The Downs and Ups of Aphid Culturing

I am cursed with living in a temperate country with defined seasons. Whilst this is great for aphid biodiversity and the autumn colours are nice, it does mean that for half the year access to study organisms in the wild can be an issue. Mine in particular is woolly apple aphid (WAA), and in order to supply a source of material year-round, I’ve set about trying to set up a culture under controlled conditions.

For small, rapidly-reproducing insects like aphids, once a culture has been established they can be kept alive indefinitely. Harper Adams University had, at my last count, twelve aphid species in culture. The issues involved with establishing and maintaining cultures varies between species, but these are my experiences trying to establish a culture of WAA to last the winter.

Conditions

Insect lifecycle is heavily influenced by temperature, meaning that to rear them, temperature has to be carefully selected to what you want to achieve. WAA performs best between 13–25°C and the controlled condition room my aphids are kept in has a constant temperature of approximately 20°C. I want to keep my aphids under summer conditions to keep them feeding and reproducing regularly to hopefully build up aphid numbers which I can use for experiments in the winter, and during any ‘dry spells’ in other months when numbers in the field are in a dip.

Another important factor which determines stage of the aphid lifecycle is day length, or photoperiod, specifically the ratio of light and dark in a 24 hour period. To keep aphids in summer conditions a short-night photoperiod of 16:8 L:D is used, that is 16 hours of light to 8 hours of darkness. At a later stage of my PhD project I want to look into whether I can induce them to form their autumn winged form in the lab, so for that I might consider putting them into a long-night time period, for example 12:12, which could even be combined with a lower temperature to give a seasonal change in aphid morphology.

Diet

Whilst controlling the rearing conditions is important to get aphids to behave how you want them, without suitable food there won’t be any aphids to control. If you have a generalist aphid there is much more leeway with host plant; potted pak choi is often used because it is easy and quick to grow and a lot of species will feed on it.

WAA, however, is a specialist and only feeds on American elm, apple, and some other Rosaceous species. Apple is slow-growing and deciduous. I did try cutting apple shoots and keeping them in beakers of water but if apple is slow, woollies are glacial! The shoots were wilting and dying off before they aphids would move onto the fresh tissue I provided. Using potted seedlings has been much more successful, although wrestling them into a bugdorm isn’t, I think, very good for the tree or for me.

This has been working out well over the past few months. I have a lot of potted trees from the resistance breeding part of my project- the rootstock M.9 is beautifully susceptible to WAA. As we approach winter here in the UK, the culture is faced with a dwindling supply of apple material suitable for aphid feeding. Apple trees require a long vernalization, or chilling period, before they will resume spring growth and flowering. Luckily, I’ve had rootstocks in the coldstore for months, providing this vital chill. They’ve now been moved into the same long day conditions as the aphids and have already started putting out new growth, hopefully full of tasty photosynthates for WAA to feed on.

Aphid health

The problem with aphids is that no matter how careful you are with their diet or growth conditions, there is always the risk of parasitism. Although aphids are particularly susceptible, it’s not just an aphid issue, however. Fellow Insects and That writer Gary is still waking up in cold sweats thinking about his run-in with the prolific ladybird-manipulating parasitoid wasp Dinocampus coccinellae, which ran rampant through his masters project seven-spot ladybird colony.

I digress. For WAA, the threat is from the wasp Aphelinus mali. Unfortunately, the populations of WAA I have been using have all been infested with A. mali by the time I took them from the field into culture. Aphelinus mali mummies are black and don’t have wax which does make them easy to see but at earlier stages of development it’s hard to tell the difference between them and normal WAA.

To try to prevent this I tried setting up smaller colonies in Blackman boxes to check daily for wasps to make sure they were clean before introduction into the main culture. It didn’t work — there were always wasps. Then I put yellow sticky traps in the culture to catch wasps as they emerged. It did work, but not enough to wipe out the parasitoids.

So how do you overcome this issue? The Blackman boxes seem like the best solution, as long as you collect populations which don’t have obvious mummies and from a field collection site, not glasshouses. Establishing small cultures which can be monitored before introducing to a main culture will probably work for most.

What did I do? I got lucky. I found an isolated glasshouse population of WAA that didn’t seem to have any parasitoids, sprinkled them on a clean susceptible potted tree and the rest, as they say, is history.