Tag Archives: parasitic wasp

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: 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