In recent years, mites have become major pests of many plants, including the following:
One reason for this is the widespread use of newer organic insecticides such as DDT, heptachlor, aldrin, and dieldrin.
While these chemicals give remarkable control of insects, they also control the natural insect predators of mites and thus indirectly help to increase the mite population.
There are, of course, many other reasons for the increase. For example, mites can infest plants that have not been sprayed with newer insecticides.
Even the nutrition of the plant may influence its prevalence. One investigator noted, for example, that three times more mites developed on rose leaves low in nitrogen than on leaves high in this element.
Plant-Infesting Mite Groups
Plant-infesting mites fail roughly into four major groups. The largest, and the one I shall discuss in the greatest detail, is the spider mite group.
In it are included the following:
- Common red spider spider
- The spruce mite
- The European red mite
- The Pacific mite
- The oak mite
- The boxwood mite
Other groups not covered at this time include the cyclamen mite, which is so common on cyclamen and delphinium, the bulb-infesting mites, and those which produce galls on leaves and stems.
Mites are not true insects. They belong to that class of animals, including scorpions, spiders, and ticks, all of which have four pairs of legs. True insects have but three pairs.
Most mites vary from 1/64” to 1/32” of an inch in length. They may be green, orange, red, brown, or black in color.
The life cycle of most spider mites is pretty much the same.
Winter is passed around leaf scars and the rough and protected places on the smaller twigs in the egg stage.
Eggs are 1/64” of an inch in diameter, oval in shape, and pink or red. During the summer, eggs are deposited on the leaves, chiefly on the lower surface.
Over-wintered eggs usually hatch in late April or early May. The young mites feed on the leaves by sticking out the juices.
In a cool spring, the complete life cycle takes about 3 weeks, and in slimmer, it takes from 10 days to 2 weeks.
Each female may lay 75 to 100 or more eggs. In mid-summer, as many as 500 eggs and mites may be found on a single rose leaf.
Injury caused by spider mites is rather distinctive, and one soon learns how to distinguish it from that caused by true insects.
The green coloring matter, chlorophyll, is destroyed wherever the mites insert their mouth parts to suck out the juices.
This gives the leaves a stippled or mottled appearance. At first, the leaf color is grayish green, and then it turns yellow and finally brown.
Mite injury is not discernible in May, even though mites may be present.
It becomes more evident as the season advances, the mite population increases, and the weather becomes warmer and drier.
This is why it is essential to learn how to spot mite infestations early and to apply the proper spray in time to keep injury to a minimum.
Most species of mites are so tiny that a magnifying glass is of great help in detecting their presence.
If one is unavailable, collect a small branch of the plant suspected of having mites and tap it vigorously onto a piece of white paper or cardboard.
If mites are present, they will fall on the paper and will be visible as tiny, moving specks against the white background.
Easy Mite Control Method
Pesticide manufacturers have gone “all out” to develop mite-killing chemicals during the past several years.
As a result, we now have several acaricides that are far more effective than sulfur dusts and lime-sulfur sprays used formerly.
The four best known mite-killing chemicals are the following:
Aramite comes as a 15% percent wettable powder and is used at 2 level tablespoons in a gallon of water.
It destroys the immature and mature mites but not the eggs. Hence it must be used a second time about 5 to 7 days after the first application.
Aramite is relatively non-toxic to humans and pets. It is compatible with the following:
- Lead arsenate
Dimite is a liquid and is usually diluted one teaspoon for each gallon of water. It is not toxic to humans and has a long residual value against mites.
It is compatible with the following:
- Lead arsenate
It has been instrumental in controlling cyclamen mites on delphinium.
Malathion comes as a 4% percent dust, a 25% percent wettable powder, and a 50% percent emulsion.
The 4% percent dust can be applied directly to susceptible plants. The dilution rate for the 25% percent wettable powder is 4 level teaspoons in a gallon of water, and for the 50% percent emulsion, 2 level teaspoons per gallon.
Because it controls many other pests, most gardeners usually prefer malathion.
It is relatively safe to use provided directions on the container are followed carefully.
Malathion is compatible with the following:
Ovotran is perhaps the most effective acaricide of all because it destroys mite eggs in addition to the mites.
It is especially recommended to control the spruce mite on conifers, particularly on arborvitae, cypress, fir, hemlock, juniper, and spruce.
It is more toxic to certain plants than the other mite-killers discussed. In addition, it will injure the tender foliage of buds and certain plants in early spring.
It should never be used before June 15 on the following plants:
- Black raspberries
- Flowering fruits
Ovotran is the preferred treatment after the middle of June, when only a single application can be made.
It is compatible with the following:
- Lead arsenate
- Lime sulfur
- Bordeaux mixture
Iris is usually considered relatively resistant to insect and disease attacks. There are times, however, when certain pests give trouble.
Borers (pinkish caterpillars 1 1/2″ inches long at maturity) are the major pests.
Inexperienced gardeners rarely detect them before they have hollowed out the rhizomes and caused the leaves to wilt.
The borers get started in early spring after hatching from eggs deposited by moths on old iris leaves the previous fall.
At first, the tiny borers crawl up new leaves, bore in, and then tunnel down inside the leaves and finally into the rhizomes.
Spraying or dusting once a week, starting when new leaves emerge, with a mixture of DDT or methoxychlor with zineb, gives good control of borers.
Associated with borers is a soft rotting of rhizomes. The decay is caused by bacteria that enter through wounds made by borers and other agents.
The disease is always most severe where the rhizomes are planted too deeply and where air circulation is poor.
When soft rot appears in an iris planting, drastic measures must be taken.
Here’s what you need to do:
- First, severely rotted divisions should be destroyed.
- Choice ones may be salvaged by cutting out the rotted areas and thoroughly drying the sound portions before replanting.
- As an additional precaution, the salvaged roots should be soaked for 30 minutes in a bichloride of mercury solution, prepared by dissolving one tablet of the poison in one pint of water.
- In replanting, allow the “back” of the root to remain above the soil.
Spotting Leaves Caused By Fungus
A fungus causing spotting of the leaves is more unsightly than destructive.
As a rule, removing old infected leaves and cutting back the new leaves to 5” or 6” inches is sufficient to keep this disease in check.
Where the spotting is severe and persists despite the above practices, one should resort to frequent sprayings with zineb until the leaves are fully grown.
Good drainage is essential for the bearded iris. They should never be planted in a low, poorly drained spot.
If one must do so, it is wise to remove the soil to a 12-inch depth and replace the lower 6” inches with coarse cinders, gravel, or broken crockery before filling the remainder with good soil.
This practice will help to keep standing water away from the roots.