Chapter 10

PESTS AND DISEASES OF ORCHIDS

The damage that can be done by pests and diseases in an orchid greenhouse is so appalling that no effort should be spared to pre­vent their initial entrance. The first line of defense is a roomy, airy greenhouse, in which balance is scrupulously kept. The grower who inspects plants frequently should be able to halt an invasion of pests before it gets under way, while the preservation of plants and the immediate segregation of sickly ones prevent pests and diseases from gaining an easy foothold. The successful grower is a good housekeeper—picking up dead leaves, removing dead plants, and isolating sick ones. Adequate bench room for each plant is also an advisable precaution. The greenhouse can be kept clear of pests and diseases only if the grower recognizes signs of trouble and is acquainted with effective means of control.

There are a few merely bothersome pests whose appearance need occasion no alarm, although the grower may wish to be rid of them. Ants and fruit flies are the most common members of this category. Many growers dislike ants. In the jungle ants have friendly relations with orchids—stinging ants often preventing human desecration of the plants—but in the greenhouse ants are unsightly and unnecessary. They have never been proved guilty of damaging orchids, but as sometimes they live in co-operation with the mealy bug and may assist aphis, it is just as well to prevent their entrance into the greenhouse. Ant-proofing outside the house is the safest and simplest method of dealing with them. Fruit flies are often found in the greenhouse. They feed on decomposing Os-munda and, although at times difficult to distinguish from the scale fly, they do no harm. Routine spray will dispose of them. Thousand-legged insects, mites, and sow-bugs are occasionally found in the greenhouse but apparently do no harm. The tiny plant lice that thrive in decomposing potting materials are harm­less though disturbing to a grower's sense of neatness and order, and almost impossible to eradicate. Where possible, garden snakes, toads, frogs, and salamanders should be encouraged, as they feed on pests.

The coming of spring and warm days brings thrips, red spiders, and aphis.

The maintenance of humidity in the greenhouse dur­ing this period will not only keep plants from being burned, but will also control these pests as they are susceptible to pneumonia. Such control is especially important because these pests are ex­tremely small and may do damage before they are seen. Nearly all flower-lovers have seen thrips-marked gladiolas; the petals and leaves of orchids attacked by thrips are disfigured in much the same manner. Red spiders leave ugly rust marks, particularly ap­parent on leaves of the Cymbidiums, and they can kill Miltonias.

In the past, systematic spraying with rotenone, pyrethrum, or nicotine provided a fair control for thrips, red spider, and aphis. Some growers even burned nicofume (a poisonous gas, difficult to handle) two or three times a year. In spite of all precautions the pests seemed to persist.

Since the war, what penicillin and sulfa drugs have done for the medical profession, D.D.T. and other insecticides have done for the orchid grower. Once a considerable problem, pest control has been made simple. While D.D.T. has not proved specific for thrips, red spider, and aphis, as it has for scale, O.O. di-erhyl-o.p. nitrophenyl thiophosphate, a poison sold under the trade names of Parathion or Orchid-thion and requiring caution in handling, and Tetra-ethyl pyrophosphate or Vapotone, also requiring deli­cate handling, are most efficient controls.

Any form of extermination should be repeated at intervals, for, although the first application kills flying pests, the dormant stages are rarely touched and must be killed at maturity. D.D.T. has the apparent advantage over other insecticides of maintaining its effec­tiveness over long intervals—from three to six months.

One of the most deadly orchid pests, but fortunately not very common, is the Cattleya fly (Eurytoma orchidearum). This insect lays its eggs in the tender new leads of the Cattleya. The lead, when the eggs have hatched and the nymphs begin to feed, swells grotesquely and becomes soft to the touch. The nymph finally eats its way out of the lead, emerging as a fly about a quarter of an inch long with a coal-black body and light transparent wings. A plant that has been attacked should be isolated and the injured growth cut off and burned. Careful observation of the new growths should lead to discovery of the condition before the nymph emerges as a fly to cause more damage. Burning nicofume in the house will kill the flies but will have no effect on the larvae. Pest B-Gon (D.D.T. 20%) is reported to have been successful in han­dling severe infestations needing drastic measures.

Another vicious pest much dreaded by growers is the Cattleya or Dendrobium beetle (Diorymellus laevimargo), which does not, incidentally, limit its operations to the species for which it is named. This hard-shelled, shiny black beetle, about a sixteenth of an inch long, has a curving beak with which it attacks flowers, buds, and leaves. It lays its eggs in the new green tips of the roots, where the larvae then hatch and, by feeding, destroy the root growth. The mature beetle is quick and agile, and when disturbed will usually drop to the potting material, where it is almost in­visible. These beetles are less active in the early morning and late at night, and hand picking may get rid of many of them at these times.

Means suggested for extermination of the Cattleya fly are also effective in dealing with this beetle. Pyrethrum powder is mildly effective and is harmless to the plant. A Tartar-emetic spray (3 ounces Tartar-emetic, 3 ounces sugar, 5 gallons water) gives good results. Before D.D.T. was discovered, growers were forced to have recourse to Paradichlorobenzine for severe infestations. This was a very complicated process, involving the drying off of plants, closing of greenhouse vents, and raising the temperature to 700 F., but, despite all precautions, there was frequent damage. D.D.T. 50% wettable is preferable when strong measures are needed.

Although not so destructive (except in cases of neglect) nor so difficult to discover as the beetle and the fly, scale is one of the most persistent threats in the orchid greenhouse. No house seems entirely free of it and only constant treatment can ward off serious damage. There is a bewildering number of types of scale, some thirty-three having been intercepted at the U. S. Quarantine De­partment in San Francisco in 1941 alone. The type most common to orchids is Cocciidai diaspis Boisduvalli, which attacks Cattleyas and sometimes Oncidiums and Cymbidiums. Parlatoria proteus, a scale attacking Vandas, Cymbidiums, and Cypripediums, causes leaves to die back at their base. Coccus pseudohesperidum, black and soft, attacks Cymbidiums, Odontoglossums, and Cattleyas. A light armored scale, Pseudoparlatoria parlatorioides, may heavily infest Cypripediums, causing the leaves to die back. Pulvinaria Phaiae, 'cottony orchid scale,' attacks Phaius, Phalaenopsis, Calan-thes, and Odontoglossum. Although not so destructive as some scale, it is disfiguring in that it turns leaves soft and yellow. There seems to be a particular type of scale for practically every species of orchid, but fortunately control measures are the same for each type.

The males are a flying form and can be killed by spray, but the females, which are hard-shelled will not be affected by it, and partlienogenetic broods can be hatched that do not require the male to carry on the line. The eggs hatch and the nymphs develop under the female's armor. The males pass through a quiescent cocoon stage, often collecting in colonies under a powdery cover­ing (sometimes mistaken for mealy bug) on the under side or in the apex of a leaf, in the dry tissue of the bulb, or, more rarely, at the base of the bulb around the new growth and dormant eyes.

When the males emerge from their cocoons they go on a mating flight, during which they do not eat; afterwards they disappear. That the nymphs

can crawl about, together with the free flight of the male, accounts for the extremely rapid spread of scale.

Unremitting attention is necessary to keep scale in check, al­though amateurs should be warned that some cures can be as bad as the pests and only proved methods should be used. Before the advent of D.D.T., spraying with rotenone, Extrax, or Wilson's O.K. Orchid Spray was found fairly effective in controlling scale. Destruxol was used but because of its oil base, needed caution; some orchids, especially Miltonias, are extremely susceptible to oil. Spraying, however, was not sufEcient to keep the scale infestations down. The use of a toothbrush was required to loosen the adult scale, which is protected by a heavy armored shell, under which it is impervious to the spray. The cottony deposit of young scale would build up in the axil of the leaf, and it was necessary to loosen this by the aid of some sharpened instrument, or stick of bamboo.

Prior to the development of D.D.T., keeping scale in check was the most that could be hoped for, but the future seems to offer the prospect of complete eradication. Its advocates claim that it will control all forms of the pest. After two or three applications, the quarantining of new plants until treated will make it unneces­sary to spray more than once in six months.

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