Eradication of Scale Insects

New Zealand Journal of Agriculture, Volume 60, Issue 1, 15 January 1940, Page 57

Eradication of Scale Insects

Citrus Notes

FEBRUARY and March each year are , the best months for the eradication of scale insects, of which the most serious is the citrus red scale, Aonidiella aurantii. This sucking insect has caused considerable damage

in many citrus groves, resulting in weakened and dying trees. ' The citrus red scale is > a shielded insect, the live insect living under two or more moulted skins. Reproduction is by living crawlers, and when it

is realised that one female can produce up to 300 crawlers—and the hotter the summer the more crawlers produced the reason for the possible rapid spead of this insect will be easily realised, Probably three or four life cycles occur

each year, but the most important reproductive period is during the next two months., Thus, this insect should be attacked at its most vulnerable period when in the “crawler” stage. The red scale attacks . fruit, leaves, and wood. ... On young trees the main trunk is often found encrusted with red scale, and many growers do not realise that this is often the cause of stunted, hidebound trees with the bark often split longitudinally. Such young unthrifty trees should be uprooted and burnt. For other and better trees the standard recommendation is two close ratio sprays of 3 per cent, certified summer oil. • 1 Apply the first summer oil under fine weather conditions towards 1 the middle of February, and the second three weeks later. Adequate coverage is essential, as oil sprays • are contact sprays and, to be effectual, must enclose the insect. The effect is to smother it by blocking its breathing pores, or tracheae.. , ■ x Value of Oils } ■ ! • ■ The efficacy and value of the summer oils may be discussed at this stage. These spraying oils are obtained from crude petroleum by fractional, distillation. They belong to the same frac-

tion as the lubricating oils, with a boiling point range of from 250 deg. to 400 deg. The composition of the fraction varies according to the source of the 'crude oil, .the process of distillation, and degree of .refinement. They consist of paraffin hydrocarbons, unsaturated hydrocarbons, and the aromatic hydrocarbons. The unsaturated hydrocarbons present in an oil are injurious to growing.plant tissues. Hence, these must be removed by treatment with. concentrated sulphuric acid, known as the sulphonation process. The unsaturated hydrocarbon content must be known, and this is generally done by the iodine value, as iodine will combine with the unsaturated to give saturated oils. The relative viscosity and the volatility of an oil are important factors. The viscosity is the rate of flow of an oil, while the volatility is the rate of evaporation. Both of these factors are of importance, and govern the degree of permanence of the oil film. As oil and water cannot be mixed unless the oil is treated with an emulsifier, this is added , in order to . keep the oil globules dissociated. . Colour is ' not a reliable, factor for the determination of the suitability of

a spraying oil, the terms “winter” and “summer” oil being preferable. The time to apply the 3 per cent, certified summer, oil for the red scale on citrus trees is after these notes appear. The toxicity, of this spray depends upon the permanence of the oil film and its action in blocking the tracheae of the insects. -y.-.; ’ . . Cultivation Cultivation of citrus groves should be continued until the- green manuring cover crop is sown in March. In order to be prepared it is desirable to order the necessary seed and manure early, as too often the : green manuring crop is . sown later than intended, with the result that next spring’s ploughing and seasonal operations are delayed or the full growth from the green manuring crop is not obtained. ■ ■ During the hot summer months a good, clean circle should be maintained around each citrus tree or the mulching material retained in order to suppress weed growth. Pruning operations may be continued at this period of the year, and every opportunity should be taken to : have trees opened out before the applicaation of the summer oil sprays.

A. M. W. GREIG,

Citriculturist, Auckland,