Pests and diseases in the greenhouse have a way of keeping one very humble. You think that you have a reasonably good understanding of how they work and what you might expect from one year to the next, and then they turn around and do something completely unexpected.
Take last year for instance. In 2005, southern Ontario experienced one of its hottest summers on record, with the resultant expectations that it would be an equally hot summer for all manner of greenhouse pests. And in some cases, the predictions were accurate. For example, mites as always, revelled in the sweltering conditions.
However, for other pests, such as thrips, we were at the opposite end of the accuracy spectrum. It was a very quiet year, at least for western flower thrips. I was contacted by a number of growers waving sticky cards that were covered with thrips, but on closer inspection, they were found to be onion thrips, a pest of less concern.
This year, increasing western flower thrips populations were noted in early June, along with some very large and sudden increases in July, suggested that outdoor populations had built up and moved into greenhouses.
Why the difference from last year?
It’s difficult to say for sure, although the mild winter in 2006 may have allowed outdoor thrips populations to survive outside more successfully than in previous years. Although it is nice to be able to understand why pests act as they do (because it helps us to better predict what will happen next year), we still have to deal with the problem facing us now, which is the purpose of this article.
Over the past four to five years, thrips have to some extent fallen off the leader board of pests of greatest concern. In large part, this has been due to the registration of spinosad (Success) in other parts of the world at that time, and more recently in Canada. However, lurking in the background (as it so often does) has been the spectre of resistance developing to this product. In Canada, resistance seems to have even greater impact than it does in other countries for several reasons:
1We have fewer other products to rotate with new pesticides and fall back on if control fails.
2We are often several years or more behind other countries in getting new products registered.
3After registration of a new product, resistance often seems to develop more quickly because we have been importing pests on plant material for several years that have already been exposed to these products.
Resistance to spinosad is just as likely to develop in thrips as it has in the past to other pesticides.
Indeed, resistance of western flower thrips to spinosad has been documented for some time in other parts of the world. And if it does show up here, Canadian greenhouse growers are in a very poor position to respond. Our choice of alternative thrips control products is confined to a very few, very old (and not very effective) products. And one of the better alternatives, Trumpet, is being taken off the market at the end of this year.
There haven been some indications already that Success is not working as well this year as it has in the past. While it is too soon to suggest that resistance is already a problem in Ontario, there is no question that growers need to use Success very carefully.
So what to do about thrips this year?
Chances are that by the time this is published, the worst of it will be over (hopefully), and these words will be consigned to the “hindsight is 20-20” file. However, I will go ahead and say them anyway.
• Consider screening as an option. I know this is something definitely too late for this year, but consider it for the future. It is not necessarily suitable for all greenhouse structures, but growers who have installed it have noticed significant improvements (especially if the source of pests is from outside).
• Consider the use of large quantities of yellow sticky tape. Yes, it looks ugly, but so do thrips-damaged plants. Yellow tape catches a lot of thrips, and on its own may not control a thrips population, but it can be a useful contributor to a pest management program.
• If using pesticides, use Success with caution. Use only when absolutely necessary. The label stipulates to use only three times per crop cycle; however, the less it is used the longer it will last. Use other registered pesticides in rotation.
• If certain crops or varieties are obviously more susceptible to thrips than other crops in your greenhouse, use them as an indicator or a trap crop. Group susceptible crops together if possible so that thrips can be treated and controlled in a smaller area before they flow over into the rest of the greenhouse. Perhaps increase the attraction into this area with yellow sticky tape, just over these crops. Treatment in this fashion may include the use of biological controls.
• Nematodes have been used in recent years to control thrips. It was thought that foliar sprays were controlling thrips adults and larvae feeding on the plant. However, recent work by Dr. Rose Buitenhuis at the Harrow Research Station in Ontario, and by Jude Bennison in the UK, suggest that control is more likely to have come from runoff into the substrate controlling the thrips pupae. Heavy sprays directed at the soil surface would have the same effect and may provide some measure of control.
• Biological control of thrips is difficult, but more and more ornamentals growers are finding a way to make it work. The key is wanting to (or needing to) badly enough, and being prepared to be a little creative and a little patient in finding the best way to get results. Every greenhouse situation is individual, and there is no recipe that will work for all. Biological control of thrips has been given a recent boost with the approval to import a new predatory mite into Canada. Amblyseius swirskii (Swirski), is reported to be a much better predator than the long-time stalwart of thrips control Amblyseius cucumeris, although experience in ornamentals crops is still needed to understand how it can be put to best use. It has the added advantage of controlling whitefly.
So, to wrap up the thrips story: be vigilant, monitor carefully, and use pesticides wisely and biological controls creatively. There is no easy solution, but the more you understand the insect and its behaviour, the better prepared you will be to fight it.
Graeme Murphy is the greenhouse floriculture IPM specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs at Vineland. • 905-562-4141, ext. 106;
GREENHOUSE GROWER NOTES September 2006 3
Is 2006 shaping up to be… the year of the thrips? The mild winter may have allowed outdoor thrips populations to survive more successfully than in previous years.
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