Sweet potato

Small (1–3 mm) soft-bodied insects, green, black or pink, feeding in clusters on young shoots and the underside of leaves. They excrete sticky honeydew. The diagnostic feature that distinguishes aphids from other small sucking insects is the pair of siphunculi (tubular projections) at the rear of the abdomen together with a tail-like cauda; a single colony usually holds wingless forms (parthenogenetic females) alongside winged migrants that fly onto new plants in summer. In summer aphids reproduce parthenogenetically and viviparously over many generations, so the population builds up explosively; in autumn many species produce a sexual generation that lays overwintering eggs on the host, and host-alternating species move between a winter host (often a tree or shrub) and a summer host (a herbaceous plant). Feeding symptoms include curling, crinkling and distortion of young leaves, stunted growth, yellowing, and the shedding of buds and fruitlets. Abundant honeydew coats the leaves, and the sooty mould fungus that develops on it forms a black film which limits photosynthesis; the honeydew is fed on by ants, which in return defend the colony against predators. Aphids are also important vectors of plant viruses (mosaics among them), transmitting them from plant to plant even during a brief probing puncture.

Larvae of click beetles: narrow, hard and shiny, yellow-brown (hence the name 'wireworms'), up to 2-3 cm long, with three pairs of short legs just behind the head. They develop in the soil for 3-5 years. Symptoms: narrow tunnels bored into potato tubers, carrot roots, bulbs and the base of stems, together with gnawed roots of seedlings that wilt and fail in patches - especially on ground freshly broken from turf. The adult beetles (click beetles) are elongated, dark, 7-12 mm long, known for their characteristic ability to spring up with a click after being flipped onto their backs - they themselves cause no significant damage to crop plants; only the larvae are the pest. Females lay their eggs shallowly in the soil, preferably under grassed, moist turf, which is why the threat is greatest in the first 2-3 years after ploughing up a long-established lawn, fallow or field infested with couch grass. The larvae move through the soil profile depending on temperature and moisture: in spring and autumn they feed shallowly, in the root zone, while in heat and drought they descend deeper. The damage takes the form of characteristic narrow, smooth channels and tunnels bored into tubers, storage roots and bulbs; it lowers the market value of the crop and opens the way to secondary bacterial and fungal rots. In seedbeds and young seedlings (maize, vegetables, bedding plants) the larvae chew through the roots and the base of the stem at the root collar, causing the plants to wilt and die off in patches, leaving empty gaps in the rows (breaks in the stand). The damage intensifies on heavy, moist, humus-rich and heavily weed-infested soils.

Tiny (0.3–0.5 mm) arachnids, hard to spot with the naked eye, feeding on the underside of leaves. Symptoms: fine, pale speckling (feeding punctures); over time the leaves turn grey, yellow and dry out. Under heavy infestation a fine webbing becomes visible. Warm, dry air favours their development — a common problem for houseplants in winter near radiators. Spider mites are not insects but mites of the family Tetranychidae — adults have 8 legs (larvae only 6). The most common in cultivation is the two-spotted spider mite (Tetranychus urticae): yellowish-green females carry two dark spots on the sides of the body, while overwintering females turn orange-red. In orchards the fruit tree red spider mite (Panonychus ulmi) is frequent, whose red eggs overwinter in clusters on the bark of the shoots. Feeding mechanism: the mites puncture the cells of the epidermis and mesophyll on the underside of the leaf and suck out their contents, and the emptied cells fill with air — hence the characteristic, silvery-grey, fine speckling visible from the upper side of the blade (resembling a mosaic discolouration). At high numbers the leaves turn brown, curl and drop prematurely, and the whole plant weakens and loses its growth. Dense webbing spun between the leaves and shoot tips appears only under an advanced attack and is a sign of the risk of the pest spreading to neighbouring plants. The development cycle is very fast in warmth and drought — at a temperature of 25–30°C a single generation can be completed in 1–2 weeks, hence the numerous generations per season and the rapid, exponential build-up of the population. Females lay spherical, transparent eggs on the underside of the leaves. Fertilised females overwinter (in bark crevices, leaf litter and plant debris) or — in the fruit tree red spider mite — the eggs on the bark. Early infestation is easy to miss; diagnosis is helped by shaking a leaf over a white sheet of paper (moving specks become visible) or examining the underside of the leaf with a magnifying glass.