Chewing insects can cause serious yield and quality losses in potatoes by feeding on the leaves, stems or tubers. Sucking insects can cause direct losses from feeding and indirect losses by transmitting viral diseases. The most important of these insects are described below. A table at the end of the chapter lists insecticides currently registered for potatoes.
Colorado Potato Beetle, Leptinotarsa decemlineata
Adult beetles have 10 lengthwise black stripes on yellow-orange wing covers and are approximately 3/8 to 1/2 inch long. They are stout and strongly convex in shape. The yellow-orange spindle-shaped eggs are laid in clusters of 10 to 30 on the undersides of leaves. They are very similar to ladybird beetle eggs but are larger. The larvae are humpbacked, red to orange, and have two rows of black spots on each side of their soft bodies.
The major food plant of the Colorado potato beetle is potato. Other crop hosts include tomato and eggplant. Wild hosts found in Florida include horsenettle, groundcherry, and tropical soda apple. Horsenettle is found mainly in North Central and North Florida, which is also where the beetle is generally found. Tropical soda apple is found throughout the state, but it is not clear if the distribution of CPB has changed following the introduction of this invasive weed. Adults overwinter in fields where they developed as larvae or in uncultivated areas adjacent to fields. They can also overwinter in wooded areas. Only a small proportion of a population leaves its field of origin by flying. Each adult female can produce about 450 eggs. Larvae, which pass through 4 instars, are generally found near the top of the plant and they seldom move far from the plant on which they hatch unless all the leaves are eaten. About two-thirds of all feeding by larvae occurs in the fourth or last instar. When the larvae have completed their development they enter an inactive pupal stage in the soil. After 5 to 7 days, adults emerge and begin to feed on the potato plants. Under ideal conditions, the life cycle can be completed in three weeks. Potato beetles are unaffected by high concentrations of toxic glycoalkaloids, the naturally occurring bitter compounds in potatoes. The efficient detoxification system of the beetle may also play a part in detoxifying insecticides and in the development of insecticide resistance.
The Colorado potato beetle is a significant problem only in North Florida production regions. High numbers of late instar larvae can defoliate plants. Yield loss is greatest if heavy damage occurs during tuber formation. Bacterial ring rot and potato spindle tuber disease, which are easily spread by mechanical means, can also be transmitted by Colorado potato beetle.
Wireworms, Melanotus communis, Conoderus spp.
Wireworms are the larvae of the click beetle. They are shiny, slender, hard-bodied and yellow to brown. Adults are large brown beetles that make a clicking sound when they try to right themselves after being turned over.
Depending on species and soil temperature, wireworm larvae can take from 1 to 5 years to develop. Corn wireworm (Melanotus communis), common in Florida, may complete its development in 2 to 3 years in South Florida. Most flight activity occurs in May and June. Females lay eggs in cracks or crevices or burrow into the soil. Larvae tend to move deeper as soil temperatures become hotter and move closer to the soil surface when it is cooler. If temperatures drop further, larvae will again move deeper into the soil. Other wireworms found in Florida (Conoderus spp.) can complete their development in a year or less, resulting in up to three generations per year in South Florida. These species tend to stay close to the soil surface.
The adult wireworms do not attack potatoes. However, the larvae ("wireworms") feed on potato seed pieces and developing tubers. Wounds to seed pieces allow disease organisms, such as fungi and bacteria, to enter. The greatest damage occurs when larvae tunnel into developing tubers, reducing their quality and value. Damaged tubers are often malformed.
Description and Biology
The adult is a small fly, approximately 1/8 inch long, with a black head, yellow between the eyes, a black thorax and a tube-like "ovipositor" at the end of the abdomen used to puncture the upper leaf surface for egg laying. The white, oval egg is inserted in the leaf tissue, but many punctures (called stipples) are used by the adult for feeding and do not contain eggs. The larva, a yellow maggot with black, sickle-shaped mouth hooks, feeds between the upper and lower leaf surface for approximately seven days, leaving a serpentine mine containing a string of black frass (fecal matter). The mature larva exits from the mine and falls to the ground where it molts to a pupa within a golden brown, barrel-shaped, and ribbed puparium from which the adult emerges in seven to 14 days. Generation time is 15 to 28 days depending upon temperature.
Leafminer damage is only foliar, caused by serpentine mines carved in leaves by feeding leafminer larvae. Heavy damage can reduce photosynthesis and cause leaf desiccation and abscission.
Flea Beetle, Epitrix hirtipennis, others
Tobacco flea beetle (Figure 4) is a fairly typical flea beetle pest of potatoes in Florida. Adults are very small, 1/12 to 1/20 of an inch long. They are reddish-yellow with a brown abdomen and a brown patch crossing the wing covers. Eggs are elongate and slightly pointed at one end. They change from white to lemon yellow as they get close to hatching. Larvae are whitish except for their yellow or yellow-brown heads and reach a length of 1/6 of an inch long.
Tobacco flea beetle feeds on tobacco, potato, tomato, eggplant and other plants in the family Solanaceae. In Florida, at least 4 generations a year can develop. Beetles overwinter as adults under plant debris. If the weather is warm enough, they may remain active all winter. Eggs are laid in the soil near the base of the host plant, in clusters of 5 or 6. Overwintering females can produce up to 200 eggs with later generations producing about 100 eggs per female. Larvae develop through three instars and feed mainly on fine roots near the soil surface. They pupate near the soil surface also. Adults feed on leaves.
The adult beetle eats small holes partly or completely through the leaves, resulting in the formation of many small "shot holes" in the leaves. Seedlings are most vulnerable to severe damage. Feeding wounds may serve as a point of entry for pathogens.
Leafhopper, Empoasca fabae and close relatives
The adult potato leafhopper (Figure 5) is pale green with a row of white spots just behind its head. It has a slender body form and is about 1/8 to 1/7 inch long. Eggs are transparent to pale yellow and are inserted into the veins and petioles of leaves. Young nymphs are very small (about 1/25 inch). Wing pads develop from the third through fifth instars.
The adult potato leafhopper overwinters in Gulf Coast States, including Florida, and disperses northward. In Florida, it can complete six generations a year. It feeds on many wild and cultivated plants, but potato is a particularly good host plant. Females can produce 200 to 300 eggs. These hatch in from 7 to 20 days depending on temperature. The average developmental time for nymphs is about 15 days. Adults can live from one to two months. Leafhoppers seem to have few effective natural enemies.
Leafhopper damage (hopper burn) late in the season is often confused with maturity of the plants (damaged leaves first turn brown along the margins but remaining foliage is often green). The adults and nymphs attack the underside of the leaves and suck the sap. They secrete a toxin into the plant as they feed. This causes the leaves to curl, yellow, and exhibit hopper burn symptoms. Plants may be stunted and yields reduced.
Aphids, Myzus persicae and Macrosiphum euphorbiae
Aphids are small, soft-bodied insects that reproduce rapidly and feed on plant sap. In Florida, green peach aphid (Myzus persicae) (Figure 6) is the most common aphid on potatoes, but potato aphid (Macrosiphum euphorbiae) (Figure 7) can also be found. Aphids occur in both winged and wingless forms. The mature wingless form of green peach aphid is egg-shaped, the tubercles at the base of the antennae are prominent and point inward, and the cornicles, tube-like structures on the back of the aphid, are long and unevenly swollen. They can range in color from light green to pink to almost translucent. Potato aphid is larger and more elongated with longer, straight cornicles and can be green, yellow, or pink. The pink form is common in the Hastings area. Its antennal tubercles point outward.
In Florida, aphids can reproduce without mating all year, as long as host plants are available. They give birth to nymphs rather than laying eggs and their offspring can be producing nymphs of their own in 7 to 10 days depending on temperature. High populations can develop very quickly as plants get crowded. Winged forms develop and fly to new host plants. Aphids have many natural enemies, both general predators such as lacewing and ladybeetle larvae, and more specific parasitoid wasps. Fungi can kill large numbers of aphids in a short period of time.
Aphids cause damage by sucking juices from the underside of leaves on the above ground portion of the potato plant. Feeding by potato aphids can cause distortion of young leaves and the dying back of the shoot or stem. Green peach aphid is more common in Florida. Early season infestation is the most damaging and can result in yield loss. Green peach aphid is also an excellent virus vector, transmitting viruses from plant to plant. At this time, potato leafroll virus and potato virus Y, the most important of the aphid-transmitted plant viruses affecting potato, are not common in Florida potato fields. Currently, aphids are managed with systemic insecticides applied at planting.
Caterpillar Pests (beet armyworm, fall armyworm, southern armyworm, cutworms, cabbage looper, etc.)
Larvae of moths can damage and occasionally defoliate potato plants. Two examples are described below.
Cabbage looper, Trichoplusia ni (Hübner) (Figure 8), feeds on a variety of crops. The adults (Figure 9) are night-flying moths with brown, mottled forewings marked in the center with a small, silver figure eight. They lay their eggs (small, ridged, round, greenish-white) singly on both upper and lower leaf surfaces. The eggs hatch into larvae that are green with white stripes running the length of their bodies. The caterpillar has three pairs of slender legs near its head and then three pairs of thick prolegs near the end of its body. It moves in a characteristic looping motion, alternately stretching forward and arching its back as it brings the back prolegs close to its front legs. After feeding for two to four weeks, the caterpillar, about 1.25 inches long when fully grown, spins a cocoon and pupates. The adults emerge 10 days to two weeks later. There can be several generations per year depending on climate. They tend to feed on older leaves.
Beet armyworm, Spodoptera exigua (Hübner) (Figure 10), also feeds on many crops and weeds. The highly mobile adult moth (Figure 11) has dark forewings with mottled lighter markings and hind wings thinly covered with whitish scales. Each female can lay over 600 eggs, generally in masses of about 100 on the undersides of leaves in the lower plant canopy. Very young caterpillars feed in groups, and then disperse as they grow older (third instar). The dull green caterpillars have wavy, light-colored stripes lengthwise down the back and broader stripes on each side. After feeding from one to three weeks, they construct a cocoon and pupate, emerging as adults about one week later. Beet armyworm survives the winter in South Florida and can complete many generations a year there. From South Florida, adults migrate into North Florida and other parts of the Southeast.