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Greenhouse Grower notes: Thrips in spring crops

It is about two years since I last wrote about thrips control in
ornamental crops, and in that time there have been changes in the way
growers are approaching biocontrol and in some of the biocontrol
products available.

It is about two years since I last wrote about thrips control in ornamental crops, and in that time there have been changes in the way growers are approaching biocontrol and in some of the biocontrol products available. In spring crops and especially in hanging baskets, growers have more options available.

Hanging baskets can be a difficult target for biocontrol of thrips. PHOTO BY GRAEME MURPHY, OMAFRA Advertisement

Thrips remains a key pest for most growers. The wide range of crops it feeds on and the damage it can cause, its role as a vector of tospoviruses and its resistance to most registered pesticides make it very difficult to control. For these reasons, large numbers of Ontario growers have turned to biocontrol as their primary control strategy, and many of them are developing very successful programs. However, it needs to be emphasized that there is no standard recipe for biocontrol; growers have a number of options at their disposal and adapt according to their experience and comfort level with different strategies.

Spring crops are grown either from seed or from vegetative cuttings. For cuttings coming in from offshore propagators, there are two concerns:

• Thrips may be present on cuttings, which can result in an established population very early in the crop and the biocontrol agents having to catch up, which can be very difficult.
• Secondly, and perhaps of more concern, is the potential for pesticide residues on the plant material. Such residues can be harmful to biocontrol agents for a number of weeks, effectively delaying the start of the biocontrol program and allowing thrips to develop through one or more generations before natural enemies can become established.

The other point is that both these situations could potentially occur at the same time. If resistance is present in thrips populations at the propagators, it could result in thrips on the cuttings and greater problems with residues.

CROPS GROWN FROM SEED BEGIN WITH A CLEAN SHEET
■ Crops grown from seed begin with a clean sheet. There are no issues with either thrips or pesticide residues. Any thrips that are found on seeded crops have come from elsewhere in the greenhouse.

I don’t want to belabour this point too much; Ronald Valentin has written an excellent article on the issue of pesticide residues in cuttings, which will be published in the April issue of Greenhouse Canada.

As soon as cuttings are planted and on the rooting bench, growers are adopting a number of different approaches, depending on the crop being grown, its susceptibility to thrips, and the history of thrips problems in the greenhouse. Cuttings on a rooting bench provide an ideal opportunity to begin a biocontrol program:

• The environment is excellent, with high humidity and warm temperatures.
• It is very intensive with large volumes of plant material located in a small area, allowing for very efficient use and distribution of biocontrol agents.
• Individual plants consist of a small leaf area and (hopefully) very low levels of thrips, the ideal situation in which to start a biocontrol program.
• The strategies that growers are adopting at this stage include:
• Weekly applications of nematodes and/or BotaniGard – the moist, warm conditions on the rooting bench are perfect for these two products.
• Weekly applications of predatory mites such as Amblyseius cucumeris. Growers usually apply bulk product that is sprinkled over the bench. Because of the closely packed plant material, there is very little wastage. Almost everything applied reaches the target.
• One-time applications of predators such as Hypoaspis or Atheta, that live in the growing medium and feed on insects such as thrips pupae and fungus gnats.

When plants are off the rooting bench, different strategies are used. Many of the early plantings are used for hanging baskets, and these baskets can play an important role in the development of thrips populations in spring crops.

• For a start, they are a much longer term crop than most spring crops, giving the thrips more developmental time to build up populations.
• They are usually started on the bench, and hung up when bench space is needed for later bedding plant crops. When they are hung, they are at a slightly higher temperature than other plant material on the benches or floors; as a result, thrips development is more rapid.
• Also when they are hung, it is much more difficult to monitor them, and as a result, pest problems develop unnoticed.
• Finally, when thrips pupate, they often drop from the plant to the growing medium or to the ground. As plant material in baskets often overhangs the basket perimeter, this means that the thrips can drop to the crops on benches and floors below, spreading the infestation to other areas.

Until recently, this has been a difficult crop for growers to treat using biocontrol. While on the bench, predatory mites could be sprinkled over the baskets, but when hung, it became much more difficult and time-consuming to treat. Slow-release sachets were an option, but at $0.30 each (approx.), they were economically difficult to justify, especially considering that each basket would probably need to be treated twice: once when first planted up and still on the bench, and a second time just before being hung.

In the spring of 2010, the biocontrol companies started producing a new packaging/distribution method for the predatory mite Amblyseius cucumeris, the most commonly used thrips biocontrol agent. This was a slow-release sachet with a lower starting population of mites (between 100-250) and in a smaller sachet. Dubbed the “mini-sachet,” it sells for approximately $0.10 each and makes biocontrol of thrips much more viable. The number of mites produced over a five- to six-week period can be in the range of 1,000 to 1,500, which is many more than could ever be applied (economically) by sprinkling.

The mini-sachets developed an instant following in Ontario, with some growers even using them in crops other than hanging baskets, e.g., potted chrysanthemum and potted gerbera. Many growers are planning to use them again in 2011.

Thrips is still one of the most significant pests for greenhouse flower growers and the lack of effective pesticides has been a major motivating factor for growers to adopt biocontrol as their first line of defence. The success that many are having in controlling this pest is evidence of their perseverance and creativity. It is still a battle and growers are never happy until the crop is finally out of the greenhouse, but there is more optimism now that thrips control can be achieved.

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, or graeme.murphy@ontario.ca.