Integrated Pest Management at Cramers’: Focus on Dahlias

Integrated pest management, or IPM, as I understand it is simply the realization that if you are only using pesticides to control pest then you will not be as successful as you can be. Instead, those who use IPM rely on cultural methods in addition to chemical ones and this leads to less chemical use overall. At Cramer’s we take a lot of cultural practices into consideration when dealing with pests. The following is a non-exhaustive list of cultural practices that we use.

  • We have found that it is important to remove peony leaf residue from the field at the end of the season as the dried-up leaves can carry diseases over into the next season.
  • Zinnias are prone to powdery mildew. Rather than spray the zinnia field every week, we plant seeds every two weeks so that when one field becomes infected with powdery mildew we simply abandon it and move on to the next field.
  • Having enough space between plants is important with many crops such as ranunculus because the additional air flow keeps the leaves dryer which is a less favorable environment for diseases such as botrytis.
  • Fans in the greenhouse also increase air circulation. This reduces damping-off diseases of seedlings.
  • The raised ranunculus beds provide better draining soil for the roots which reduces root diseases.
  • Ranunculus beds are steamed to kill disease spores and weed seeds.
  • To some extent, amending the soil with compost improves the health of our plants and perhaps their ability to fend off diseases.

But today I want to focus on Dahlias. Dahlias, particularly dahlias grown in a high tunnel, are a spider mite favorite. In the past we really struggled to find an effective chemical control for the mite problem. And I think I know at least part of the reason why. Look at the picture below. This is what Naomi dahlias in a high tunnel look like. How do you penetrate that canopy with insecticides?

Several years ago, broad mites distorted the leaves of our blackberry plants and stunted shoot growth. In fact, the broad mites were a result of excessive spraying to combat the newly invasive species spotted winged drosophila. All that spraying killed the mites’ predators but not the mites and they flourished. So we purchased predatory mites. Predatory mites are the good mites that eat the bad mites that eat our plants. And they worked.

Anyway, it was only natural for me to use some of those predators on the dahlias. And when we discontinued the blackberry experiment, I continued to buy the predatory mites. I now release 3 times over the season and use 2 predatory mite species. The beauty of using predatory mites is that they easily penetrate the dense dahlia canopy. Besides that, once the predatory mites establish themselves they reproduce and stay with the crop which prevents flare ups of spider mite populations.

A common concern about using predatory mites is that they are easily killed by broad-spectrum insecticides. While this is partially true, I have found that occasional spraying for Japanese beetles or whatever is ok. Probably this is partly because these sprays only need to be focused on the top of the plant canopy, because that is where the flowers and pests are, while the interior of the plant serves as a. reservoir for the predatory mites. Also, we switch from our systemic insecticide with remains toxic for a long period of time to an insecticide which is toxic for only a short amount of time. This still kills the bad bugs on contact but spares the beneficial insects that might transverse that same area a little later.

Another common problem of dahlias grown in high tunnels is thrips. Thrips cause blemishes on the petals that are especially noticeable in white dahlias. Thrips also seem to be impossible to completely control with insecticides. I don’t really know why. Maybe because they are so small and can hide so deep in the flower, maybe because they have such a short life cycle, maybe because they hang out in the youngest parts of the plant (the flowers) which are least likely to have been sprayed, maybe because they are ubiquitous in the landscape….

Because I was already buying predatory mites and paying for shipping I decided to buy some minute pirate bugs as well. Minute pirate bugs (MPBs) are, you guessed it, predators of thrips. However, I think MPBs are more vulnerable to insecticides then the predatory mites. The solution is ornamental peppers; another Cramers’ crop. Every year I place the ornamental peppers centrally among the dahlias. Peppers are a good host plant for the MPBs because they provide pollen which is an alternative food source for MPBs and because we do not spray the peppers with broad-spectrum insecticides. Therefore, if we spray the dahlias and kill most of the MPBs, there is still a reservoir of MPBs in the nearby peppers. MPBs are comparatively mobile and can migrate back into the dahlias. Also, peppers in high tunnels always seem to attract thrips.

Both the predatory mites and the minute pirate bugs have been very effective. This means you are receiving a more consistently high-quality product than if we were relying on chemicals alone. And the product will have less chemical residue.

 

John Thomas

Production Managerblog

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