Tag Archives: pests

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…

Ecology and gaffer tape

Will a hard winter mean fewer pests this year? I’m not holding out much hope. It all depends whether you believe the path to redemption lies in ordered hygiene or dynamic balance. In favour of hygiene, the cold will have cleaned things up; a lot of aphids will have perished and leaves and roots harbouring disease will have been killed, thus breaking the disease-carrying bridge between seasons.

Unfortunately my experience of cold winters past is that any benefit will be short lived. Taking an ecological “balance” perspective, this is easily explained. Most pests that make a meal of our crops are also a meal for someone else: aphids are eaten by ladybirds, lacewing and hover fly larvae and parasitized by certain wasps, slugs are eaten by carabid beetles and toads and predated by nematodes. Red spider mites are controlled by the predatory mite phytoseiulus. Unfortunately these farmer friendly “beneficial” organisms will have also suffered in the cold; in fact they tend to be more affected by the cold than the pests (not only do many die, the survivors get dopey and less hungry).

Some pests always survive and, after a cold winter, there are fewer predators to keep them in check. As pests tend to get going sooner and breed faster, a cold winter might be expected to result in a higher population peak before the predators catch up. Hence cold winters may help the hygiene approach to pest management (as propounded by pesticide salesmen) but are not much help to those looking for balance.

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

it’s bug eat bug on an organic farm

Integrated Pest Management
Our photographer Martin Ellis was out visiting Adrian Izzard, one of the suppliers of veg to our East Anglian sister farm, River Nene, and we thought you might like an insight into what goes on inside an organic polytunnel…

Here is a pic of what is called “integrated pest management”, basically bugs, and bug’s eggs, which will eat, mummify or act as parasites on the pests that want to chomp on our veg. It’s a natural process – and much better for wildlife than using the chemicals employed on conventional farms. If any of the bugs escape from the polytunnels, then they will quickly die because of a lack of food.