Description:
Two spotted spider mites, Tetranychus spp., are widespread on melon crops throughout the southwestern U.S, but only occasionally cause significant damage. Spider mites are very small and are difficult to see with the naked eye. Adults are about 0.45 mm long, have eight legs and an oval body. Tiny spherical eggs may be present as well as webbing. Spider mites develop numerous generations throughout each melon growing season. Under optimal conditions of high temperatures and low humidity, mites can complete their life cycle in about seven days.
Damage:
Spider mites injure melons by puncturing the surface cells on the underside of leaves where they feed. This results in the destruction of chlorophyll and reduction in photosynthetic activity. Injured leaves become pale, stippled and can dry up and die under heavy infestations. Injury often is not noticed until reddish brown patches of affected plants appear in the field. Injury is most common in hot, dry weather from late spring to early fall when temperatures are favorable for rapid development. Light infestations can be tolerated, but severe injury can result in reduced yields and fruit quality.
Management:
Spider mites feed on a large number of crops and weeds and will overwinter in soil and debris on the ground. Infestations in melons often begin with adults carried by wind from adjacent crops. Because dust favors spider mite populations, minimize dust by watering field roadways. Several natural enemies (including predatory mites and thrips, minute pirate bugs and lacewings) play an important role in regulating mite populations below economic injury levels. Predator populations should be encouraged by limiting chemical rates and using selective insecticides for other pests. Good irrigation and fertilization management can enhance plant tolerance to mites. No economic thresholds have been established for spider mites in the desert, however treatment with an acaricide is recommended when webbing occurs, damage is evident and predatory mites and thrips are absent. Although spider mites inhabit the undersurface of leaves, most of the new miticides are translaminar and can effectively reach the pests when properly applied to upper leaf surfaces. A number of effective miticides are available for melons including Agri-Mek, Oberon, Acramite, Miteus, Zeal and sulfur. If leafminers are present in the field, Agri-Mek is a good option. If whiteflies nymphs are beginning to build on older leaves, Oberon may be an effective option.
Description:
Leafminers have a relatively short life cycle. The optimal temperature for development is about 85-90 °F and development ceases below 50 °F. The entire life cycle can be completed in less than 3 weeks under ideal conditions. Several generations may be produced during each growing season in Arizona. The eggs are very small and laid into the leaf just beneath the upper surface. After 2 to 4 days, larvae hatch and begin feeding on plant mesophyll tissue just below the upper surface of the leaf. Black hook-like mouthparts are apparent in all instars, and can be used to differentiate the larvae. The winding tunnels (mines) that result from their feeding are initially small and narrow, but increase in size as the larvae grow. Larvae emerge from the mines after completing three instars in as little as 7 days in fall growing conditions. The mature larva cuts a slit in the upper surface of the mined leaf, emerges from the mine and drops to the soil where it often burrows into the soil to form a puparium. The reddish brown puparium requires about a week to mature. Adults emerge as small, shiny black and yellow flies with a bright yellow triangular spot on the upper thorax between the wings. Subtle differences in color exist between the two species. L. sativae adults have a shiny black mesonotum whereas, L. trifolii has a grayish black mesonotum. Also, the black hind margin of the eyes of L. sativa distinguishes it from L. trifolii, which has eyes with yellow hind margins. Female flies’ make numerous punctures (stipules) of the leaf surface with her ovipositor, and then uses these punctures for egg laying. Both female and adult flies feed on the plant secretions caused by the oviposition stipules. Female flies can produce as many as 600 eggs over their life span. Adults are weak fliers, and often are blown by the wind. Female flies can live for as long as 4 weeks.
Damage:
The numerous oviposition sites by adult female flies can result in a stippled appearance on foliage. However, the primary damage caused by leafminers is the mining of leaves by larvae. Larvae mine between upper and lower leaf surfaces creating winding tunnels that are initially small and narrow, but increase in size as the larvae grow This not only results in destruction of leaf tissue, but both leaf mining and stippling can greatly reduce the photosynthetic activity in leaves. Extensive mining may cause premature leaf drop, which can result in lack of shading and sun scalding of fruit. Wounding also allows entry of bacterial and fungal diseases. In melons, Liriomyza leafminers can cause significant economic damage to small and large plants alike, particularly in L. sativae on later-planted spring and fall plantings. Mining of leaves by the larvae can cause direct injury to seedling plants by removing chlorophyl and reducing the plants photosynthetic capacity (Plate 2C). Mines and feeding punctures also produce an entrance for pathogenic organisms. Excessive leaf mining in older plants can cause leaves to dry, resulting in sun burning of fruit and reduction in yield and quality. In severe infestations, leafmining may cause plant death, particularly to seedlings or transplant watermelons. Damage to mature plants can occur when attempting to hold the crop longer for a second or third harvest.
Description:
Flea beetle adults are small, shiny, hard beetles with enlarged hind legs that allow them to jump like fleas (Plates 1, 2). The pale stripped flea beetle is the most destructive flea beetle on vegetables and melons. It has a pair of pale yellow stripes along the back, one on each elytron and can be confused with stripped cucumber beetles. The stripe is broad, usually measuring one-third the width of the elytron. Although the background color of the elytron is usually dark, in some specimens it is much paler, approaching yellow. The western black flea beetles are a shiny black beetle and much smaller. Flea beetles have a broad host range and can be found on most important field crops and many common agricultural weeds. The adult beetle is highly mobile and disperses readily onto emerging crops, particularly in the fall. They are most active on warm and sunny days.
Damage:
Adults do most of the damage on desert crops; flea beetle larvae may mine leaves or feed on roots, but generally do not damage vegetables crop or melons. The one exception is carrots where larvae have been found to feed on developing carrots and rendering them unmarketable. Flea beetles feed on both sides of leaves, creating small scars or irregularly shaped holes (Plates 2, 3). Large populations can kill or significantly stunt seedlings. Damage to cotyledons and young leaves is the major cause of crop loss, usually through uneven stand development. Older plants rarely suffer economic damage although their older, lower leaves may be damaged. Damage to cole crops and melons can be severe during September and October and preventative treatments are recommended.
Management and Control:
It is important to control weeds in and around the field which could act as a host for flea beetles. Sudan grass, alfalfa and cotton can harbor large number of beetles so it is important to destroy plant residue immediately after harvest if vegetable are to be grown nearby. There are no natural predators or parasites that can effectively control flea beetle populations. Monitor newly emerged seedlings frequently during stand establishment for flea beetle damage until plants are well established. Relatively low populations can cause economic damage when plants are in the cotyledon or firstleaf stages. Treat if you find several damaged plants or if populations of beetles are high. A single insecticide application is usually sufficient, but under heavy pressure repeated sprays may be needed to control new beetles that re-invade the field. Lannate, pyrethroids and foliar neonicotinoid sprays (Venom, Scorpion, Belay, Assail, Endigo, Admire Pro, etc.) will effectively control adult flea beetles. Pyrethroids can be effectively applied by chemigation, but timing is important. Soil-applied, neonicotinoid insecticides may be used at planting in areas where flea beetles are a chronic problem, but deep soil placement or delayed application through drip irrigation may reduce their effectiveness. Insecticidal baits will not provide adequate control. Neonicotinoid seed treatments (Nipsit) are available that provide good activity against adults.
Description:
Darkling beetle adults chew off seedlings, feed on foliage, and occasionally on melons fruit laying on the soil. The adults are from 3-6 mm long and are black or brown (Plate 3A). Darkling beetles, which are in the family Tenebrionidae, can be easily confused with predaceous ground beetles (family Carabidae), which prey on various soil insects. The two beetles can generally be distinguished by the carabids lack of clubbed antennae. Darkling beetles are most active at night but can be spotted moving on the ground during the day. They generally stay hidden in the soil or within field debris (Plate 3A). Larvae are cylindrical soil-inhabiting worms that are yellow and range from 1-8 mm in length. They are often referred to as false wireworms, and are not considered economically important.
Damage:
Darkling beetles are generally not a problem unless large populations move into a field when plants are emerging. They usually invade fields from weedy areas or crops such as cotton and alfalfa, so damage is often first observed on field edges. Seedling plants may be girdled or cut off at the soil surface. Once the plants have 5-6 leaves, the beetles are usually not a problem. As the season progresses, feeding can occur on flowers, on the undersides of leaves and on the netting or rind of mature melons. Under moist soil conditions, they can also bore into fruit where it rests on the seed bed.
Management and Control:
Several cultural practices can help reduce potential problems associated with darkling beetles. Maintain fields and ditches free of weeds. Water barriers placed around the field can aid in reducing migrating populations. Reducing organic matter in the soil by fallowing or deep-plowing may minimize beetle reproduction. When beetles are observed migrating into melons from surrounding fields, insecticide baits placed around the edges of the field may provide adequate control. Treat fields with insecticides (pyrethroids or Lannate,) whenever beetles are readily observed feeding on plants, flowers or fruit.
Description:
Often referred to as “wooly worms”, saltmarsh caterpillars are not normally a pest of fall grown vegetables but will often migrate in as larvae from neighboring cotton or alfalfa. Large populations can be extremely damaging to melon plants. The larvae are usually yellowish to brown in color and covered with long, dark black and orange hair (Plate 4). Full grown larvae may be 2 in. long. Adult moths have white to yellowish wings and are peppered with black spots. Their wing span is approximately 2 in. Eggs are laid in clusters of 20 or more on the leaves
Damage:
Most damage occurs to early planted seedling plants. Large populations of larvae will move out of newly defoliated cotton or cut alfalfa and devour the young plants. After thinning, saltmarsh caterpillars are generally not a problem. However, they should be included in counts for Lepidopterous larvae. On older plants damage is distinctive. They prefer to feed in groups and will completely skeletonize several adjacent plants.
Management:
Scout adjacent cotton fields prior to melon plant emergence. It is best to control saltmarsh caterpillars before they enter the field. If possible, treat the population in the cotton field during or just before defoliation. Larvae are particularly sensitive to Bacillus thuringiensis (B.t.). Physical barriers are effective at preventing larvae from entering a field. Saltmarsh caterpillar larvae do not like to cross fence type barriers of aluminum sheeting and can be used to direct populations into holes containing cups of oil. Carbaryl or permethrin baits can be spread on edges of cotton fields or along ditches to kill migrating populations. Lannate, Radiant, Intrepid, Lorsban and pyrethroids are all effective against saltmarsh larvae.
