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Vol. 11(9), pp. 11-16The McAllen International Orchid Society JournalSeptember 2010

Orchid Problems and Pests : Mealybugs and Brown Soft Scales

R J Ferry

This article starts out with some jaw-breaker words, but don't get alarmed: things become more simple as the article progresses. First of all, there are more mealybug and scale species than will be referenced here, but for our purposes the following few representatives will be sufficient:

Family Diaspididae : Armored Scales
Quadraspidiotus perniciosus Comstock -- San Jose scale
Lepidosaphes ulmi Linnaeus -- oystershell scale

Family Conchaspididae
Conchaspis angraeci Cockrell -- mainly California & Florida (!) on orchids

Family Pseudococcidae : Mealybugs
Planococcus citri Risso -- citrus mealybug
Pseudococcus fragilis Brain -- the citrophilus mealybug
Pseudococcus longispinus Targioni-Tozzetti -- long-tailed greenhouse mealybug

Scale

Fig. 1. Cattleya leaf with oystershell scale infestation. Digital photo DSC_2852a; Mon-25May09.

Shortly after they permanently attach themselves to the plant's vulnerable surfaces, both scale and mealybug insects, feed on the vital sap of the leaves and stems of the orchid plant. The initial appearance of the scale pests normally shows a few minute translucent "turtle-shelled" objects, usually on the leaf's underside (abaxial) surface. Unchecked, these will soon develop into veritable patches of the small shelled objects (Fig. 1), which--on close inspection may completely cover particularly vulnerable areas such as the eyes from which new shoots develop, and other particularly tender plant parts.

The floral rachis, the peduncles of individual flowers, and the flowers themselves may all be feeding grounds. Unchecked, one is presented with weakened, disfigured, or deformed flowers, and infestation can result in severe weakening and ultimately death to affected plants.

In the short term, continuing to leave these pests unchecked results in leaf damage, the visual indication of which of is a characteristic series of small brown pits in the leaf surface. The cumulative effect of this infestation can easily kill off new shoots and entire leaves and, with time, the entire plant can be rendered unable to photosynthesize. In addition, ancillary consequences of a scale infestation may be the growth of molds and attraction of scavenger insects to the sticky honeydew excreted as waste by the insect. Particularly within the generally closed environmental conditions of a greenhouse, the appearance of a scale infestation must not be taken lightly!

Fig. 2. Individual scale insect, dorsal aspect. Photomicrograph, insect ca. 2mm long; Tues-04Aug09.

The individual mature scale insect exhibited (Fig. 2) was slightly less than 2mm in length, but when magnified, one can see indications of the legs beneath the insect's shell-covering. The adult scale insect travels little or not at all. Eggs are hatched and the motile young exit from beneath the adult's shell, mate, develop their own protective shell, and continue their parasitic ways.

Fig. 3. Individual scale insect, ventral aspect, note egg masses upper right area. Photomicrograph, insect ca. 2mm long; Tues-04Aug09.

As they "glue" themselves to the leaf surface, it's not an easy task to flip one of these insects over for examination. One method is to take a very sharp razor blade and gently slide it under the affixed insect. This causes some damage, but at least one gets a look at the insect's underside (Fig. 3).

So much for a close look at a scale insect. Now some details. Reproduction may be bisexual or parthenogenetic. Some species produce eggs while others give birth to living young. The eggs are laid under the scale and the first instar young, or crawlers, are active insects and may travel some distance. These are able to live for several days without food. At this stage, a species is spread either by locomotion of the crawler itself or by being transported on some appendage of a bird or another insect (e.g. cockroach, grasshopper). Eventually the crawlers settle down and insert their moutparts into the host plant. The females remain sessile for the rest of their lives. Outdoors, some species overwinter as eggs under the scale of the female. One point: because one is culturing orchids, doesn't preclude scale being transported from some other plant!

Mealybugs (Family Pseudococcidae)

Fig. 4. Mealybug on Cattleya leaf. Digital photo DSC_2822a; Thur-21May09.

The name "mealybug" is derived from the mealy or waxy secretions covering the bodies of these insects. The female's body is elongate-oval and segmented and has well-developed legs. A colony may exhibit an insect or two, or have most covered with their cottony shelter (Fig. 4).

Fig. 5. Mealybugs and egg cases on Coleus leaf. Digital photo DSC_2841a; Mon-25May09.

Fig. 6. Mealybugs and opened egg cases on Coleus leaf. Digital photo DSC_2845a; Mon-25May09.

The photographs (Fig. 5) (Fig. 6) of a Coleus leaf infested with mealybugs shows individual insects and a couple of egg cases burgeoning with eggs. Mealybugs can move quite rapidly, and particularly so when exposed to intense light used for photographs.

Mealybugs and scales are closely related insects but whereas female scale insects are sessile, mealybug females move about freely. Scale eggs hatch under a shell from which the juveniles exit and are motile. Mealybug juveniles hatch within a free-standing casing and are sufficiently motile to be found on almost any part of the host plant. For all their differences, both share the common trait of being plant parasites. Orchids and Coleus are by no means, their only host plants. The San Jose scale first appeared in California about 1880, probably from the Orient, and has since spread throughout the United States. It attacks a number of different trees and shrubs, including orchard trees, shade trees, and ornamental shrubs, and when numerous it may kill the host plant.

Management

Over the years several insecticides of various potencies have been tried; all with some degree of success, but with the shortcoming of having to return and respray in order to contend with eggs that subsequently hatched, juveniles missed by being protected under a shell, or ones missed by contact with the spray. One needs to attack both the adults and juveniles as well as the eggs!

Some weeks ago, in contact with a Florida orchid grower also having problems with scale and mealybugs, your editor decided to experiment with employing a few different chemicals mixed together as a "cocktail."

The Bayer Company puts out a product designed to be poured around the dripline of a tree. This systemic product contains iminocloprid; a growth regulator. Another product (a water soluble powder) on the market is Acephate. This is an Orthene chemical that has proven to be highly effective when sprinkled on fire ant mounds. It was decided to try a mix of these two chemicals.

Fig. 7. Containers: Bayer Tree & Shrub, 75% Acephate, and Consan 20. Digital photo DSC_3762; Fri-10Sept10.

A one-gallon pump-spray tank was bought and labeled for use only with this chemical concoction. The initial work was done with adding one level teaspoon of Acephate power of 97% strength, but Acephate of 75% was more readily available locally, so to one gallon of water was added one-and a third level teaspoons of this Acephate powder, along with one level teaspoon of the Bayer Tree and Shrub chemical. Then, in order to have the mix adhere to the leaf's surface, a slightly less-than-level teaspoon of Consan (a fungicide) was added (Fig. 7).

This "cocktail" was sprayed on an orchid heavily infested with both scale and mealybugs. Normally one would need to re-spray in a week, and also in another week or two to hit any juveniles or other pests missed, but this plant was given no further treatment. It was hung with others, in the patio shade-house area. It was given no further treatment, but was inspected periodically.

To be sure, the "cocktail" coped well with the adult parasites, but what about the eggs and juveniles? Here the iminicloprid (growth regulator) came to the fore. It halted egg development and juvenile growth, and addition of the Consan not only acted as a fungicide (a fringe benefit), but caused the "cocktail mix" to stick to the leaf's surface to provide a coating which would last until the next good rain shower or watering. To say this one-time spray was effective is an understatement! Two months later, this orchid is flourishing with no sign of scale or mealybugs! The cocktail mix took care of the parasites on the plant, and, as some of the spray went onto the bark-substrate and soaked in, it acted on pests that may have been within the substrate not just on its surface.

As with all work with chemicals, follow the directions and keep your tools clean! This "cocktail" concoction may not be the "cure-all for every orchid pest, but it's worked here, and all constructive suggestions are welcomed.

Copyright © 2010 R J Ferry