Pests -- Spider Mites

 Mites are not true insects. As adults the have 8 legs and are actually related to spiders and daddy longlegs. They may be one of the most damaging agricultural pests. One reason is that they're nearly impossible to see so you don’t know you have spider mites until it is a serious problem. In fact, they usually appear as no more than specks of dust the size of the period at the end of this sentence. Spider mites are not only hard to see, they're nearly impossible to get rid of. If you have spider mites you will notice yellow dots on leaves, curled or distorted plant leaves, which is caused by the mites' piercing mouth parts. Sadly, most growers notice the webbing between leaves and other plants. This signals you have a serious infestation.

Like white flies, spider mitess life cycle is temperature dependent. A female lays about 100 eggs, but under 60 F she will do it over two months. At 70F there will be over 10,000 mites after a month, and over 80 F there will be over 10,000,000 mites in your grow area.

How to deal with mites – First, If you lower the temperature even just during the night cycle it will help. You can submerge plants under water as I mentioned in a the general pests post or spray the plant with insecticidal soap. Misting your plants often with just water also helps, since mites thrive in a warm and dry atmosphere. The higher the humidity the slower their metabolism and life cycle. The good thing is mites can’t fly. Look and see if some plants are not infested, or if you can see some that are only lightly infested vs. heavy infestation. (webbing is a heavy infestation). Since mites walk everywhere, you can put double sided tape around the plants to prevent mite migration. You may need to trim your plants so that the mites can’t walk from leaf to leaf, or leaf to wall to leaf etc. Mites can blow in the wind so keep that in mind if you have air flow (Which you should for optimal plant growth).

As with white flies there are predators of spider mites, there are predator mites. Unlike predatory wasps of white flies predator mites are not so efficient, since being mites they can’t fly. It is a non-chemical option, and I know many of you have asked for suggestions for that type of control. The type of mite that infests greenhouses is called the red mite or the two spotted mite Tetranychus urticae . The predator mite Phytoseiulus persimilis feeds on all stages of the two-spotted mites and is the most commonly used predatory mite in greenhouses. It does well under humid conditions so if your plants can tolerate high humidity this may be the choice for you. There are other species of predator mites, each has its own environmental preferences. You want to try and match up your grow environment with the best predator mite

If you are using a chemical you should know many miticides work by coming in direct physical contact with the mite.  This means you need thorough coverage to the underside of the leaves where some spider mites are feeding and most lay their eggs. Often eggs are more tolerant to many miticides so repeated applications are often needed.

Ultimately how you deal with mites depends on your growing situation. If you have prized plants like bonsai you may want to try chemicals several times. If you are growing plants to harvest you may want to keep the humidity as high as possible and the temperature as low as possible until your plants are done, then sterilize everything with alcohol, bleach or peroxide. If you've tried everything and you can’t get rid of the mites, you may have to kill most plants and clean out the grow area and start new. Odds are the older plants are a source of mites spreading to younger plants so you may have to get rid of the oldest plants.   Two years ago when I had a spider mite infestation I finally got rid of them at the end of the semester when I killed most of the plants and only kept a couple plants which I cleaned thoroughly two times with a 10% isopropyl alcohol mixture. Each time I carefully cleaned the plants I moved them to a room that did not have plants in it previously. Two years mite free!

If you or someone you know has mites, keep in mind mites can be spread easily on your hands and clothing (they even blow around with puffs of air). When it comes to spider mites, prevention is far easier than the cure!

Good Growing,

Dr. E.R. Myers

Pests -- White Flies

White flies may be one of the most common greenhouse and indoor pests. This tiny little white insect sucks plant juices for meals and can be seen flying off the plants when they are disturbed. They lay their white eggs on the underside of leaves, and their larva are green and almost invisible to the naked eye. White flies under go several stages of development each called an instar. (Similar to how a caterpillar turns into a butterfly). The larva do damage as they eat the plant too. If white flies are left unchecked indoors they can KILL your plants. I’ve read that some people say white flies can’ kill your plants, don’t believe it. Example, they secrete a substance like aphids which is often called honey dew. (See picture at bottom of post) This shiny sticky substance can be seen and felt on the leaves and is a great environment for molds to grow which also harm your plants. So, if the whiteflies don’t suck your plant to death, they make an environment where mold can harm/kill your plants.
A very important thing to know about white flies is that their life cycle is regulated by temperature.  In general, as the average temperature increases from under 60F to over 85F the number of days in a white flies life (egg to adult) decrease from 100 to 20 days. Below 60 degrees Fahrenheit an adult white fly can live for two months, but over 80F the adult fly will live just over a week. Egg production is also regulated by temperature, at lower temperatures females can lay over 300 eggs in its lifetime. With high temps the number of eggs drops and the average eggs per life cycle is around 30.
If you are into non-chemical cures the interesting thing about white flies is that there are predator wasps which eat the white fly larva. (No, they won’t sting you they are too small). You can find many suppliers if you do a web search for Encarsia formosa, the scientific name of a solitary wasp that preys upon white flies exclusively. Adult wasps eat the eggs and first instars and lay their own eggs in the third instar of whiteflies. This causes the green larva to turn black. In fact whey you buy these predatory wasps they are sold in larval form inside white fly larva that are glued to paper. Encarsia Formosa is also regulated by temperature, perhaps a good example of coevolution. Below 60F degrees it takes almost a month for an egg to reach adulthood. Above 80F it takes 10 days. Below 60F adults live a month above 80 they live just over a week. The number of eggs laid by a female remains at about 30 regardless of temperature, she just lays them faster the hotter it is. Most companies that sell the white flies say you should release them three or more times over a time period. This would allow several generations of white flies to be parasitized. I did not do this, I released them all at once and cleaned out the grow room when the plants were done flowering.

From my experience when I used Encarsia formosa it works to keep the white flies in check, but did not completely get rid of them. For a month, there were almost no white flies. Then they came back, I think because the wasps died of starvation before they found the last few eggs and white fly instars. I could have ordered more wasps but it was the end of the semester so I just harvest/killed all the plants and cleaned out the greenhouse. The ultimate solution to any infestation is you end up killing many plants and hand washing every inch of the plants you keep... A lot of work and not much fun.

Pyrethrum which is sold by HTGSupply.com  works well on white flies, but I am hesitant to use any chemical on plants I consume. If you are growing flowers/plants for show or decoration pyrethrum would be a good choice. You should know the predator wasps are more susceptible to chemical insecticides than white flies so you can’t use them if you have already tried a chemical insecticide. White flies are also attracted to yellow so HTGSupply.com sells yellow sticky tabs that white flies stick to and die. This may help keep down the number of flies but I think is better used for early detection of white flies or to see if they are reoccurring after a treatment.

In my greenhouse the white flies seem to get in every summer since I started working here 6 years ago. I am not sure if some get through the air exchanger/UV filter or if they catch a ride in on cloths. I have seen them flying around my back yard some years so anything is possible. This year I had a new plant and they went for this hibiscus plant almost exclusively. I had to snip off all the leaves and carefully threw them away to get rid of the eggs and larval instars. The bad thing is that a white fly female can fly around and land on any other plant in the room. The other plants I do not eat and are perennial and they will be in the greenhouse every summer. So, I resorted to a systemic chemical insecticide that you water the plants with so it gets into the leaves and keeps insects from completing their life cycle. This is not something you can do with plants you consume. I will never ever reuse the soil in these plants and I will have to wear gloves when I pot up the plants. But, I won’t have to worry about white flies getting into my experimental plants -- I am testing out a LED and 600 W HPS from htgsupply.com.  Also, I am betting next summer will be my first with out a NEW infestation. I’ll let you know.
Good growing,

Dr. E.R. Myers.