Whitefly, & Weedkiller Woes

Answering a question about whitefly and a problem from using weed and feed on the lawn grass.

After I send out the newsletter I’m swamped with emails back asking questions. Now to be quite honest, I can’t reply individually to everyone (although I try) or I’d get nothing done. A lot of questions are best asked on the forums, not only does it get a variety of answers from a range of gardeners but it also helps people with the same problem.

Anyway, a couple of Questions and Answers

Controlling Whitefly

I’m looking for a good tip to control whitefly – we have a veg area inside a polytunnel frame with .4 mm net which keeps most things out ( no pollination required) yet we still suffer from the pesky whitefly , so any suggestions appreciated

Like a lot of insects, whitefly reproduce at a frightening rate. One adult female whitefly getting under the net can lay hundreds of eggs which will turn into adult whitefly in as little as three weeks – depending on the exact species and weather conditions.  So just one fly getting in can mean a veritable plague in a couple of months. Do the math! (Warning it’s scary)

Often – almost usually – a pest in the greenhouse where conditions are great for them but they can be a major problem on brassicas outside in a good summer. They live by sucking sustenance from the plant which weakens it and can allow infection in.

By encouraging predators as covered here, Aphids – Natural Helpers & Organic Controls for Aphids the problem can be controlled but if they’ve got the upper hand you need to take direct action.

Sticky yellow card traps have some effect in a greenhouse and biological controls such as Encarsia formosa but are quite effective but both are pretty much useless outside. This leaves us with sprays.

Sprays for Whitefly

Pesticide sprays like Bug Clear can be very effective. They act fast and have a dual action. Contact with the pest kills it and the plant absorbs the chemical giving systemic control. Be aware that pesticide resistance is becoming a problem with whitefly so it may not actually work for you. The main question is if you want to go down the non-organic route and eat trace amounts of pesticide.

Agralan Whitefly Killer and SB Invigorator are both reasonably effective ‘safe sprays’ but require more frequent and more thorough application to work well. You really need to follow the instructions carefully, a quick careless squirt will not do the job.

Weedkiller Woes

I haven’t used any weed and feed on my lawn since last year, probably in September. My first cut this year I have added to my compost heap working on the theory that so much rain over the past quarter would have washed away everything. Do you think I have made a grave error?

These persistent selective herbicides remain in the grass itself, so the rain will not make much difference. If this is the first cut since you applied it then I fear you’ve most likely contaminated your compost. You could try potting some compost and growing a few beans as a trial to see if it is contaminated. If they fail or come up distorted then you know not to use the compost on flower borders or vegetable plots.

If the compost is contaminated, when it’s rotted down riddle it and apply as a top dressing to the lawn. This way you still get a benefit from the compost until the chemicals have gone.

There’s usually a warning on the packet or product guidance label that states how long the product will be around and how to handle the clippings. It’s very much the same as the problem we have with aminopyralids in manure.

Posted in Allotment Garden Diary
One comment on “Whitefly, & Weedkiller Woes
  1. Mike S says:

    My mother’s lawn is infested with field woodrush & unfortunately, liming has had little effect over the past two years.
    I’m loath to use chemicals but in this case, I’ll be applying Roundup lawn weedkiller. Active chemicals appear to be Mecoprop & Dicamba, rather than aminopyralids.
    Instructions state leave the first clippings on the lawn, then compost the next four for a minimum of 6 months – I’ll put them on top of her rampant ground elder to see if it has any effect over & above mulching.

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