Cucurbits
Cucurbits: Aphids, Hemiptera: Aphididae
Biology: Aphids are small soft-bodied insects that feed on plants by inserting their stylet into the plant and removing large amounts of plant sap (Photo 33). Not only do aphids adversely affect plants by feeding, they transmit viral diseases that result in total loss of harvest. In Arkansas much of the cucurbit production area was devastated by potyvirus diseases during the late 1980’s. These diseases are still prevalent. Aphid reproduction is both sexual and asexual and numerous generations occur each year. As a result population increases can be dramatic. But just as dramatic, populations often crash due to the action of natural enemies including fungal pathogens, parasitic and predatory insects. Insecticide application often decreases the effects of the beneficial organisms and large increases in aphid populations may result.
Management: Aphids are often naturally controlled by beneficial insects (predators and parasites) and fungi diseases. Use of foliar insecticide sprays disrupts the action of the beneficials and should only be used against very high and increasing levels of aphids. Of the insecticides currently available imidacloprid applied into the soil prior to planting or used as a foliar spray offers excellent aphid control. Many other traditional insecticides are effective against aphids but will have little impact on reducing the incidence of viral diseases in cucurbits.
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Cucurbits: Spotted cucumber beetle, Diabrotica undecimpunctata howardi Berber, and striped cucumber beetle, Acalymma vittatum (Fabricius). Coleoptera: Chrysomelidae
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Photo 34 |
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Photo 35 |
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Photo 36 |
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Photo 37 |
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Photo 38 |
Biology: Soon after cucurbit seedlings emerge, the new plants are attacked by cucumber beetles. Adult beetles pass the winter and early spring months in weeds and brush near production fields. Although beetles can be found feeding on several non-cucurbit hosts, they prefer cucurbits and are capable of inflicting severe damage to emerging seedlings. When numbers are high, as often occurs in the Arkansas River Valley, their feeding may result in death of cucurbit seedlings. Adult beetles are about 5mm long and emerge from pupa within the soil, mate and deposit up to a few hundred eggs near plants but in the soil. Larvae develop within the soil by feeding on plant roots but damage from feeding by the larvae is usually minimal. The spotted cucumber beetle adult (Photo 34) is slightly larger and has 6 black spots on each outer wing while the striped cucumber beetle (Photo 35) is smaller and has 3 black lines on the upper abdomen. Adult spotted cucumber beetles feed on numerous host plants including cucurbits, beans, peas, corn and many others. Cucurbits, however, are their preferred host plant along with the striped cucumber beetle. When adult populations are high, damage to cucurbit seedlings may be excessive and plants may be killed (Photo 36 and 37). This often requires management of the adult beetles. Once plants develop a few true leaves, plants can generally tolerate additional damage and management is usually not required. Adults may also feed on the outer skin of fruit but this is generally minimal (Photo 38). Multiple generations occur each year.
Management: The most susceptible stage of cucurbits to cucumber beetle attack is just after plants emerge from the soil. Planting when temperatures are warm and use of irrigation and fertilization will promote rapid plant growth and resulting damage from adult beetles will be minimized. If beetle numbers increase beyond acceptable levels, use of insecticides may be warranted. Traditional insecticides include pyrethroids, methomyl, carbaryl, and endosulfan. Systemic insecticides such as imidacloprid, that are applied for other insects will likely provide effective control of cucumber beetles. In areas with a history of high cucumber beetle populations, treating the soil or seed with neonicotinoid insecticides will protect emerging seedlings. Once plants have a few true leaves, low numbers of adult beetles may be tolerated with a minimal effect on yield.
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Cucurbits: Pickleworm, Diaphania nitidalis (Stoll) and melonworm, D. hyalinata L. Lepidoptera: Pyralidae
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Photo 39 |
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Photo 40 |
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Photo 41 |
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Photo 42 |
Biology: Adult pickleworm and melonworm moths lay eggs individually or in small groups on the leaf surface of cucurbits (Photo 39). Eggs generally hatch in 3 to 4 days and emerging larvae initially feed on cucurbit foliage. While melonworm larvae continue to develop on foliage, pickleworm larvae move to fruit after about 1 week. Here the half inch long larvae chew an entrance hole and enter the fruit (Photo 40). Not only does the tunneling cause fruit injury but fungi is introduced that further destroys the fruit. Infested or damaged fruit is unmarketable. In areas with high pickleworm populations, entire fields can be lost. Melonworm larvae generally feed only on foliage where development is completed in about 2 weeks. Upon reaching maturity larvae approach 30 mm in length. Mature pickleworm larvae are light green and without stripes (Photo 41). Mature melonworm larvae are dark green and possess two lateral white stripes (Photo 42). Both species usually pupate on the foliage within a chamber composed of rolled leaves.
Management: First, location of cucurbit fields in areas of low Diaphania populations should be attempted. Immediately after cucurbit seedlings emerge, plants should be scouted for Diaphania. If detected use of insecticides may be warranted. However, the use of “hard insecticides” should be delayed for as long as possible. Bacillus thrugiensis (BT) likely offers the best alternative to the “hard insecticide”. Also, several newer (Biobit) and more effective types of BT should improve management. Again, the objective of early season use of “soft” insecticides is to promote the development and prolong the effectiveness of beneficial organisms. Traditional insecticides include carbamates, organo-phosphates, pyrethroids and spinosad and many of these effectively control Diaphania. In small fields searching for damaged fruit and removal may reduce the insect population to an acceptable level. Sanitation including destruction of crops immediately following the last harvest will likely reduce damage in subsequent plantings.
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Cucurbits: Squash bug, Anasa tristis (De Geer). Hemiptera: Coreidae
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Photo 43 |
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Photo 44 |
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Photo 45 |
Biology: The squash bug is often a major pest of squash in Arkansas. Each year reports surface of significant populations, particularly on pumpkin, summer and winter squash. Adult squash bugs are about 18 mm long and dark gray (Photo 43). Like other true bugs, adults have outer wings with the distal half membraneous and the proximal portion hardened. Adults overwinter in old cucurbit fields or in nearby boarders. With the arrival of spring, squash bugs migrate into recently planted cucurbit fields. They feed by inserting their piercing mouthparts into the plant, injecting a digestive toxin and extracting plant fluids. Shortly after mating an egg mass is laid, typically in a pattern, on the leaf surface (Photo 44). Eggs are a shiny bronze and hatch in 6 to 14 days. Nymphs are wingless and feed gregariously on all parts of the plant (Photo 45). Numbers of nymphs and adults may reach several hundred on a single plant. Heavily infested plants are severely weakened and fruit production and quality are greatly reduced. Egg laying by a single adult may continue for several weeks. Thus, all stages of nymphs may be present along with adults. One generation occurs each year in Arkansas.
Management: Of utmost importance is field sanitation. Squash bugs feed only on cucurbits and reducing the length of time host plants are available greatly impacts the population. Thus, destruction of crops immediately following the last harvest will likely reduce damage in subsequent plantings and will reduce overwintering habitat. Also, elimination of cucurbit weeds in adjacent areas will aid in population reduction. As plants develop in the spring and early summer, they should be searched for adult squash bugs which are often detected on the plant base or on soil near the plant. Low numbers of adults may be tolerated, particularly during periods of rapid plant development. Plants may simply outgrow the damage from low squash bug numbers. Proper fertilization and irrigation are important. In small gardens, egg masses can be removed and destroyed. Impact of beneficial insects on squash bug in Arkansas is minimal and as squash bug numbers increase, use of synthetic insecticides are often needed. The squash bug is tolerant to most insecticides and currently the only effective insecticide registered for commercially produced cucurbits is bifenthrin.
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Cucurbits: Squash vine borer, Melittia cucurbitae (Harris). Lepidoptera: Sesiidae
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Photo 46 |
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Photo 47 |
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Photo 48 |
Biology: The squash vine borer is a noteworthy problem in areas where cucurbits, esp. pumpkins, are grown annually in Arkansas. Adult moths emerge from overwintering larvae or pupae from the soil in early spring, mate and lay eggs singly on cucurbit stems. Caterpillars emerge in 4 to 10 days and enter the stem to feed (Photo 46). Initially, feeding has little effect on the plant but as larvae mature within 3 to 4 weeks, the infested plant or a portion of a plant, may suddenly wilt. Close examination of the stem close to the soil will reveal a mass of caterpillar excrement expelled from the stem (Photo 47). By splitting the stem with a knife the larvae can be located. Mature larvae are white with a brown head and reach about 28 mm in length (Photo 48). Prior to pupation, larvae exit the plant and burrow into the soil. Little is known of the squash vine borer biology in Arkansas, but two generations likely occur annually.
Management: Of utmost importance is field sanitation. Squash vine borers feed only on cucurbits and reducing the length of time host plants are available greatly impacts population. Thus, destruction of crops immediately following the last harvest will likely reduce damage in subsequent plantings. Disking cucurbit fields after harvest and during the fall and winter will expose them to the adverse effects of winter and kill overwintering stages. Low numbers of adults may be tolerated, particularly during periods of rapid plant development. Plants may simply outgrow the damage from low squash vine borer numbers. Proper fertilization and irrigation are important. In small gardens, damaged stems can be split with a knife and exposed larvae destroyed. By killing the larvae and covering the damaged stalk with soil, the plant may recover sufficiently to produce fruit. Use of synthetic insecticides may be warranted in commercial operations but once larvae enter the stem, traditional contact insecticides are not effective. The value of systemic insecticides is not well documented.
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Cucurbits: Whitefly, Bemisis spp., Homoptera: Aleyrodidae
Biology: Whiteflies have become increasingly important in cucurbit production in Arkansas. In years with hot dry summers, numerous reports of whitefly problems are received from cucurbit producers, especially those who produce pumpkins in the fall (Photo 49). Adult whiteflies are minute (about 1 mm in length) and white in color (Photo 50). Eggs are inserted into leaf tissue and nymphs also known as “crawlers” emerge and feed by extracting sap from plants. Not only does this feeding damage the plant but whiteflies are also responsible for virus disease transmission. Shortly after hatching, nymphs attach themselves to the underside of the leaf surface while they feed one to four weeks. Developmental time is greatly dependent on temperature. Whiteflies can be found on most vegetable crops and also occur on numerous alternative hosts including weeds.
Management: Current management of whitefly often relies on repeated application of insecticides. Materials presently used include bifenthrin and others. With high whitefly populations repeated insecticide use will not be effective and whiteflies are likely to develop resistance to the insecticides. Several other practices should improve whitefly control and include the following. First, location of cucurbit fields in areas of low whitefly populations should be attempted. In dry areas or during periods of drought white fly management is difficult and may not be successful. Many weeds, esp. velvet leaf, harbor large whitefly populations. Removal of these alternate host plants several weeks prior to the cucurbit production season will reduce the likelihood of high whitefly populations at planting. In areas with a history of high whitefly populations, application of neonicotinoid insecticides at planting either as a seed treatment (Crusier) or directed into the furrow (Centric) should provide some protection for about 2 months. Immediately after plants emerge, cucurbits should be scouted for whiteflies. If detected, use of insecticides is warranted. Proper selection of insecticides such as imidacloprid, should be effective against whiteflies and its use during early parts of the season may preserve natural enemies and reduce the occurrence of secondary pests. Sanitation including destruction of crops immediately following the last harvest will likely reduce damage in subsequent plantings.
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