Bacillus thuringiensis-based biopesticides are well known to growers of many organic products. While they can be an effective tool in reducing populations of certain insect pests, a lack of understanding of how these products work and how they are best applied can lead to insufficient control. Over the last couple of years we have received a number of calls from growers who have experienced problems associated with applying these products either too late in a pest infestation, or in a way that did not adequately target the pest. Now that we are in the lull between growing seasons, it seems like a good time to review how these products work and how they are most effectively used.
Bacillus thuringiensis, or Bt, is a naturally-occurring bacterium that is commonly found in the environment. The main activity of Bt comes not from the bacterial cells themselves, but from protein crystals, called endotoxins, which are produced as by-products of cell growth and sporulation. These crystals are inactive in the environment and can be rapidly degraded by exposure to ultraviolet radiation. However, when they are eaten by an insect, they dissolve in the alkaline conditions of the insect gut and the protein toxins become activated. The toxins then bind to specific receptors on the insect's gut cells, punching holes in the stomach wall through which the gut contents and Bt spores can leak out into the body cavity. As the spores and toxins multiply in the insect's body, it stops feeding and dies in 2-5 days.
Bt toxins must bind to specific gut wall receptors which allows Bt to be specific to only certain species or groups of insects possessing receptors the toxins are capable of attaching to. This makes it possible to target specific groups of pests without affecting beneficial insects that do not possess the same gut receptors. Furthermore, there are different strains of Bt, each binding to the specific receptors of different groups of insects. Bacillus thuringiensis subsp. kurstaki affects a number of Lepidoptera (caterpillar) pests, Bacillus thuringiensis subsp. israelensis affects certain species of flies, and Bacillus thuringiensis subsp. tenebrionis certain beetles. The kurstaki strain is currently the only one with products permitted for use on organic crops in Canada.
Bt products do not contain bacterial cells, but rather the crystal proteins, sometimes in combination with spores. While Bt itself is considered organically acceptable, it is important to note that sometimes Bt is formulated with inert ingredients that may not be organically acceptable, so consequently not all Bt products are allowed for use on organic crops. A list of potentially acceptable organic Bt products can be found in OMAFRA's vegetable and fruit protection guides (Publications 838 and 360, respectively), however growers should always check with their certifying bodies before applying any product.
The fact that Bt proteins only become activated in an insect's gut and that they can be degraded by sunlight has important implications on the effective application of Bt products. First, they must be eaten by the insect to take effect. This makes it somewhat different from many other insecticides, which kill insects on contact. Second, because many insect pests only feed at certain times of day and on certain locations on the plant, it becomes very important that Bt products be applied where and when the target pest is actually feeding. We have seen examples of growers encountering problems when a Bt application against leafrollers is made during the morning. Why? Leafrollers feed only at night. Applying Bt in the morning, several hours before feeding begins, allows time for sunlight to break down some of the proteins before the pest encounters them.
For a similar reason, adequate spray coverage will also be very
important when using these products. Sprays need to be directed
at the part of the plant on which pests are feeding. For some caterpillar
pests, this can be the leaf underside. There are a number of articles
available on the OMAFRA website on the topic of spray coverage.
Because these focus more on conventional products, they may receive
less attention from organic growers. However because the success
of Bt and certain other organic products depends on adequately targeting
the pests, spray coverage is equally important to organic growers
planning to use them. Many of the practices for achieving good spray
coverage with conventional products also apply to organic. For instance,
placing water-sensitive paper in the locations you want to target
(for example, the underside of leaves) will allow you to evaluate
what your coverage actually looks like. Selection of adequate nozzles,
adjusting water volumes and spray droplets are other factors that
can influence spray coverage. These topics are too broad to cover
in this article, however a number of articles written on these subjects
by OMAFRA's Application Technology Specialist, Jason Deveau, can
be found in past issues of the OMAFRA newsletter, HortMatters.
Other general guidelines to ensure best results with Bt products include: applying products in the evening, on cloudy days, and when no rain is forecast for 24-48 hours before application; avoiding storing Bt products under extremely hot (> ca. 25°C) or cold (< ca. 0°C) (specific temperatures vary with the product) and ensuring spray mixtures are applied within 12 hours of preparation. Due to the specific enzymes required to activate the proteins, Bt products are not equally effective against all pests within a group (e.g. not all caterpillar pests are equally susceptible). Remember that Bt products are not registered on all organic crops, and may not be labelled against all pests you want to control. Refer to the product label for instructions on pests, crops and rates.
For more information:
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