Know Your Bugs: Phytoseiulus persimilis

Since the popularization of the modern greenhouse in Victorian England, non-chemical, greenhouse pest-control, as we know it, has been practiced. It was before modern chemical pesticides were available, and it was at this same time that Phytoseiulus persimilis (common name: Persimilis) became cosmopolitan and the standard for two spotted spider mite control. 


Without the sophistication of our time, the principle of spider mite control then was summarized by the term “pest-in-first.” The principle is simple: early in the season, intentionally introduce spider mites. Once they have established (but long before they get out of control), introduce the Persimilis at a very low rate. Then monitor their ratios throughout the year.

The idea of Pest in First was to inoculate ones crop with pest and predator and maintain balance. You will always have plants, which are food for spider mite; and you’ll always have spider mite which is food for persimilis. The result is you will have spider mite all season, but will never have spider mite at dense-enough populations to substantially damage your crop. 

Pest in First was used primarily in Holland, where it is still used today. We even provide a specialty product to Holland we call “Phyto-combi” which is Persimilis on bean leaves, but with a ratio of (sometimes over) 20 spider mite to one persimilis. These leaves are put directly onto their crops. The ratio allows for spider mite to leave the bean leave and lay some eggs before the Persimilis eventually catch-up. It establishes several generations of Persimilis with just one application. 

Many North American’s are repulsed by the idea of voluntarily putting spider mite into their crop – and there are some crops we would never do this (high tunnel cucumbers or some quick growing tomatoes) because the rate of growth can exceed the rate of spread of persimilis. -The mites are incredibly fast, but if there is food on lower areas they may choose to stay low. There are also some plants or some growing conditions that are so perfect for the growth of spider mite that we avoid the “pest in first” concept. But we know first hand how effective persimilis is because of how we rear them. We get bean plants so loaded with spider mite that they are on the brink of death – literally dripping with spider mite. Then the tiny inoculation of persimilis – which is at a ratio far more dramatic than 20:1 – slowly catches up. Our system ends by collecting the persimilis that voluntarily leave the crop because there are no spider mite left. The beans, that once had browned and were entirely mottled from spider mite damage, will be sending up fresh new green growth at the end.

This crop has just been inoculated with persimilis. The spider mites were in such large numbers that they “drip” off the leaves.
Only persimilis are left on this crop. New growth compared to spider mite damaged leaves.

Our rearing system is proof of their effectiveness. Persimilis eat only* two spotted spider mite, and will eat all of the spider mites before leaving the plants – which they do so by climbing up to the top, piggy-backing on one another to achieve a differential in electro-magnetic field, charge themselves and get carried off by the slightest breeze – hopefully to where there is more spider mite. 


*”Only” in this case means “effectively,” almost all mites are generalist when they have to be. One study found that Persimilis would eat 10% of the whitefly scale it came across compared to true whitefly predators. But, a population of persimilis will not survive without spider mite.

The most common use is to buy pure persimilis adults in a media and sprinkle it directly onto affected plants. If this product is fresh, the adults will be active and hungry and will immediately begin to wipe-out the spider mite. However, because this is pure adults, there is little or no food available for them,  there are no persimilis eggs, or young, and the persimilis adults become stressed. As they are arachnids, they exhibit some similar behaviours and thus the females begin to eat the males. In some cases, when you buy this and it is not fresh, you are getting a bunch of hungry females, but very few or no eggs will be laid, and you’ll be left needing to buy more. 


While I sell the pure adults on vermiculite, I try to talk most growers into using the leaf-product. The typical ratio in our regular leaf product is often less than 10 spider mites to one persimilis. This product adopts some of the “Pest in First” principle – in that it comes with spider mite – but the ratio is so low that the persimilis will either be finished feeding on spider mite when leaf pieces are distributed, or there still won’t be enough spider mite to establish on the crop. In either case, you get both sexes, all life-stages and the immediate spread of persimilis in search of spider mite that may already have been on your crop.


To give you an idea of how successful persimilis is in wiping out spider mite, consider this: In our rearing-system we can pick leaves for the dutch “Phyto-Combi” about one week from the entire series running out of food. We can only pick leaves for our “regular” leaf product about 3 days before finishing. When it finishes, all the persimilis leave the crop, and we collect them. So one week out we’re looking at ratios of over 100:1. Three days out, we start to see 10:1, and only a couple days later there is no spider mite left. 

The success of persimilis is not only from how many spider mite they consume, but also because they will walk along the webbing created by the spider mite (some other predators will not), and their life cycle is much quicker than spider mite meaning you can simply out-breed the spider mite. While spider mites can take one week from egg to egg-laying adult, persimilis will do that in three days.

Unless you are an experienced grower and comfortable with the “Pest-in-First” approach, do not use persimilis as a preventative. For that we use tiny amounts Neoseiulus fallacis or the beetle Stethorus punctillum and – of course – an in-soil predatory mite to prevent spider mites from successfully overwintering in the ground. 

In a large area, where many plants may, or do have some spider mite and the plants are not touching, Stethorus is your best choice – it flies and cleans up spider mites as well as persimilis. But, it is more expensive. So use persimilis when you have spider mite, and you’re able to distribute it onto each infected plant (and those around it for good measure).

persimilis title

For more information, visit our website:

or: Stethorus – The Ulimate Spider Mite Predator

Stratiolaelaps – a MUST HAVE

Biological Pest Control 101

Using Beneficial Insects in your Home Garden


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