The control of grass grubs damaging the roots of strawberry and other plants is a difficult matter, though some benefit is to be derived by making holes about four inches deep with a stick in the soil near to the infested plants and pouring in a little carbon bisulphide; the holes should be closed immediately. To protect strawberry beds, if they are not too extensive, the most satisfactory method is to cover the plants with scrim, stretched on frames, at dusk during November and early December, when the beetles are flying; this will prevent the insects from infesting the ground with their eggs. The use of sulphur smudges, already referred to, is of great importance in this respect.
Subterranean Grass Caterpillars.—These caterpillars are the larvæ of native moths ([Fig. 12], f1) belonging to the genus Porina, and when they become epidemic they cause much more extensive damage to pasture and lawns than do the grass grubs. When full grown, the greyish-black caterpillars ([Fig. 12], f2) reach a length of about three inches; they are soft-bodied and rather flaccid, and live in underground burrows of varying depth. After dark, these caterpillars come to the surface and devour the grass, eating it close to the ground, much soil being swallowed by the larvæ during the feeding. This soil is evacuated, and resembles earthworm castings, but is mixed with silk spun by the caterpillars; the emergence holes of the caterpillars, about the diameter of a lead pencil, are conspicuous on the surface denuded of its covering of grass. Pupation takes place underground, and when the moths emerge the pupæ first move to and project beyond the surface of the ground; these pupæ are large and easily recognised by the wing-cases, which are very short compared with the length of the body. The moths are on the wing during spring and summer, the rest of the year being spent in the larval stage. The moths are night-flyers, and are amongst the largest species in New Zealand, their wings having an expanse of from one to over two inches; they are heavy-bodied insects, and vary considerably in colour. One of the commonest, species is brownish-yellow, or sometimes a smoky-grey, with a white streak bordered with black on the fore wings; the hind wings may be pinkish.
The most satisfactory method of controlling the insect is to roll infested lawns after dark, in order to crush the caterpillars whilst feeding on the surface. Flooding an infested lawn with water will bring most of the caterpillars to the surface, when they can be collected and destroyed. Spraying grass in spring and early summer with arsenate of lead will tend to poison the immature caterpillars. There are at least three species of insect parasites that attack these larvæ, and there is also a fungus which invades and destroys the whole body, taking the shape of the insect; such fungus-infested caterpillars are commonly called “vegetable caterpillars.”
Wireworms.—The roots of garden plants and germinating seeds are often damaged by hard, wiry beetle grubs, reddish-brown or whitish in colour, called “wireworms,” so named from their resemblance to short pieces of wire; they have three pairs of legs behind the head and a sucker-like appendage on the last body-segment ([Fig. 12], g2). These grubs transform to narrow-bodied, brownish or blackish beetles, known as “click-beetles” ([Fig. 12], g1) from their habit, when overturned, of righting themselves by a springing action, during which a distinct and sharp clicking sound is made; the spring apparatus consists of a spine, the tip of which fits into a notch on the under side of the thorax.
Practically nothing is known as yet in regard to the biology of the New Zealand click-beetles. They are extremely difficult to control, and the larval stage covers a period of two or more years.
Narcissus Flies.—There are two species of narcissus flies—the larger (Merodon equestris) and the smaller (Eumerus strigatus) both occur in New Zealand. The larvæ of these flies attack bulbs of various kinds, the hosts of the larger fly being narcissus, hyacinth, tulip, amaryllis, habranthus, vallota, galtonia, scylla, and leucojum; and of the smaller fly, narcissus, hyacinth, onion and shallot. These flies are two-winged insects, the hind wings being wanting as such, and belong to a group called the syrphid, or hover flies.
The larger narcissus fly ([Fig. 12], h1) resembles somewhat a humble-bee (which, however, has four wings); its stout and very hairy body measures about half an inch long. There is considerable variation in colour, though black or brown predominates, with greyish or yellowish hairs, and bands of the same colour; the bands, however, are absent in some individuals.
During spring the insects fly about in the sun, and lay their eggs at the leaf bases of the host plants, or on the exposed neck of bulbs, or in the soil close by. The larvæ, which are legless, yellowish grubs, enter the bulb, and may completely destroy it. Infested bulbs may be detected by an unnatural softness near the neck when pressed between the fingers.
The smaller narcissus fly ([Fig. 12], h2) is about half the length of the larger, of a shiny black colour, with metallic reflections, and is not clothed with hair. The eggs are laid in the ground, or at times upon the plant itself. Several larvæ of this fly may be found in the one bulb; the larvæ resemble those of the larger fly, but are smaller, and have three small processes at the end of the body. The smaller narcissus fly usually attacks the bulbs already damaged by some other agent, though it has been known to infect sound bulbs.