Recently, I had an interaction with a grower where their long-standing biocontrol program for thrips suddenly seemed to be failing. After a (too long) investigation by myself, the grower, and several consultants, we found out the horrible truth: their predatory mite were being MURDERED (Duh dun DUHNNN!)… By improper storage.
This post focuses on whether YOU might also be guilty of mite murder. (And yes, I’ve stooped to the level of click-bait titles).
Special thanks to Graeme Murphy (BioLogical Consulting) Dave Neal (Koppert), Rose Buitenhuis (Vineland Research and Innovation Centre) and especially Ronald Valentin (BioWorks) for all their knowledge that went into this post!
1. Storing mites in the wrong spot:
In a perfect world, everyone would put their biological control agents out the second they received them. But I know that shipping and other duties sometimes take precedent. However, WHERE you store your mites until you put them out has a huge impact on their quality.
The grower in question was using Cucumeris mite sachets that were stored in the temperature-controlled office before use, to protect them from too high temperatures in the greenhouse. At the time, this made perfect sense everyone involved.
Turns out, that was the wrong move.
Sachets contain 2 types of mites: the predatory mites (Cucumeris or Swirskii) and “food mites” that the Cucumeris/Swirskii prey on while in the sachet (providing a self-renewing food source so you get repeated generations of predators from the sachet).
We found out through some discussions with Ronald Valentin (formerly BioLine; now with BioWorks!), that to feed the food mites and keep them reproducing over time, sachets actually have yeasts and fungi added to the bran inside. Lower the humidity too much – by putting them in an office with forced heating/cooling (around 40-50% RH), for example – and you kill the yeasts/molds, starving the cereal mites. And voila, your sachet stops reproducing and you have sachets filled with bran (and not much else) in about a week.
Similarly, coolers also pose a danger to mites. Not only can they be very dry, they are often set below the proper temperature for mite storage (min. 15 C for Swirskii and 12 C for Cucumeris).
According to Ronald’s years of experience, the better place to store your mite shipments in an active compartment in the greenhouse. The higher the humidity, the better – try not to go below 60% RH. Place the boxes under a bench to shield them from sun. Although the compartment might be on the hot side, sachets are affected less by temperature than by humidity.
You should also open up the boxes if you can’t put the mites out for a few days, as CO2 buildup can also threaten mite survival.
2. Enlarging the exit holes on the sachet:
I know – it’s very tempting. That hole is SO SMALL! But it’s been specially designed to juuustt be large enough to let mites out, while keeping humidity in. Resist the urge to mess with it.
3. Placing sachets incorrectly on plants:
One reason I love Rose Buitenhuis at the Vineland Research and Innovation Centre is that she brings us some darn good practical research.
One of her first projects was to look at optimal mite sachet placement on plants. And she found some very clear results.
When mite sachets were nestled within the plant canopy – shaded from the sun and within a higher humidity “micro” environment – LOTS more mites emerged than when sachets were exposed (i.e. placed above the plant canopy). And the mites kept emerging, with sachets peaking at around 3 weeks post-placement, and lasting for up to 6 weeks. Conversely, sachets exposed to the greater greenhouse environment only provided enough mites for 1 or 2 weeks of thrips control.
One reason for this is that both beneficial AND prey mites need high humidity for their eggs to hatch. For example, the %RH needed for 50% of eggs laid by Cucumeris and Swirskii to hatch is >60%! Thus, lower humidity = lower reproduction. Once the mites are out of the sachet, however, the humidity level at the leaf boundary layer is generally sufficient to keep them happy.
So, yes, humidity really is THAT important for breeder sachets of mites, even once they’re in the greenhouse.
4. Using incompatible pesticides
Predatory mites generally tend to be less susceptible to pesticide residues than other biocontrol agents, but that doesn’t mean they’re invincible. Obviously, the old-school organophosphates and carbamates are hard on any bio. But mites can also be affected by spray applications of the following insecticides:
- Insecticidal soaps (low persistence)
- Kontos (spirotetremat; 14 days persistence*)
- Landscape oil (mineral oil; 2 weeks persistence)
- Pylon (chlorfenapyr; 14 days persistence*)
- Success (spinosad; 2 weeks persistence)
As well as the following miticides (i.e. for spider mite and/or broad mite):
- Avid (abamectin; 3-6 weeks persistence time)
- Dyno-Mite (pyridaben; unknown persistence)
- Floramite (bifenazate; up to 1 week persistence)
- Forbid (spiromesifen; unknown persistence)
- FujiMite (fenpyroximate; unknown persistence)
- Pylon (chlorfenapyr; 14 days persistence*)
(* Data courtesy of Plant Products. Other data taken from Biobest/Koppert compatibility databases).
Of the products available for spider mites, Vendex (fenubatin oxide) and Floramite (bifenzate) are generally the gentlest on predatory mites (Floramite being harder on P. persimilis and Swirskii than Cucumeris).
With all products, avoid hitting the sachets themselves, and always check the viability of sachets post-spray. Be prepared to re-apply after the persistence period if necessary. Lose broadcasts of mites will need to be re-applied.
How to Know You Have a Problem:
As with any bio, while it’s always good to check for life upon receipt. But with mite sachets, you also need to monitor their actual emergence in your crop over time.
By keeping an eye on them, you can a) get ahead of any problems and work with the bio companies to resolve them before things turn critical, and b) determine how long sachets last for YOUR crop under YOUR greenhouse conditions. This will help you plan your thrips IPM program according.
As I don’t believe in re-inventing the wheel, I will simply direct you to this amazing video by the Vineland Research and Innovation Centre on how to do weekly mite walk-out counts. This is truly the best way to determine what bang you’re getting for your buck when it comes to mites, and is considered the “gold standard” by biocontrol companies.
With these pitfalls outlined, and a handy way to monitor your sachets established, you’re on your way to effective thrips control.