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