I have been growing plants indoors and outdoors. Outdoors in Iceland is always cold for some plants, so I must grow indoors.
This year spider mites have attacked many plants in my conservatory. I tried keeping moist conditions but they are still alive. They devastated almost every bean plant (including winged beans, cow peas, runner beans, and lima beans). They are not like aphids, once they arrive, no hosing fixes the problem, the plant eventually dried and dies within a few days, very quickly. They remain alive well after the plant died. I tried spraying them with soapy water, essential oils, nothing worked.
They arrive, multiply by thousands, create a spider-net like around the stems and leaves, they are very tiny and reddish, just like mites. The leaves dry within days of their infestation. It is way worse than aphids.
What can I do to control them? Is there another plant that could repel them?
in Portugal, sheltered terraces facing eastwards, high water table, uphill original forest of pines, oaks and chestnuts. 2000m2
in Iceland: converted flat lawn, compacted poor soil, cold, windy, humid climate, cold, short summer. 50m2
The best thing for any "pest" in a greenhouse is to be patient, and let nature take care of it. This process can also be facilitated, and that certainly could help in a place like Iceland where insects are dormant for a large portion of the year.
The best spider mite predator that I've found is predator mites. It's important to let the spider mites get so prolific that the predator mites will never be able to eat them all. There will always be eggs left behind. If there are not enough spider mites the predator mites will eat themselves to starvation and then you have the whole problem again.
It requires patience, but if you are patient nature will reward you, the cycle will find a balance, and you won't have a problem with spider mites anymore, yet you are never doing anything to prevent them. This beats the alternative by a WIDE margin. For people like yourself you can order these predator mites by mail if you wish to facilitate the process. There are lots of good places, I'd recommend finding the closest breeder to your location. A good one in the united states is Nature's Control.
Other alternatives are lavender, I've heard of people putting lavender blossoms in the bottom of a spray bottle and letting them soak for a couple days then using this on mites. Neem oil is another solution for spider mites. It is the pressed seed oil from a rain-forest tree. Not really sustainable, especially with all of the embedded energy in distribution, but it is a somewhat natural solution. The oil leaves behind a waxy residue that the bugs don't like the taste of. I'm not sure if it's edible for humans, but it has a funny taste. I wouldn't recommend using it on the fruit but it won't damage the plant if used properly.
A good Neem oil is a fine art. First off you want the pure Neem oil, it should be solid at colder temperatures. The Neem oil that you buy from the garden store is full of a ton of other things that I am not comfortable used, I would only use something that is 100% Pressed Neem Oil. Because it is solid at colder temperatures it is best to apply in a warm spray. Here's the mix for a 3 gallon sprayer:
Heat 2 gallons of water, put bottle of Neem Oil in water so it warms up as well. Put warm water is sprayer, add 1 shot of Neem and one tea spoon of a natural surfactant (like the soapy stuff you wash off of Quinoa or something like coco-wet) per Gallon water. So for a 3 gallon mix, 3 shots Neem, 3 teaspoons sufactant. Then close the sprayer and mix all of the Neem and warm water up. The surfactant breaks up the Neem (which binds together in the water because they are both strong dipoles) and the warm water speeds up the process. Then you open up the sprayer, add the last gallon of COOL water (to cool the spray to room temp so that it does not harm the plants) then spray away.
I really recommend the natural approach as opposed to spraying. If you start spraying you are always going to have to spray. It's a lot of work. If you let nature take place then you won't have to work nearly as hard. But I know everyone is not as patient as I am.
For example in my greenhouse there were aphids EVERYWHERE, you haven't ever seen so many aphids. What I did was nothing, and now there are a ton of aphid destroying larvae (that I think are hover fly larvae?) totally decimating the aphid population. I did absolutely nothing to make this happen other than letting it happen. I actually even ordered bugs from Planet Natural, but they have yet to come. And so even before they arrived nature already provided a much simpler solution. The larvae that I now have didn't cost me anything, the bugs that I'm still waiting for, $400. I think I'm just gonna be more patient next time an "infestation" arises. It's really not an infestation at all, its a bloom of bugs because the system is healthy.
Most people would have seen these worms and probably bombed the whole place for fear they were going to eat all of the plants. I watched what this creature was actually doing, and it was going around gobbling up aphids as quick as it can. And now there are probably 6 of these guys for ever square foot in the greenhouse.
The best way to control the small sap sucking mites is to maintain a high relative humidity in air. Since plant sap is mostly water, sap sucking mites have to suck and process a lot of plant sap to extract enough proteins to fuel their growth. The excess plant sap and sugars get extruded from the mite's body and its water content is evaporated away. But when the relative humidity is very high, the extruded plant sap doesn't evaporate away very fast and the mite has to stop pumping plant sap (and stop growing) or risk drowning in the accumulating plant sap it is extruding. This is why mite populations explode when the relative humidity is low, but they aren't a problem when it is very humid.
Second the humidity. A rh of 70% eliminated the need for miticides in. my greenhouse, even among extremely vulnerable species. Pot growers purportedly call them "the borg" so hold no illusions about elimination vs control.
Hi; Neem oil is the best bet, apply at nite it kills eggs & larva and interups their breeding cycle. Lady bugs released can also control spider mite infestation. neem oil can be used at the same time as it will not hurt the ladybug population. Remember that your ladybugs will leave and head back to california as soon as they are hungry,so use both. Tom