Identification, Biology and Management of Insects Attacking Vegetables in Arkansas

Leafy vegetables (spinach)

Leafy vegetables (spinach): Beet armyworm, Spodoptera exigua (Hubner), Lepedoptera: Noctuidae

Photo 86
Photo 86
Photo 87
Photo 87

                     
Biology:  In recent years spinach producers in Arkansas and Oklahoma have experienced an increase in the frequency of beet armyworms.  This increase may be related to the widespread use of minimal tillage in soybean.  Farmers are now permitting weeds to grow longer in emerging soybean fields before applying herbicides.  Pigweed, in particular, is now more abundant in late spring and early summer.  Pigweed is a preferred host plant for the beet armyworm.  Populations are building on the abundance of host plants during early season and continue increasing throughout the summer.  When spinach plants emerge during early fall months, they serve as the only available hosts for beet armyworms.  Adult moths emerge in spring and deposit eggs on foliage of favorable plants including pigweed.  Eggs hatch in 3 to 7 days and larvae feed for about 3 weeks (Photo 86). At maturity, larvae (Photo 87) are about 30 mm long and pupate just under the soil surface.  Several generations occur annually in Arkansas.

Management:  Use of Bacillus thuringiensis may provide some benefit early in the production season.   In late season, insecticides are generally the tactic of choice.  Beet armyworm populations vary greatly in their susceptibility to insecticides.  Methomyl may provide acceptable control.  Others include spinosad, cypermethrin and other pyrethroids.  Proper spray coverage is important and to achieve this, plant spacing may need to be increased.  Also, use of a surfactant with the spray may increase effectiveness.  Due to the plant shape, beet armyworms are difficult to manage with insecticides on large spinach plants.  Insecticide application through center pivot irrigators has often provided improved control.

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Leafy vegetables (spinach): Corn earworm, Helicoverpa zea (Boddie), Lepidoptera: Noctuidae

Photo 88
Photo 88

Biology: Spinach is not a typical host plant for the corn earworm, also known as the cotton bollworm.  However, corn earworm populations build throughout the summer on many agronomic and vegetable crops.  As these crops are harvested in late summer and fall, earworm adults are forced to deposit eggs on any available plant and this may include spinach.  Adults of the corn earworm are light tan in color and are about 1.25 in. long.  Moths generally have green eyes.  Eggs are near white when laid but darken just prior to larvae emergence.   Larvae initially are minute, about 1 mm in length.  Three pair of true legs occur on the thorax and four pair plus an anal pair are found on the abdomen (Photo 88).  Color of larvae varies greatly.  Larvae cannot complete their development on spinach and eventually die as 3rd or 4th instar larvae from lack of nourishment.  Larvae have almost no impact on spinach quality or yield.  However, their presence at harvest on spinach grown for processing may result in contamination within the can.     

Management:  Generally, the only problem from corn earworm occurs on early planted fall spinach.  Thus, delaying planting date may avoid having larvae present at harvest.  When corn earworm larvae are present just before plants are harvested, use of foliar insecticides is likely the only management tool available.  As mentioned for management of other insects on spinach, increasing plant spacing, use of surfactants and insecticide application through center pivot irrigators should improve management.

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Leafy vegetables (spinach):  Grasshoppers and katydids, Orthoptera: Arctiidae and Tettigoniidae

Photo 89
Photo 89
Photo 90
Photo 90
Photo 91
Photo 91
Photo 92
Photo 92
Photo 93
Photo 93
Photo 94
Photo 94
Photo 95
Photo 95

Biology:  Large insects such as grasshoppers present major problems to the vegetable canner.  The large flat leaves of spinach can easily obscure detection during processing.  Also, if even a single grasshopper makes its way into the can, its large size will almost guarantee detection by the consumer.  In Arkansas common grasshoppers include the American (Photo 89), Carolina (Photo 90), differential (Photo 91), red legged (Photo 92), green striped (Photo 93), and Hispiscus (Photo 94).  Katydids (Photo 95) and several additional grasshoppers occur in lower numbers.  As the “grasshopper” name implies, grasses are the preferred host plants and while spinach is not preferred, its production during the cool seasons of spring, fall and winter constitutes a “green island” for grasshoppers.  Although most grasshoppers overwinter as eggs in the soil, several species are capable of surviving winter in Arkansas as adults and nymphs.  Green striped grasshoppers are commonly observed throughout warm periods in winter.  In spring, overwintering grasshoppers move from resting sites into the first green area regardless of host plant.  In fall months, grasshoppers move from mature or harvested agronomic crops like soybean and corn and dormant Bermuda grass into adjacent spinach.  Boarder areas of these fields generally hold the greatest numbers of grasshoppers although low numbers can be detected throughout the field.  Although most grasshopper species have only one generation per year in Arkansas, stages overlap and both nymphs and adults are often found.

Management: In small commercial fields produced for fresh market and in home gardens, low numbers of grasshoppers may be tolerated with no effect on yield.  A slight reduction in quality due to consumption of leaves may occur.  However, in commercial fields the lack of tolerance for even low numbers of grasshoppers and their characteristic of moving  into greens, requires that grasshoppers be intensively managed and this generally is in the form of insecticide applications.  Luckily, grasshoppers in the Arkansas/Oklahoma region are highly susceptible to several currently labeled insecticides.   Reasons for this susceptibility are not well documented but are likely related to the historical lack of exposure to modern insecticides.  Insecticide effectiveness against grasshoppers may be greatly reduced in regions in which the insecticide load is greater, esp. cotton production areas.

Pyrethroid insecticides are currently the insecticide of choice for grasshopper management.  Following application, grasshopper mortality can be detected in a few hours and most pyrethroids will persist for a few days.  Grasshopper movement back into spinach, however, may require additional insecticide applications.  Other efforts at grasshopper management include location of fields away from grasses, esp. Bermuda in turf farms and hay fields; mechanical removal with “bug knockers”, i.e., boards attached to the front of harvest machines; and even the use of “bug vacuums” attached to harvesters.  Finally, truckers should be advised not to park under lights during warm nights.  This may be a source of night flying katydids.

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Leafy vegetables (spinach): Green peach aphid, Myzus persicae (Sulzer).  Hemiptera: Aphididae

Photo 96
Photo 96
Photo 97
Photo 97
Photo 98
Photo 98

Biology:  Locally processed spinach is produced as a cool season crop in western Arkansas and eastern Oklahoma along the Arkansas River (Alma, AR to Webbers Falls, OK); in western Oklahoma around Hydro; and in southwestern Missouri near Stark City. In Arkansas and Oklahoma, spinach is generally planted three times, i.e., September for fall spinach harvested in November; late fall for overwintered spinach harvested in March; and in February for spring spinach harvested in May.  In these areas each field is generally harvested only once.  In Missouri, spinach is planted in late summer and recently was able to grow throughout the winter and be harvested multiple times. As a side note, aphids are easily washed off flat-leaf spinach during processing but very difficult to wash from savoyed (crinkle-leaf) spinach.  Spinach that is harvested multiple times may experience very high levels of aphid infestation (4000 to 5000 aphids per plant) and plants may be severely affected and killed.  In Arkansas and Oklahoma, aphids rarely reach numbers greater than 1000 per plant and appear to have little direct impact on the spinach plant.  The impact of aphids on virus movement in spinach is not fully understood.  Their main impact occurs as food contamination when aphid infested spinach is processed resulting in aphids being detected in the can.  Economic thresholds are not clearly defined but canners generally will not accept spinach with more than 150 aphids on a large plant.  Processors much prefer 0 levels.  Also, the only aphid species consistently colonizing local spinach is the green peach aphid.

The green peach aphid is a small soft-bodied insect that feeds on plants by inserting its stylet into the plant and removing large amounts of plant sap.  Both winged and wingless adults occur (Photos 96 and 97).  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.  The green peach aphid prefers the lower leaf surface of inner leaves on the spinach plant.  Thus, contact insecticides applied with typical sprayers are usually ineffective.  Also, application of contact insecticides often decreases the effects of the beneficial organisms and large increases in aphid populations may result.

Management: Two to three weeks prior to the expected harvest, an inner fully expanded leaf from at least 20 spinach plants from randomly selected locations in each field, should be visually searched for aphids.  If the average number of aphids per leaf exceeds 5, imidacloprid (Provado) should be applied.  Delay of planting date for fall and overwintered spinach can also aid in reducing aphid numbers.

Much concern exists with the potential for resistance development in green peach aphid populations to imidacloprid.  One objective of the current recommendation described above, is to avoid “overexposing” aphid populations to the highly effective aphicides.  By monitoring field populations, tolerating low numbers of aphids and applying aphicides only when needed, i.e., IPM, susceptible aphid populations should be maintained for an extended period.  In addition to insecticides, aphids are often naturally controlled by beneficial insects (predators and parasites) and fungi diseases (Photo 98).  Imidacloprid is less toxic to these beneficials than standard contact insecticides.

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Leafy vegetables (spinach): Seedcorn maggot, Delia platura (Meigen), Diptera: Anthomyiidae

Photo 99
Photo 99

                       
Biology:  Adult seedcorn maggots resemble houseflies but their body is somewhat slimmer and at rest, wings are held more backward directly over the abdomen.  The damaging form is the larval stage.  A distinct head and legs are lacking on the larvae (Photo 99).  All stages can be found during mild winters in Arkansas.  As spring nears, adults seek decaying organic matter for egg deposition.  After hatching, larvae feed for 1 to 3 weeks and pupate within the soil.  Adults emerge and renew the cycle.  Multiple generations occur each year in Arkansas.
In spinach, larvae can be found attacking the inner part of the spinach crown.  As the plant grows, a blackened area of leaf damage becomes noticeable. Seedcorn maggots occur worldwide and throughout Arkansas.  Many crops are attacked.  Problems in spinach may be more common in fields high in organic content and in freshly plowed fields with decaying grass and weeds.  However, even clean spinach fields with low organic content may be attacked.  

Management:  Planting in soils with fully decomposed plant material should reduce the attractiveness of the field to adult flies.  Damage also is more severe in fields where seed are slow to germinate and where seedling growth is retarded.  Thus, delaying planting until soil is warmer will reduce the impact of the seedcorn maggot.  Treated seed and soil insecticides applied at planting are effective tactics.  Use of insecticide sprays after maggot problems are detected on germinating seed or seedlings is of no benefit.

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Leafy vegetables (Swiss chard and spinach): Spinach flea beetle, Disonycha xanthomelas (Dalman), Coleoptera: Chrysomelidae

Photo 100
Photo 100

Biology: The spinach flea beetle is a relatively common pest of Swiss chard in south central Arkansas.  Adults apparently overwinter in plant debris, mate and deposit eggs in early spring.  Alternative host plants include several weeds, especially pigweed.  When Swiss chard develops in spring and summer, eggs are laid on foliage.  Larvae emerge and consume large amounts of foliage (Photo 100).  Small larvae feed gregariously and plant damage may be severe.                        

Management:  Low numbers of adult and immature spinach flea beetles may be tolerated, particularly in home gardens, with a minimal effect on yield.  If beetle numbers increase beyond acceptable levels, use of insecticides may be warranted.  Traditional insecticides include carbaryl and endosulfan.    Systemic insecticides such as imidacloprid, that are applied for other insects, will likely provide effective control of spinach flea beetle larvae.   

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Leafy vegetables (spinach):  Garden webworm, Achyra rantalis (Guenee) (Photo 101), Hawaiian beet webworm, Spoladea recurvalis (Fabricius) (Photo 102), and Southern beet webworm, Herpetogramma bipunctalis (Fabricius) (Photo 103), Lepedoptera: Pyralidae

Photo 101
Photo 101
Photo 102
Photo 102
Photo 103
Photo 103
Photo 104
Photo 104

                     
Biology:  In recent years, spinach producers in Arkansas and Oklahoma have experienced an increase in the frequency of webworms.  This increase may be related to the widespread use of minimal tillage in soybean.  Farmers are now permitting weeds to grow longer in emerging soybean fields before applying herbicides.  Pigweed, in particular, is now more abundant in late spring and early summer.  Pigweed is a preferred host plant for webworms, especially, the garden webworm.  Populations are building during early season and following herbicide application, webworms are moving into non-choice crops like soybean and other vegetables including spinach and greens.  Three webworm species are now common on spinach and greens grown in the Arkansas River Valley during the fall.  All have somewhat similar biologies.  Adult moths emerge in spring and deposit eggs on foliage of favorable plants including pigweed.  Eggs hatch in 3 to 7 days and larvae feed for about 3 weeks.  Pigweed can be severely damaged.  Spinach and greens are somewhat less susceptible but foliar damage is common especially from the Hawaiian and southern beet webworms (Photo 104).  At maturity, larvae are about 25 mm long and pupate on or just under the soil surface.  Several generations occur annually in Arkansas.

Management:  Beneficial insects often play a substantial role in webworm regulation and their effects should be prolonged by delaying the use of synthetic insecticides.  Use of Bacillus thuringiensis may provide some benefit early in the production season.   In late season, insecticides are generally the tactic of choice.  Webworm populations vary greatly in their susceptibility to insecticides.  Methomyl may provide acceptable control.  Others include methoxyfenozide and tebufenozide.  Proper spray coverage is important and to achieve this, plant spacing may need to be increased.  Also, use of a surfactant with the spray may increase effectiveness.  Due to the plant shape, webworms are difficult to manage with insecticides on large spinach plants.  Insecticide application through center pivot irrigators has often provided improved control.

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Contact Dr. McLeod
479-575-3397
© 2008 Dr. Paul McLeod, Department of Entomology
University of Arkansas, Fayetteville, Arkansas