Plant Care Articles

Controlling pests in the garden and on houseplants has typically been a bit of a dilemma for many plant loving aficionados.  Options have most often been a mix of chemical pesticides and clumsy physical processes like washing leaves with dilute soap solutions.   While these methods are effective for controlling unwanted pests they also harm beneficial insect populations and can also harm the environment as well.  When used properly chemical pesticides are an effective option.  Unfortunately, many people assume more is better and over use pesticides.  In a perfect world everyone would follow the instructions but that’s just not the case.  Another option has been a long list of physical measures such as washing plants, picking the bugs off, dunking plants in water, spraying with horticultural oils and so on.  All of these options work to a degree but are normally ineffective unless done on a regular basis.  A good option to chemicals and physical controls is the use of beneficial or predatory insects.  Beneficial or predatory insects are effective because they feed on the pest insects.  In nature pests are kept in balance by different types of predatory species.  We lose those beneficial bugs when we bring plants into artificial environments like our homes or gardens.   Once the beneficial predators are lost pest populations can explode.

Recently, we had just that type of explosion occur in our Botany lab here at Plantsense.  We grow various types of plants in our Botany lab to test sensors and conduct numerous types of experiments.  Everything from house plants to fruit trees finds a home in the lab located inside our office here in San Francisco.  The pesky critters that decided to take up residence inside our cozy lab are the Two Spotted Spider Mite (Tetranychus urticae).  This particular species of Spider Mite is the most common for causing problems for home and commercial horticulturists alike.  Personally, I detest them and they are my least favorite pest to find on plants.  This disdain for these little buggers is mainly because they are so common and difficult to control using conventional techniques.  I have to admit a plant or two has found itself in the trash at my house just because I haven’t wanted to take up the task of getting rid of a spider mite infestation.  Because of the extremely rapid lifecycle and reproduction rate of the Two Spotted Spider Mite, problems can go from non-existent to out of control in a couple weeks.  One adult female spider mite lays 12-20 eggs per day and lives anywhere from 2-4 weeks depending on temperature.  At optimal temps of 80 Fahrenheit it takes 3 days for an egg to hatch and the resulting mite will be sexually mature in 5 days.  Most common pesticides are ineffective on spider mites and the fact that they live on the undersides of the leaves makes it even harder to control using sprays.  Also, using chemicals that are supposed to kill things around the home or office can sometimes be an unacceptable option.  Due to these draw backs we decided to go green and use beneficial insects to control our happy new Spider Mite colony in the Botany Lab.

For the home gardener using predatory mites is a very good beneficial insect option for controlling spider mite outbreaks.  Predatory mites feed on pest spider mites and eggs while not harming plants, pets or people.  Predator insects are raised at insectaries (commercial insect farms) specifically for commercial or home use.  They are purchased on-line or through select garden centers then released on plants.  Predatory mites can be purchased in varying quantities and are typically transported mixed in with corn cob meal to make release easier.  There are generally 4 different species of predatory mites sold to control spider mites. Environmental conditions such as humidity and temperature are the critical factors that determine what species of predatory mites should be purchased.  The four most common species sold are:

1. Phytoseiulus persimilis- Are effective at temps between 70-100 Fahrenheit and 60-90% humidity.  This species is good for using in greenhouses.
2. Galendromus occidentalis- Prefer higher temps of 80-110 Fahrenheit and humidity around 30% to be most effective.
3. Mesoseiulus longipes- Like lower temps of 55-105 Fahrenheit and low humidity around 45%.
4. Neoseiulus californicus- Is the best species for controlling spider mites on house plants.  Works well in a wide range of conditions.  Temps of 50-90 Fahrenheit and humidity between 40-80%.  They move and consume less spider mites then the other species but can be used effectively under a wide range of conditions.

Because it can be a bit difficult figuring out your exact environmental conditions many distributors sell mixes that contain all species of predatory mites.  Releasing all the species will ensure that at least one or hopefully more is right for your particular environmental conditions.  Predatory mites should be released a second time a few weeks after the first to make sure a good population is established.  On average predator mites such as P. persimilis live up to 1 month and lay about 50 eggs in a life time.   Depending on temperature they normally go from egg to adult in 7 days.  Predators do not reproduce as rapidly as the pest mites but can eat up to 5 adults or 20 eggs a day.  Control is not immediate but once populations are established spider mite problems can be kept in check quite well.  We used a mix of P. persimilis, N. californicus and M. longipes in our Botany lab here at Plantsense.

Predators should be released the following way to ensure success with establishing a reproductive colony and have effective control.  Distribution of the marauding mites should be done at a cool time of the day.  Higher temps at release can shock the predators.  Be sure to gently turn the container with the mites for about a minute before release to evenly mix them throughout the carrier agent.  Prior to the release mist plants with clean water to help the corn cob meal and mites stick to the plants.  Gently dust the plants with the mixture being careful to not dump too much in one place.  Cover all the affected leaves and parts of the plants that have webbing or high concentrations well so predators have food immediately when released.  Also be sure to sprinkle a bit at the base of the plants so lower leaves will be gone over as the predator mites move up the plants.  Several weeks after the initial release a secondary one should be considered to ensure control.  Always be certain to monitor populations of pests and predators using a magnifying lens.  Predator species are smaller and do not have two spots on their backs like pests species.  Populations need to be at least 1 predator for every 10 pests.

While control is not immediate like with the properly applied miticides, using predator species has many advantages.  Other than the obvious benefits of not having to use dangerous chemicals around the home, predators offer a more effective long term solution to the problem.  Chemicals will kill both beneficial species and the intended pests.  Eliminating the beneficial species helps to ensure that future outbreaks by the more rapidly reproducing pests will be much worse than the initial outbreak.  More chemicals will need to be purchased and applied when future problems persist.  Getting a predator population established early will help control any future outbreaks requiring less action in the future with chemicals.  Not all bugs are bad so killing them all to get rid of just one species isn’t your best option in all cases.  For the home gardener beneficial insects can be just as effective as chemicals and also provide a green solution to an ongoing dilemma all plant lovers face.