Tag Archives: aphids

Guy’s Newsletter: an aphid’s view

If things are this good why grow wings, why even move? Why have sex and risk producing variable babies that may not be as good as me? Sexual reproduction is so full of uncertainty. Why not just stay put, plug in, suck that sweet, sweet sap and pour out a stream of babies identical to me through parthenogenesis; they need only shake free of my abdomen, plug in and enjoy the same good life. Within five days the young’uns will be squeezing out their own; it’s perfect.

Two weeks ago, looking around the peppers on our farm in France I calculated that about 20 million wingless aphids were sucking the life out of my crop; each leaf had up to ten mothers with a stream of look-a-likes plugging in within millimetres of their mother. Marco, my ever-calm agronomist, told me not to worry; “I’m on top of it,” he said. The temptation for the macho and inexperienced would be to wade in with some soap spray (restricted but permissible under organic regulations) which effectively suffocates the aphids it touches by invading their spiracles, but this would also risk killing the predators already feasting on the aphids and destroy our chances of reaching the holy grail of organic pest control; balance. Marco’s policy was to wash off the worst colonies with water and introduce more ladybirds to mop up the rest. I was nervous; a ladybird can eat 5000 aphids in its life but can’t compete with their reproduction rate. Who would eat their way to the top? As well as ladybirds we often seek help from my favourite aphid predator, Aphidius colemani. This tiny parasitic wasp oviposits a single egg in each aphid which slowly digests them from within before emerging two weeks later, alien style, as an adult wasp ready to lay another 200 eggs; we introduced some of them for good measure.

Two weeks later, Marco was proved right; the ladybirds won and it looks like we will have a good, if slightly delayed, crop of peppers. Having seen the scenario played out so many times since we gave up spraying soap on aphids 15 years ago, I should have had more faith in the under-promoted virtue of using less and understanding more. If a fraction of the money spent on pesticides and GM went into studying agro-ecology, most insecticide use could be avoided.

Guy Watson

guy’s newsletter: plants, pests & the search for balance

The first basil and cucumbers were harvested from our polytunnels last week, and very fine they were too. We grow mainly mini cucumbers as they taste better, are easier to grow and avoid you having that soggy-ended cucumber half lurking in the back of the fridge, so I can’t really understand why anyone grows anything else.

Outside we are in the hands of the Gods with a difficult start to the season, but the protection of our flimsy tunnels can give dangerous delusions of omnipotence. We can manipulate the temperature, humidity and ventilation to promote growth and avoid fungal disease and our team of pickers, pruners and tomato trainers are experts at identifying and monitoring aphids and spider mites. Rather than turning to chemicals as a means of pest control, a dynamic balance of pests and predators is our aim, but when an aphid gets its proboscis plugged into a good stream of plant sap they can squeeze out babies at an alarming rate. If life is good they give up on sex and egg production altogether; why bother with the complications, wasted energy and variable offspring when you can just replicate more like mamma via parthenogenesis. The trick is to introduce enough of the right predators and parasites before the explosion happens and to get the balance at an acceptable level where crops do not suffer significantly.

We are struggling to find that balance out in the fields too. Aphids in the lettuce and flea beetle on rocket, mustard and spinach have forced us to abandon a number of crops, just when we need them most for your boxes. It could be that low temperatures are disproportionately slowing predator activity, but I feel more inclined to attribute our problems to stressed crops emerging from a miserable winter. Just as with humans, stress leads to vulnerability. Later crops are looking happier however, and past experience would suggest that predator appetites rise with temperature faster than pest fecundity does. So, as we enter summer proper, we expect the balance to come outside in our fields as it has in the tunnels, and all will be well on the farm (for the time being at least).

Guy Watson

guy’s newsletter: a bad end for aphids

As the lettuce and spinach season starts in Devon, we are clearing up the last stragglers on our farm in France and are busy harvesting cabbage, kohl rabi and the first courgettes. When the first female courgette flowers opened two weeks ago there was not enough pollen about (weirdly the male flowers seem to open a little later) and not enough pollinating insects to carry it to female flowers. Poor pollination produces aborted or misshapen fruit, which must be picked off by hand to divert the plant’s efforts into filling better fruit. After the first pick, the ground is littered with rejects, with only 20% making it into the barrow. Next week will be better and we will be picking until early July when the UK crop starts.

In the tunnels, the ramiro peppers are looking fantastic. There is a nail-biting, ecological race going on between the peppers, aphids and their predators and parasites. Most plants now have sizeable colonies of aphids. Having settled on a suitable plant, the winged aphids plug one end (their proboscis) into a pepper vein and produce a stream of babies from the other. If the food source is good, sex and wings are abandoned in favour of efficient, flightless, genetically identical asexual reproduction. Such efficiency can quickly result in a truly scary population explosion; the peppers would quickly be sucked into a premature, stunted death if it weren’t for the intervention of a tiny, midge sized parasitic wasp. Each adult wasp deftly oviposits a single egg in over a hundred aphids. The egg hatches and devours the aphid from within, emerging two weeks later as an adult which immediately mates and starts the cycle again. Maybe those aphids should have kept their wings. There is a photo of the mummified aphids on www.riverford.co.uk/blog/ and you can watch the gruesome business on YouTube (search ‘National Geographic parasitic wasps and aphids’).

It’s a race of relative fecundity, but provided we introduce enough wasps early enough (I think we have) they will establish a balance within a few weeks and the peppers will be fine. We should be picking green ramiros in July and red from August through to October.

Guy Watson

Biological warfare

With a cold wet summer such as we’re experiencing this year it can be a bit of a relief to go down to the polytunnels where it’s nice and dry and we have much greater influence over the growing environment. These warmer conditions can bring problems of their own, however, as what is good for something like a cucumber can also be good for pests such as aphids and red spider mite, which can rip through a crop if nothing is done about it. Aphids have a life cycle of 3-4weeks (depending on climatic conditions) and during that time can give birth to 40-100 live young who emerge with the next generation already inside them!

Some predators will follow these pests through the doors: ladybirds, lacewings and hoverflies are all welcome visitors and we have some plants dotted around to encourage them (lacewings love fennel, for example) but this isn’t always enough and so we boost their numbers by distributing extra pest-specific, insects and bugs through the crop.

These fall into two main categories: predators and parasites. Predators (like phytoseiulus persimilis for red spider mite and aphidoletes aphidimyza for aphids) will attack and eat the pest, then lay eggs which hatch into a new generation to continue the process. Parasites are, if anything, more gruesome: aphidius colemani, for example, will lay an egg inside the aphid itself. This obviously kills the pest as the larva grows and when it hatches, carries on the process. Parasites tend to be much more host-specific than predators, which aren’t too fussy (within reason) what they go for. In both cases, the second generation tend to be more active and vigorous than the parents we introduced as they are more acclimatised to the conditions in the tunnels.

Some battles you win and some you lose: to date there are no signs of red spider mite, but our peppers have a few green aphid and one of our cucumber tunnels is fairly heavily infested with black aphid. We have ordered extra insects to help in the war and I have even been introducing the odd ladybird I have found in the fields! Hopefully this will be enough and we can get on top of the problem.

ImageBack outside, meanwhile, we are beginning to harvest our globe artichokes. These highly architectural plants, a relative of the humble thistle, are one of the many crops to have taken a bit of a battering from the elements: they can suffer from browning leaves if conditions are too humid but are worth persevering with as they’re relatively low maintenance for a perennial crop and have a great and unique flavour. I tend to just steam them and eat as a starter with loads of melted butter, though I’m sure Rob in our Field Kitchen restaurant has far more imaginative uses for them…

Cotton aphid

The cucumbers on our Devon farm have been under attack from the cotton aphid. We are usually prepared for it in late June but this year high numbers of them came early.  The aphid isn’t interested in the cucumbers but feeds on the leaf to take in sugars. Anything they can’t use is secreted onto the leaf as honeydews and invaded by sotty mould. The leaves then can’t photosynthesise so the plant struggles to grow.

sooty mould on the leaves

sooty mould on the leaves

To keep aphids down we send in a parasitic wasp, aphidious colemani, that stings up to 300 aphids in two days, injecting an egg. This parasitises the aphid so the egg can use the aphid’s body to feed on. We’ve got them under control now but it’s too late for some of the plants and we’ll get around 50% of expected yield of cucumbers this year. Next year we’ll anticipate an early attack!
Hoverfly lay eggs on leaves and the larvae is another natural predator of aphids:
aphidious colemani

hoverfly