Clearly the natural predators are not controlling this pest, so as an organic gardener, what are my options?
Spray them with a mix of oil, soap and water, having checked for beneficial insects first. The soap destroys the waterproofing over the insect’s cuticle, while the oil blocks the breathing holes (spiracles). Mix a few tablespoons of oil into 5 litres of water along with a teaspoon of liquid soap. Shake and spray over the aphids. Alternatively, you could use pure soap which you shake with water and then spray. If you are in a hurry, you could always dump a bowl of soapy water from washing up over the plants, but I wouldn’t do this for crop plants you are going to eat.
Another product to consider is SB plant invigorator. This is a mix of organic ingredients, such as seaweed and fish oils, in a soap mix. It is recommended for use on soft bodied insects, such as aphids, whitefly, spider mite, mealy bug, and scale insect. It is not classed as a pesticide as it lacks an active ingredient. Instead, it relies on soap-based surfactant (a surfactant acts to reduce the surface tension of a liquid to improve its spreading and wetting properties) to damage the insect’s waterproofing (as above). It also contains a range of nutrients and micronutrients that act as a foliar feed and can encourage plant growth, which can be useful for a plant that has been set back by a pest. It can be very effective, and I have used it in my greenhouse to help tackle whitefly. But you do need to think about its affect on other insects, especially if you are using biocontrols. It is only effective against small insects, so pollinators and predators are unaffected, but what about the soft-bodied larval stages? Joy and Michael at Sea Spring Seeds have done a very detailed analysis of its effectiveness vs cost, which is well worth a read.
Biocontrols You can use a wide range of predators to control insect pests but this problem is outside rather than in a greenhouse, so most of the options are not suitable. But there is one. Studies indicate that lacewing larvae are good predators and as well as attracting adult lacewings to your garden, you can buy lacewing larvae to release in late spring and summer if you see the aphids. The larvae are released directly onto the aphids. I don’t see many lacewings in my walled garden, although they are present in numbers in my orchard, so this is something to consider next year.
Essential oils. A number of essential oils have been shown to have insecticidal properties (see below). The use of essential oils is of great interest to organic growers and farmers, as they are a far safer option to using so called ‘natural products’ such as pyrethrum and neem (Azadirachtin) which is now known to be toxic to bees and has been banned in the UK.
What about next year?
I really don’t want the same problem next year and I’m going to have to be more proactive. When I saw the aphids on my Welsh onions I assumed, wrongly, that the predators would appear and tackle the problem but they didn’t. And during my holiday, the chives and garlic chives got infected – so I need to be more attentive and wash them off as soon as I spot them rather than leave them and hope a predator will come along.
I am concerned that as these are perennial onions the aphids may survive on the bulbs underground and over winter. The mild winters we have had recently may have allowed these pests to survive in the UK in the first place.
If these perennial onions do recover, I am going to have to put some defences in place. I may surround the onions beds with lots of annuals that attract hoverflies and lacewings. Allysum would be good as its low growing, easy to grow from seed and would complement the garlic chives. Elsewhere in the garden, I need to grow more buckwheat, dill and tansy.
If you have had problems with onion aphids, I would love to hear about your experiences in the comments.
Rosemary and other essential oils to control pests
When reading research papers about this aphid, there were comments that these aphids detect their host plant through the pungent sulphur compounds that they release. Given that olfactory cues are so important to this pest, it is possible to use smell to disguise the onions.
Essential oils include black pepper, eucalyptus, geraniol, lavender, rosemary, tea tree and thyme. The volatiles that these plants produce can act as a deterrent or even a toxin to leaf-eating pests. Initial studies have shown that a treatment of just 1% rosemary oil has been found to be effective on pests, including two-spotted mites (Tetranychus urticae), while in another study, a dose of 5% of the essential oil was found to reduce and control green peach aphid (Myzus persicae), a pest which is found globally on more than 500 species of plants including fruits and vegetables. The treatment was even more effective when essential oils were combined, for example rosemary and tea tree oils. Some researchers believe that these oils could act as a repellent, as well as an aphicide.
There are three essential oils that you can buy which are undergoing trials; eugenol, geraniol and thymol. Eugenol is a phenol found especially in aromatic and spice plants, such as basil, clove, bay, cinnamon, marjoram, and nutmeg. Geraniol is a terpene from aromatic plants and has been found to have insecticidal and repellent properties as it affects the insect’s nervous system. Thymol is derived from thyme and used as an insect repellent and fungicide.
Protecting your plants with essential oils
Essential oils are best used on small soft-bodied insects, such as aphids and whiteflies, and mites.You can make your own spray from fresh ingredients or buy a small bottle of essential oil.
To make your own, pour 5 litres of boiling water over 500g of fresh plant material, such as fresh rosemary, thyme, eucalyptus. Cover and leave for a few hours, then strain and dilute 1:4 with more water. Spray the liquid over plants as a repellent.
Alternatively, add 10 drops of essential oil to 1 litre of water. But remember essential oils vary in their quality. For example, rosemary oil can be distilled from different components, some producers use only young leaves, while others use all the shoots including woody material. Also the geographic location of the plant material is another variable.
Happy gardening
Sally
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Talks
I am talking at a number of venues in the coming weeks. Most of these clubs allow non members to attend for a fee.
14 October Nynehead Gardening Club near Wellington
17 October Dulverton Gardening Club
19 November Mayfield Horticultural Society by zoom
21 November University of Bristol Botanic Gardens
23 November I am talking at the inaugural LandAlive conference at the Bath and West showground on Sat 23 November. The topic is on creating a climate resilient allotment.
4 December Castle Cary Gardening Club
