[Illustration: FIG. 6.--Bud Mites (_Phytoptidae_). A, Currant Bud Mite (_Phytoptus ribis_); B, Nut Bud Mite (_P. avellanae_).]
For spraying, proper instruments must be used, by means of which the liquid is sent out over the plants in as fine a mist as possible. Numerous pumps and nozzles are now made by which this end is attained. Both horse and hand machines are employed, the former for hops and large orchards, the latter for bush fruit and gardens. In America, where trees in parks as well as orchards and gardens are treated, steam-power is sometimes used. Among the most important sprayers are the Strawson horse sprayers and the smaller Eclair and Notus knapsack pumps, carried on the back (fig. 7). The nozzles for "mistifying" the wash most in use are known as the Vermorel and Riley's, which can be fitted to any length of tubing, so as to reach any height, and can be turned in any direction. The pumps in the machine keep the insecticide constantly mixed, and at the same time force the wash with great strength through the nozzle, and so to the exterior, as a fine mist; every part of the plant is thus affected.
[Illustration: FIG. 7.--Knapsack Sprayer for Liquid Insecticides.]
_Beneficial Insects_ have also to be considered in economic entomology. They are of two kinds--(1) those that help to keep down an excess of other insects by acting either as parasites or by being insectivorous in habit; and (2) insects of economic value, such as the bee and silkworm. Amongst the most important friends to the farmer and gardener are the Hymenopterous families of ichneumon flies (_Ichneumonidae_ and _Braconidae_); the Dipterous families _Syrphidae_ and _Tachinidae_; the Coleopterous families _Coccinellidae_ and _Carabidae_; and the Neuropterous _Hemerobiidae_, or lace-wing flies. Ichneumon flies lay their eggs either in the larvae or ova of other insects, and the parasites destroy their host. In this way the Hessian fly is doubtless kept in check in Europe, and the aphides meet with serious hindrance to their increase. If a number of plant-lice are examined, a few will be found looking like little pearls; these are the dried skins of those that have been killed by _Ichneumonidae_. The _Syrphidae_, or hover flies, are almost exclusively aphis-feeders in their larval stage. _Tachina_ flies attack lepidopterous larvae. One of the most notable examples of the use of insect allies is the case of the Australian lady-bird, _Vedalia cardinalis_, which, in common with all lady-birds, feeds off _Aphidae_ and _Coccidae_. The Icerya scale (_Icerya purchasi_) imported into America ruined the orange groves, but its enemy, the _Vedalia_, was also imported from Australia, and counteracted its abnormal increase with such great results that the crippled orange groves are now once more profitable. (F. V. T.)