Is it mite murder?

Here in Canada, 90% of our greenhouse growers use biological control, especially for key pests, like thrips. This makes our growers world experts in using predatory mites, which form the backbone of many biocontrol programs. But even experts can make mistakes.

A few years ago, I had an interaction with a grower when their long-standing biocontrol program for thrips suddenly seemed to be failing. After an investigation by me and several industry consultants, we found out the horrible truth. Their predatory mites were being murdered by improper storage of their mite sachets.

This article focuses on ways you might also be guilty of mite murder, and how to avoid common pitfalls when using mite-breeding sachets. By taking a little extra care, you can make sure you’re getting your money’s worth out of this biocontrol tool.

Pitfall No. 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 high temperatures in the greenhouse. At the time, this made perfect sense everyone involved. As it turns out, that was the wrong move (Fig. 2). Sachets contain two types of mites: predatory mites (e.g. Amblyseius cucumeris or A. swirskii for thrips control) and “food mites” that they prey on while in the sachet. This provides the predatory mites with a self-renewing food source, so you get repeated generations of predators emerging from the sachet week after week.

Through some discussions with a consultant that has been in the industry for 25 years (Ronald Valentin, now with Mastronardi Produce), we found out that sachets actually have yeasts and fungi added to the bran carrier inside. This is what acts as a food source for the feeder mites to keep them reproducing over time. Lower the humidity too much – by putting them in an office with forced heating/cooling (typically around 40% RH), for example — and you kill the yeasts/molds, starving the food mites, and in turn, your predatory mites. And voila, all the mites in the sachet stop 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, but they’re also often set below the proper temperature for mite storage. The minimum temperature you should ever store your A. swirskii is around 15° C (59° F) and is 12° C (54° F) for A. cucumeris.

The better place to store your mite shipments is actually in an active compartment in the greenhouse. The higher the humidity, the better; try not to store them below 60% RH. The boxes should be placed 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. If you can’t put the mites out for a few days, you should also open the top of the boxes slightly, as CO2 buildup can also threaten mite survival.

Pitfall No. 2:

Enlarging the exit holes on the sachet.

Although the hole near the top of the mite sachet which lets the mites crawl out onto your plants seems very small (Fig. 3), it’s actually the perfect size to let mites out, but keep humidity in. Although it’s tempting to want to enlarge it so mites may have an easier time exiting the sachet, resist the urge to mess with it, or you may see your mite populations crash.

Pitfall No. 3:

Placing sachets incorrectly on plants.

My colleague in Ontario examined optimal mite sachet placement on plants. Dr. Rose Buitenhuis, senior research scientist at the Vineland Research and Innovation Centre, found some very interesting (and useful results. When mite sachets are nestled within the plant canopy, they are somewhat shaded from the sun (Fig. 4), and this creates a higher humidity “micro” environment within the sachet. Under these conditions, a lot more mites will emerge over a 3-week time period (Fig. 5) than when sachets are fully exposed (i.e. hung on the side of a pot or placed above the plant canopy). When sachets are protected, mite populations will reach peak emergence at around three weeks post-placement, and last for up to six weeks. Conversely, sachets exposed to the greater greenhouse environment only provided enough mites for one or two weeks of thrips control.

Fig. 4. Data showing the number of Cucumeris mites that walk out of sachets after 3 weeks when sachets are tucked into plant foliage vs. exposed. Data courtesy of Dr. R. Buitenhuis, Vineland Research and Innovation Centre.

So just why is humidity so important to mite sachets? We already talked about one reason: keeping the food source for the feeder mites alive (yeasts and molds). However, both beneficial and prey mites also need high humidity for their eggs to hatch. For 50% of eggs laid by A. cucumeris or A. swirskii to hatch successfully, a relative humidity of over 60% is needed. It stands to reason that anything that lowers humidity in the sachet (like the sun beating on it), will immediately lead to fewer eggs hatching and fewer mites walking out. 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 out in your greenhouse. If you need to put sachets out in your crop while the canopy is still small (like in large spring baskets before they’re hung), or in crops that don’t offer much canopy protection (like Dracena spikes), consider using mite sachets made out of foil (like UltiMite sachets; Fig. 6). These are more resistant to both high and low humidity that the paper sachets, which can make them a great tool in certain crops. However, they are more expensive than regular sachets.

Pitfall No. 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 a hard no for any biocontrol agent. But mites can also be affected by newer chemistries as well.

This can include spray applications of the follow insecticides (listed in order of hardest on mites, to more moderate effects):

• Pylon (chlorfenapyr; 1 week persistence time on mites)
• Success or Conserve (spinosad; 2 weeks persistence)
• Kontos (spirotetremat; 2 weeks persistence)
• Landscape oil (mineral oil; 1 week persistence)
• Insecticidal soaps (low persistence; safe when dry)

As well as the following miticides (i.e. for spider mite and/or broad mite), also listed in order from hardest to softest on predatory mites:

• Avid (abamectin; 2 weeks persistence time on mites)
• Pylon (chlorfenapyr; 2 weeks persistence)
• Forbid (spiromesifen; 1-3 weeks persistence)
• Dyno-Mite (pyridaben; <1 week persistence)
• FujiMite (fenpyroximate; unknown persistence)
• Tetrasan (etoxazole; 2 weeks persistence)
• Floramite (bifenazate; up to 1 week persistence)
• Vendex (fenbutatin oxide; <1 week persistence)

The persistence (the time the residual pesticide on the plants is still toxic to the mites) plays an important role in determining when you can reestablish your mites. Although the data here was taken from online compatibility databases run by commercial biocontrol companies, we often find safe time frames to be much longer in real life. I always tell growers to double the persistence time, just to be safe. Or in the case of Success (Conserve) and Avid, even triple it, as we’ve seen mites take over six weeks to re-establish on plants treated with these products.

With all products, avoid hitting the sachets themselves, and always check the viability of sachets post-spray (see more on this below). Be prepared to re-apply the sachets after the persistence period if necessary.  

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, there are often few predatory mites, and more feeder mites in the sachet when you first get them. So it’s important to monitor their actual emergence in your crop over time. The Biocontrol Lab at Vineland has created some “how-to” videos to conduct mite walk-out tests yourself. You can find this helpful video (along with a slew of others) on the ONFloriculture YouTube channel under the “How-To Videos” playlist.

By using mite walk-out assessments, you can work with biocontrol companies to resolve any issues in your crop, and also determine how long sachets last for YOUR crop under YOUR greenhouse conditions. This will help you plan your thrips IPM program accordingly.

By avoiding the common pitfalls outlined above and armed with a handy way to monitor your sachets, you’re on your way to effective thrips control without the need for pesticides.

Sarah Jandricic has been the Greenhouse Floriculture IPM Specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) in Canada since 2014. She also runs the ONFloriculture.com blog, which brings timely information to floriculture growers on pest and production issues.