This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The following article is offered in competition for the $25 special prize offered by Peter Henderson, for the best essay on prevention and destruction of insects under glass:
There are few of the insects that attack plants grown under glass, but such as are to a greater or less extent injurious to the gardener; but in this, as in all other ills that affect plants, prevention is of more importance than cure. One of the most common insects attacking plants is the Aphis or Green fly. In any well regulated greenhouse this should never be seen, for a complete prevention is tobacco, either in solution made by steeping the refuse steins in water until of the color of strong tea, and syringing it on the plants twice a week, or in the form of dust or snuff, which after syringing may be dusted on the plants: or by burning the dampened stems twice a week, in quantity of, say (1) one pound to every 1000 square feet of glass. This last is rather the best method where greenhouses are detached, but when the conservatory is attached to the dwelling, either of the two former will answer.
The next best known enemy to plants under glass is the Red Spider, always found in greatest abundance in a hot and dry atmosphere. When a good force of water is obtainable, so that plants can be syringed by hose, there is little excuse for this insect, for it cannot exist to do much injury where leaves can be forcibly washed by water. When there is not sufficient head of water to syringe by hose, the next best thing is some form of garden engine or hand-syringe, in addition to which the paths and under the branches should be copiously watered so as to counteract the aridity of the air consequent from tire heat. The fumes of sulphur is also destructive to red spider, but this can only be safely applied by painting the hot water sides with sulphur, or if the greenhouse is heated by flue, it is only safe to do so at the cold end, for if sulphur is volatilized in a temperature much higher than 200° it will injure the leaves.
The next insect in importance as injurious to plants under glass is the mealy bug, a pest which has made rapid increase in nearly all greenhouse establishments, of late years, owing to the immense increase in the growing of plants in high temperature for the forcing of flowers in winter. The usual method has been the tedious one of rubbing the insects off with a soft brush; and until recently the scores of substances with which we have tried to destroy the insect either failed to do so, or else in ruining the insect they destroyed the plant. I say until recently, for there is now a certain remedy known as " Cole's insect exterminator." This put on with a Barber's atomizer at once kills the mealy bug, without injuring the most delicate or tender plant. The serious objection to the " exterminator" is its price, which is upwards of $2.00 per quart. A quart, however, is sufficient to go over at least 1000 square feet of plant surface, as it is thrown out by the atomizer in spray fine as mist.
The Black Rust on verbenas, heliotropes, petunias, etc, is caused by an insect known as the verbena mite, too small to be visible without a microscope. This, like nearly all other parasites that attack plants, is rather a consequence than a cause of disease, for we find whenever plants are neglected, by being pot-bound, or by insufficiency of water, or any other cause that lessens the vigor of the plant, it is more likely to be attacked. It is thought that the fumes of sulphur, given out by painting the sides, is destructive to it, but of this we are not certainly assured. There is but little doubt that this insect spreads quickly, and it is safest when plants are affected to at once throw them out, for if seriously affected they rarely recover.
Ants are often troublesome in greenhouses. A simple remedy is to steep pieces of bread or sponge in some solution of sugar. They will leave everything else for that, and soon thousands of them may be thus caught and destroyed.
Snails are often destructive, as they usually keep under the benches during the day and come up to feed on the leaves at night. I have found that salt strewed along the edge of the table was a complete barrier to their getting at the plants from below.
The Thrip is a troublesome insect, appearing in quite a number of varieties on different plants. The same directions may be given for its destruction as for that of the red spider, though in such plants as cannot be reached by the syringe there is nothing for it but sponging the leaves, or else in using the " exterminator" with the atomizer, as is done with mealy bug.
The Carnation Twetter, - so-called for want of a better name, - is an insect but little known, as its ravages are often ascribed to red spider or thrip, but it has no resemblance to either. Its presence is indicated on carnations or pinks (we have never seen it but on plants of this family), by the end of the shoot having a slight curve or twist. If this twist is carefully unfolded, the insect will be seen varying in length from the sixteenth to thirty-second part of an inch, and as thin as the point of a fine needle. It is either green, yellow, or black, according to its stage of development. In many places it has completely destroyed, year after year, the whole crops of carnations. It is believed to be fostered by growing the plants in too high a temperature, as we find it makes but little headway under 50°. I can give no remedy except the general one, to stimulate by manure water, or by any other means plants so affected into a vigorous growth, so, as it were to outgrow its ravages.
I now come to the Rose Bug, the insect that of all others is of interest at the present time, owing to the vast quantities of roses grown under glass, but as the subject has been recently so thoroughly discussed, I can say but little new in the matter. The rose bug, so-called, is in the perfect insect about the size of a large house fly, of a leaden-grey color, resembling in appearance somewhat the curculio that attacks the plum tree, but larger. It feeds on the leaves of the rose plant for probably a month in this stage, and then goes down and deposits its eggs in the soil. These in time develop into maggots, which at once begin to feed on the roots of the rose plant. This is its destructive stage, and a rose plant cannot long retain its vigor while this pest is sapping at its vitals. When the insect is in the maggot stage, it is believed there is no remedy. I have tried to kill it with a dozen different things that are usually destructive to insect life, with no effect whatever. The only remedy then, is to catch and kill the perfect insect on the plant. Professor Riley has suggested that folded strips of paper be inserted close to the stem of the plant in the soil, as he has discovered that the insect deposits its eggs in the rough bark near the surface.
This I tried, but so far with no results, as no eggs were found in the traps so set. There is reason to believe that many failures of roses to do well is in consequence of this pest at the roots. When this has been ascertained to be the case, there is no remedy but to take them up at once and throw out every particle of soil wherein they have been grown. It is easier to manage the pest when roses are grown in pots, then, if affected, there is no necessity to sacrifice the plants if taken in time. Some of the most extensive rose growers grow their roses in pots, so as to have full control of the rose bug. Their system is this: After growing the plants in the floweriug pots during winter, they are taken from the greenhouses in May or June and stood outside and kept rather dry until August; the ball is then shook and every particle of soil rinsed or washed from the roots. This, of course, carries off every egg and insect also adhering to the roots, and the plants are potted in fresh mould and are then entirely clear of the pest.
I have seen many thousands of plants so treated that are now (November) growing luxuriantly and flowering in greatest profusion.