Citrus Pests: (6) Scale Insects.

New Zealand Journal of Agriculture, Volume 58, Issue 2, 20 February 1939, Page 145

Citrus Pests: (6) Scale Insects.

(II) Unshielded Scales.

W. COTTIER,

Plant Diseases Division, Plant Research Bureau.

IN this group the insect is not a softbodied creature shielded by a scale which is separate from the insect. On the other hand, the scale seen is actually the hardened and thickened back of the insect itself, perhaps ornamented with a waxy covering, as in the white-wax scale. The Olive Scale (Saisseiia oleae Bern.) is typical of the unshielded scales (see Fig. 1). Life-history. In New Zealand the females of the species on citrus all produce eggs which subsequently hatch into “ crawlers,” very similar in appearance to those of the shielded scales. As the adult female lays the eggs the soft parts of her body shrink so that the lower surface ultimately comes to rest against the upper interior surface of the scale. The space made by this shrinkage is occupied by the eggs, so that should an old adult female scale be taken from its host it will be ■ seen to be little more than a hollow shell containing either eggs or egg envelopes from which the “ crawlers ” have emerged. After wandering on the food plant for a time the " crawlers ” settle down and begin feeding. In some cases feedingplaces selected by the “ crawlers ” become the permanent abode of the scale, while in others the insects may migrate later in life. Once the “ crawlers ” have settled the upper surface of; the body becomes hardened and thickened and remains so, at least in the case of females, throughout life. The female insects during growth cast their skins or moult twice, but they differ from the shielded scales in that the legs and antennae are not lost but always ! remain, although often in a comparatively reduced form. In the case of the males, which usually moult three times before reaching maturity, the first nymphal stage is similar to that of the. female, but in the second stage the . male forms a cocoon, which is usually a partly depressed, thin, glassy, or waxy covering. Beneath this covering the male moults twice more, and ultimately emerges as a two-winged insect, similar ■ in appearance to the males of the shielded scales.

The Olive Scale. The Olive Scale (Saissetia ole Bern.) is convex and about 3 mm. long when full grown. The colour of the adult scale is very dark brown or almost black, the younger stages, however, are usually distinctly brown. The surface is roughened, and a characteristic feature is the presence of a raised H-shaped marking on the scale, this feature being present in both immature and mature scales (Fig. 1). . Life-history and Habits. In New Zealand this species may be found in all stages during the winter. A large number of eggs is produced by each female, the number having been estimated at over fifteen hundred. It is commonly considered that there is only one generation each year and that the scale takes about three months to reach maturity. The egg-laying period commences about five weeks after maturity and proceeds for about six weeks. Observations in the Auckland district show that the young forms of this insect are abundant in October.

The Olive Scale infests the more woody parts of citrus—-viz., the twigs, small branches and leaves, especially along the mid-ribs. The more succulent parts, such as the fruit, are apparently not so suitable as feeding-grounds. The scale exudes copious supplies of honey-dew, on which the black sooty fungus grows. Consequently, infested trees usually show a more or less sooty coating on the woody parts, and where the scale is plentiful fruit and foliage is rendered unsightly. Hosts and Distribution. Recorded food plants include citrus, oleander, rose, guava, pear, palms, apple, apricot, plum, grape-vine, wisteria, peppertree, holly, laurel, camellia. It is also said to infest various native trees. The scale has a world-wide range, having . been recorded from North and South America, Italy, Australia, Algeria, Assam, South Africa, China, Eritraea, France, Philippines, Tunisia, Spain, French North Africa, Tunis, Morocco, Finland, Transcaucasia, Porto Rico, Bermuda.

Control. For this scale use summer grade oil at a strength of 1 part of oil to 60 parts of water. Apply in the early summer as soon as the spring growth has hardened somewhat. The. period about early December is suitable. .

The Hemispherical Scale.

The Hemispherical Scale (Saissetia coffees Wlk. hemisphaerica Targ.) when full grown (Fig. 3) is easily distinguished from others infesting citrus. The adult is brown to reddish-brown and, as its name implies, highly convex and of a shining smoothness. The surface is densely covered with lighter spots, which

can be seen plainly, however, only under the microscope. When full grown the scale is about 4 mm. to 5 mm. long. The young “ crawlers ” are very light in colour with well-developed legs and antennae. After settling, the scale becomes light brown and is ornamented with three ridges forming a raised H pattern after the style of the Olive Scale, but unlike . this latter the H disappears at maturity and the adult scale is quite smooth. Life-history and Habits. In the summer a generation probably takes about three months to reach maturity, so that it is likely that in New

Zealand there is more than one generation annually. The scale over-winters in the immature forms, and by the spring the over-wintering females are mature and begin producing eggs. Each female is said to be capable of producing well over five hundred eggs. The young, on settling down, choose the very young twigs, the insects rarely being found on hardened growth. As the time of reproduction approaches the females migrate to more exposed positions, mostly to leaves and, more rarely, fruits. Sooty mould may be associated with this scale, but it is not a characteristic of it and large colonies may be free from any such fungous growth. In the open the scale is found mostly on trees showing abundant foliage and much tender growth. It occurs also under glass. Hosts and Distribution. The scale has a wide range of food plants, among which are citrus, oleander, palms, japonica, camellia, asparagus, orchids, cucumber, currant, and eggplant.

Distribution is world-wide, the pest having been recorded from America, Australia, Netherlands, Indies, Britain, Madagascar, Philippines, Porto Rico, Africa, Ceylon, Bermuda, Hawaii, Samoa, West Indies, Jamaica, Malaya, tropical Asia, Mauritius, &c. The control of this insect is the same as that for the Olive Scale. . .

Modern Cool Storage Practice.

THE term “ long holding ” necessarily speaks for itself when dealing with fruit cool storage, and many are the pitfalls in this class of storage. These pitfalls are well known to most fruitrowers - A closer study of our problems reveals causes and their effects which may be remedied (i) by a determination to eliminate certain unprofitable varieties, (2) by making a study of conditions which are affecting the normal growth and maturity of other varieties, and (3) by varying cool-storage-temperature conditions to a degree that is found beneficial for the safe-keeping pf these varieties.

These factors are much more efficiently dealt with when there is a closer association and co-operation of the orchardists and their cool-storage management. The use of overhead cooling coil and trays, which are designed for the purpose of evenly distributing the cool air over the whole surface of the ceiling of each cool chamber in order to drain away the condensed moisture and to cool the fruit effectively by convection without the aid of forced circulation, is designed to increase the humidity of the chambers, which remains constant at about 92 per cent., thereby removing the main cause of wilt and shrivel in cool storage.

The excess moisture over the percentage mentioned is removed by condensation of the moisture from the atmosphere on the coil, from which it drips into the trays and runs to waste. This coil is rendered more effective for readily condensing the excess moisture because of its construction in a position near the ceiling of the chamber, where the heat rises naturally and is constantly coming in contact with the coil.. As the temperature is reduced it readily gives up its excess moisture to the cold surface of the evenly distributed cooling pipes.

—-A. Powell,

Cool Storage Officer,

Wellington.