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Extension Entomology

Category: Field Crops

Field crop pest and beneficial organisms.

MORE – Japanese Beetles

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Japanese beetle adults have also been emerging over the past week or two.  These adults will feed on just about any pollen, nectar, or succulent plant source for a few days then disperse to begin ovipositing into the soil.  The adults may attack emerging silks in corn or new succulent leaves in soybeans, but typically only around the edges of fields.

Dectes Stem Borers

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Dectes stem borer adults continue to emerge throughout north central Kansas.  These adults are currently aggregating mainly around field borders and most commonly on ragweed.  They will start dispersing into soybean fields within the next week to 10 days, as they do every year, to begin oviposition.  The adult females are relatively mobile and move from plant to plant inserting eggs into, or just below, the petioles of many plants.  This oviposition period may last for four weeks or more and may be spread throughout the field! This is one reason why controlling dectes stem borers with an insecticide is so difficult – timing of application.

Eggs hatch in the stem and the small larvae start feeding/boring their way to the main stem and then down this stem to the soil surface.  They usually reach the soil line in late August and larvae girdle their way around the inside of the stem, weakening the stem and often leading to lodging, especially if there are strong winds.  This lodging is responsible for most yield loss.  For more information regarding dectes stem borers, please see Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf

For more information on soybean pest management, please refer to the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

Soybean Update – Dectes Stem Borer

— by Dr. Jeff Whitworth and Dr. Holly Davis

 

Dectes stem borer adults can usually be found emerging from infested soybean stubble right around the 4th of July.  This year, despite all of the cooler, wet weather throughout north central Kansas earlier this season, is no different.  Dectes adults have been emerging throughout NC KS for about the last week or two.  They usually hang out around the field edges, aggregating for purposes of mating for just a few days up to a couple of weeks.  Then, they will disperse to soybean and/or sunflower fields to start depositing eggs in the stems of either crop.  Eggs hatch within the stems and larvae tunnel into the plant where they eventually bore their way toward the base of the plant and girdle around the interior of the stem.  This is usually completed by late August when they tunnel even further down the stem, below the soil line, for overwintering.

There are no great management tactics to avoid dectes stem borer infestations although the important damage is due to the end of the season girdling, if the plants lodge.  Harvesting fields with significant infestations as early as possible helps greatly to avoid yield losses.

For more information, please see Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf

 

Soybean Update – Bean Leaf Beetles and Webworms

— by Dr. Jeff Whitworth and Dr. Holly Davis

 

Adult bean leaf beetles (BLB) are still causing some concern for soybean producers throughout north central Kansas.  These adults have been chewing round or oval holes in the leaves of seedling plants.  However, it seems much of their leaf feeding has slowed and the females are now mostly on the ground around the base of plants depositing eggs.  These eggs will hatch in a few days and the larvae will start feeding on soybean roots/root hairs.  These larvae resemble corn rootworm larvae but BLB larvae do not feed on corn roots just as corn rootworm larvae do not feed on soybean roots.  One other major difference is that the corn rootworms eggs were deposited in fields planted to corn last year whereas the overwintering adult female BLB deposited these eggs after finding seedling soybean plants this season.

Garden webworms have been causing concern because of visible defoliation over the last couple of weeks.  However, most of these webworms, plus thistle caterpillars, have ceased leaf feeding and are in the process of pupating.  The adults will be emerging over the next couple of weeks and the females will be depositing eggs to initiate the next generation.  Thus, this first infestation of larvae of both species was just a “springboard” generation for the next one or two generations to come.

For more information relative to soybean insect management, please refer to the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

Alfalfa Update

–by Dr. Jeff Whitworth and Dr. Holly Davis

Potato leafhoppers are rapidly increasing throughout alfalfa fields in north central Kansas for two reasons:  1) potato leafhopper adults are still migrating in and 2) the eggs are hatching and nymphs seem to be everywhere.  These nymphs are very small and very shy – which means they are easily under counted as they hop to the underside of leaves, or even off the leaves, at the least disturbance.

Alfalfa weevils mating—(photo by T. Sexton)

Parasitized Alfalfa Caterpillar

Alfalfa weevil adults have mostly migrated out of alfalfa fields in north central Kansas, however there are a few that pupated late and that are just emerging out of their pupal cells.  Interesting, at least to us, was that some of these adults were mating (see picture).  Most of the literature reports alfalfa weevils mating in the late summer, fall/winter  —  not soon after becoming adults.

Alfalfa or garden webworms are also relatively common in alfalfa, where they may cause a problem in new alfalfa, and soybeans.  The next generation will probably be more problematic in small soybeans because there will probably be more webworms as this generation is more of a “spring board” generation.

Alfalfa caterpillars (see picture of larva with attached parasitoid eggs) are also quite common in alfalfa fields as are the white and/or yellow butterflies that they develop into.  However, they have not ever been found in densities great enough to cause any negative impact on yield.

 

Soybean Update – Thistle Caterpillars and Bean Leaf Beetles

–by Dr. Jeff Whitworth and Dr. Holly Davis

                Thistle caterpillars are becoming more evident around north central Kansas as they increase in size and their feeding becomes more visible.  These larvae are the result of eggs deposited by painted lady butterflies that migrated back into the state about two weeks ago.  These larvae will pupate in a couple of weeks and the adults will emerge soon after.  There will probably be even more in the next generation.

 

 

Round and/or oblong holes in seedling soybeans are indicative of adult bean leaf beetle (BLB) feeding.  Remember, these young plants are very resilient at overcoming up to about 50% defoliation in these early vegetative stages.  It takes approximately seven adult BLB/row ft. to achieve that level of defoliation.  However, adult BLB usually don’t feed for more than a few days after locating the seedling soybeans.  While this feeding can cause considerable concern because of the highly visual holes, it typically does not result in much stress to the plants, especially under good growing conditions.

 

For more information on bean leaf beetle biology and management, please see Bean Leaf Beetle:  https://www.bookstore.ksre.ksu.edu/pubs/MF2824.pdf

For more information relative to all soybean pests, please see the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

 

Corn Update – Corn Rootworms and “Ragworms”

–by Dr. Jeff Whitworth and Dr. Holly Davis

Corn rootworm larvae continue to be very active in fields of continuous corn that have been planted to susceptible varieties.

Corn earworms have been feeding in north central Kansas corn for about a week now and signs of this feeding are now becoming visible as the leave start growing out of the whorl.  The small larvae may consist of corn earworms, fall armyworms, and/or armyworms, but all may cause the same type of ragged looking leaves, earning them the name “ragworms”.

 

 

This type of leaf feeding can be highly visible, and many plants can be impacted, but the data has always indicated there is little to no effect on yield.  In addition, the larvae are well sheltered within the whorl and thus insecticides only impact them when they exit the whorls to pupate in the soil.  And, by that time, all the feeding is completed anyway.

Alfalfa Update – Potato Leafhoppers and Pea Aphids

–by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa seems to be growing very well and many fields around north central Kansas have finally dried out enough to swath and remove the hay from the field.  However, potato leafhoppers continue to migrate into the state and will continue to for about another month.  Most are still adults and have been/are now depositing eggs in stems and the tiny nymphs are just starting to emerge.  Thus, potato leafhopper feeding will become more evident as “hopper burn”, the yellowing of leaves which can reduce the health of the plants and the nutritive value of the foliage.  Therefore, if fields were just recently cut, or will be in the near future, while potato leafhoppers are still migrating into the state, they will be very vulnerable to potato leafhopper feeding damage.

Pea aphids are still plentiful throughout alfalfa fields in north central Kansas.  Populations should not reach treatable levels this late in the year, and they are a good host for many beneficial insects.

For more information regarding these and other alfalfa pests, please see the KSU Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

Bean Leaf Beetles

— by Dr. Jeff Whitworth and Dr. Holly Davis

Bean leaf beetle adults that are out and about now have successfully overwintered, probably fed a little in an alfalfa field, and are now eagerly awaiting soybean germination.  These adults are amazing at finding the first, small soybean plants where they begin feeding, causing the characteristic round and/or oblong holes in the small leaves.  These beetles will feed for just a little while and then begin depositing eggs in the soil around the stems of these plants.  These young plants are usually very resilient at overcoming this early season leaf feeding, until it reaches 50% defoliation or more. For more information on bean leaf beetle biology and management, please see Bean Leaf Beetle:  https://www.bookstore.ksre.ksu.edu/pubs/MF2824.pdf

For more information relative to all soybean pests, please see the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

 

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