|My Dwarf Alberta Sruce have brown needles and are defoliating. I have been treating them once every two weeks for spider mites with a pesticide I purchased from a local nursery. I have also been treating the soil with Miracid every two weeks. They are still withering away.
One tree is completely defoliated and a few days ago I noticed that the tree next to it has started to defoliate.
What can be done to save these trees?
What is an appropriate maintenance/preventive regimen for these conifers?
First off you need to be sure your plants actually have spruce spider mites. An easy way to check is to hold a white piece of paper under a live branch and tap the foliage: tiny specks should fall to the paper and then move. You can also see them with a hand lens (they are barely visible to the naked eye), check the underside of the needles as well as the top. If you have any doubt, you might wish to contact your County Extension for a positive identification of the pest and suggested controls.
Spruce spider mites are most active in the cooler weather of spring and fall. To understand how to control them, you need to understand their life cycle. The mites lay eggs in the fall. The eggs overwinter under bud scales or at the base of needles and hatch in spring. The larvae, nymphs and adults feed on the needles and sometimes twigs. Successive generations are produced every two or three weeks until autumn -- hence the fast and drastic build-up. The infestations tend to reoccur from year to year.
Often, damage inflicted in fall and spring goes unnoticed until the following summer. The first indication of a problem is stippling and distortion, increasing to browning and dropping needles, next you might see a silky webbing on the underside of the branches; the webbing eventually becomes quite noticeable when it catches up dead needles, dirt and debris. A bad infestation can kill branches or the entire plant.
It is very difficult to control spruce spider mites. (Even if you used a miticide before, it is possible you missed unhatched eggs or molting larvae and nymphs which would also be unaffected by the chemical.)
Horticultural oil and dormant oil can be effective in suppressing the mites if applied properly. You must apply any spray to thoroughly wet the foliage and try to contact as many mites as possible and be especially sure to cover the undersides. In other words, thorough spray coverage is absolutely critical. In addition, you must be careful to follow the label instructions.
Horticultural oil is used when the mites are active, and dormant oil is used to kill the overwintering mite eggs. It would be applied when the weather turns cold -- in late fall after there have been several light frosts or in late winter or early spring (before bud break).
When the infestation has been so severe it is worthwhile to continue to monitor the plants closely even after applying the dormant oil. Unfortunately, you might also consider removing or at least isolating the worst-affected plant(s) to cut back on possible sources of infestation next year.
Finally, healthy plants have a better chance of resisting infestation. These dwarf trees prefer a moist yet well drained, sandy, acid soil. They resent winds, reflected heat (such as that from a driveway or building), hot/dry weather and crowding. As a general guideline, you might also wish to perform some basic soil tests to determine which, if any, soil amendments or fertilization might be needed. Your County Extension can help you with the tests and with interpreting the results.