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Insects of Forage Crops 

Currently only three insects are of general concern to alfalfa producers in New York. These are the alfalfa weevil, the alfalfa snout beetle, and the potato leafhopper. The alfalfa weevil and the alfalfa snout beetle are abundant before the first cutting, and the potato leafhopper is anticipated after the first cutting.

 A.C. Magyarosy, Bugwood.org
ALFALFA WEEVIL

Frank Peairs, Colorado State University, Bugwood.org 
POTATO LEAFHOPPER

 

Alfalfa Snout Beetle Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org
ALFALFA SNOUT BEETLE

 

Alfalfa Weevil Damage, Whitney Cranshaw, Colorado State University, Bugwood.orgALFALFA WEEVIL
Description
The alfalfa weevil is an important pest of established alfalfa stands in New York. New seedings are not typically affected. The alfalfa weevil overwinters primarily as an adult in New York.

These adults return to alfalfa fields in the spring when temperatures permit and begin to feed and lay eggs in the stems of developing alfalfa plants. In an “early” spring, weevils emerge sooner, more eggs are laid, and damage to the alfalfa plant occurs in the plant’s development. In general, both the weevil and the alfalfa plant respond to favorable spring temperatures, but at lower temperatures alfalfa develops more quickly than the weevil. Normally larvae hatch from eggs in early to mid-May, and their development is completed by late May to mid-June. During this developmental process the larvae go through four distinct stages: first-stage, second-stage, third-stage, and fourth-stage larvae. The larvae in each stage are progressively larger and consume increasingly more alfalfa. Actually, the larvae in the last stage consume about 80 percent of the total foliage eaten in all the stages. Therefore, the last instar causes the most damage and is the target of the management program. Fourth-stage larvae are about 3/8 inch long, green with a white stripe down the back and sides, and have a black head.

After completing its larval development, the alfalfa weevil spins a netlike cocoon on the plant, dead leaves, and stems or litter on the ground below the plant on which it fed. In about 1-1/2 weeks the adults emerge from the cocoon and feed on alfalfa regrowth for a brief period. In early July the adults migrate from the alfalfa field to adjacent protected areas such as the edge of woods, fencerows, and old fields. They then enter a resting period. In late September the adults return to the alfalfa field, feed for a brief time, mate, and lay a few eggs. Most of these eggs never make it through the winter. In early to mid-November the adults again move to protected sites to overwinter. The alfalfa weevil goes through only one generation per year.

Management
Alfalfa weevil monitoring should begin in early May and continue through first harvest and early regrowth. While walking alfalfa fields, pick at least 50 alfalfa stems at random throughout the field. Pick stems close to the ground. After collecting stems, check terminal buds and upper leaves for the “shot-hole” signs of weevil feeding. Severely damaged stems may have minimal amounts of green leaf tissue remaining, giving the leaf and stem a grayish-white frosted appearance. An alfalfa stem is considered “positive” for weevil feeding if at least one leaf shows the shot-hole damage. Count the number of weevil injured stems as a percentage of all stems collected.

Note: Alfalfa weevil infestations may vary as much as 100-fold between adjacent fields. There is no substitute for close and frequent observation in each alfalfa field. Even in the most heavily infested regions, many fields will be well below the treatment threshold and not require treatment.

Before first cutting an action threshold has been reached if 40 percent of alfalfa stems are positive for signs of weevil feeding (20 of 50 stems). Note: Count the percentage of stems showing alfalfa weevil tip-feeding injury. Do not count the percentage of leaves with weevil feeding.

If weevil populations are approaching threshold, several management alternatives should be considered: early harvest, use of a recommended insecticide (Table 4.10.1), or biological control.

Whenever possible, harvest the first cut early for hay or silage. If the field is within 10 days of a normal harvest, cut the crop for alfalfa weevil control. Harvest the alfalfa field before the fourth-stage larvae can cause damage.

This management option is practical in a “normal” or “late” spring, but difficult in an “early” spring when fourth-stage larvae peak early and alfalfa development has barely reached peak protein yield. Figure 4.10.1 shows how these peak larval periods can vary from year to year.

Remember, if harvest or other action is not taken or is badly timed, yield losses and crop quality losses can occur. After harvest, check stubble and regrowth for signs of weevil feeding. If 50 percent of regrowth shows signs of weevil feeding, larvae are <3/8 inch long, and there are few or no weevil cocoons, the field may need to be treated with an insecticide.

Insecticidal control by a wide variety of chemicals (Table 4.10.1) is another option that provides temporary reduction of the larval population.

Cost, date until harvest, residue constraints, and weather factors can limit practical implementation of this option. Like harvesting, accurate timing of the insecticide application is necessary for optimal control.

Apply an insecticide when 40 percent of the alfalfa tips show signs of injury from larval feeding. Precise timing is critical because damage builds up very rapidly. If spraying is neglected or badly timed, yield losses and crop quality losses can occur. Remember that most insecticides require a waiting period between application and harvest.

With frequent close observation of each alfalfa field and with proper timing and careful application of insecticides, good alfalfa weevil control can be achieved with the presently labeled and recommended insecticides (see Table 4.10.1). For best results, follow these guidelines:
 ·         Do not treat any alfalfa field unless absolutely necessary as shown by an assessment of feeding damage and weevil populations during the current year. Parasites and predators controlling the alfalfa weevil are building up in these fields, and insecticide application may interfere with future weevil control.
·         Do not treat fields with an insecticide with less than 50% of the stand alfalfa. In these fields, the economic return from the insecticide is marginal and these fields serve as an important refuge for the alfalfa weevil parasites which usually do an excellent job in controlling alfalfa weevil below economic levels.
·         Treat only if necessary and only at the proper dosage and time intervals. Sometimes, two applications may be necessary.
·         Do not spray alfalfa in bloom; this is a violation of state law. Protect our bees. Do not spray weedy alfalfa fields when weeds are in bloom. Poor stands of alfalfa do not justify chemical treatment. They serve as desirable sources of weevil parasite buildup.
·         Biological control of the weevil by a number of parasites is another control option.

Thirteen species of parasites of both alfalfa weevil larvae and adults have been identified in New York alfalfa fields. As these beneficial parasites multiply and become more widespread, the alfalfa weevil will become less troublesome.

One of these parasites, Bathyplectes curculionis, a small parasitic wasp, is particularly effective. Bathyplectes wasps lay their eggs in the small alfalfa weevil larvae. As the parasite develops, it consumes the weevil larva and forms a cocoon inside that of the weevil. The parasite’s small, brown, football-shaped cocoon, generally with a white band around the middle, can be often found inside the alfalfa weevil cocoon cases.

Harvesting alfalfa early, when practical, is an effective method of minimizing risk of weevil damage. This method is also more “friendly” to populations of beneficial organisms present in the field.

Related Publications: Alfalfa Weevil Management Guide, NYS IPM

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POTATO LEAFHOPPER
DescriptionPotato Leafhopper Nymph, Frank Peairs, Colorado State University, Bugwood.org
The potato leafhopper, Empoasca fabae, is perhaps the most abundant insect pest in New York State alfalfa fields. It is potentially the most damaging of the alfalfa pests because of the debilitating effect of its feeding on newly established alfalfa stands and stunting of regrowth alfalfa.

In many respects the potato leafhopper is quite different from the other major insect pests of alfalfa in New York. It does not overwinter in New York or the Northeast. Instead, it migrates into NYS each year from the South. The numbers and time of arrival vary each year, depending on the spring climate, weather patterns, and spring crop production in the South. It can arrive from mid-May to late June.

The type of feeding is also different. Weevils are basically chewing insects, consuming leaf tissue. The leafhopper is a sucking insect, removing plant sap from the vascular system of the plant. In the process of removing sap, leafhopper insects leave in the plant a salivary secretion that causes hopper burn or tip burn, resulting in injury to the plant. The characteristic yellowing or reddening of the alfalfa leaflets during the summer months are a result of the salivary toxin injected during feeding. By adversely affecting the vascular system, leafhopper feeding reduces photosynthesis, decreases productivity, stunts the plant, and sometimes kills young seedlings. A third difference is that the leafhopper does not occur in distinct generations or peaks. The adults are very long-lived and the generations continue to overlap and increase through mid-August.

The potato leafhopper is a small, wedge-shaped green insect about 1/8 inch long. It has long hind legs that allow it to hop like a grasshopper, and it has very powerful wings that allow it to fly quickly. Adults and nymphs walk backward and sideways as well as they walk forward. The leafhopper feeds on the underside of the alfalfa leaflet and stems, sucking sap from the veins. Adult females also implant eggs in the veins with the aid of a sharp ovipositor. Females lay about three eggs a day over a six- to eight-week period. Eggs, which are not visible to the naked eye, take about nine days to hatch, depending on the prevailing temperature. Nymphs, which are very pale green and hard to see on the plant, are miniature versions of the adult but have no wings. They go through five stages, or molts, in about two weeks before they become adults. The entire life cycle takes a little longer than three weeks.

Scouting for Potato Leafhopper, http://extension.entm.purdue.edu/pestcrop/2009/issue9/graphic/popups/ScoutsSweepingAlfalfa.jpgManagement
We do not yet have a reliable method to forecast damage by the leafhopper because damage relates to the density of the leafhopper and the age and state of the stand at the time of infestation. We do know that very early detection of the leafhopper-before damage appears-is essential for good management. Very young plants and plants in very early stages of regrowth are the most sensitive to leafhopper damage. Damage is intensified by moisture stress during drought. In general, if the alfalfa plant is more than 14 inches tall before the leafhopper begins feeding on it, no reduction in yield will result. If leafhoppers infest the alfalfa regrowth when it is 2 to 4 inches tall, however, densities as low as one per sweep can cause economic damage under the right circumstances. Potato leafhopper damage is intensified by drought. Under severe drought conditions, where dry weather conditions are expected to continue, leafhopper action thresholds can be halved, especially for new seedings.

Potato leafhoppers are detected by sweeping the field using a standard 15-inch-diameter insect sweep net. While walking forward, swing the net into the tops of alfalfa stems using a pendulum motion. Count the number of sweeps taken each time the net passes in front of you (see Fig. 4.10.2). Five sets of sweeps (ten sweeps per set) collected from different areas of the field are generally used for making management decisions. Count PLH found in each individual set of ten sweeps.

The management decision is made by comparing the number of leafhoppers (adults and nymphs) per sweep with the height of the alfalfa using the following chart as a guide:

Average stem length

Leafhopper/sweep

less than 3 in. (new seedings)

0.2

3 to 7 in.

0.5

8 to 10 in.

1.0

11 to 14 in.

2.0

15 in. or above

If leafhoppers exceed 2.0 per sweep and if regrowth is within 1 week of harvest, no action needed. If not, use a short-residue insecticide.

The two lowest treatment levels are specifically for use in new seedings, which warrant protection at lower leafhopper densities.

Potato Leafhopper Resistant Alfalfa: Recent advances in the development of PLH resistant alfalfa have made the planting of resistant alfalfa a viable alternative to insecticides for the management of leafhoppers. Planting the newest generation of PLH resistant alfalfa hybrids is strongly suggested for the management of PLH in both clear alfalfa seedings and in stands mixed with grass species. Please refer to the alfalfa variety table to evaluate the different available PLH resistant alfalfa varieties.

Related Publications: Potato Leafhopper on Alfalfa Management Guide, NYS IPM

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Alfalfa Snout Beetle, Kent Loeffler, Cornell University, Bugwood.orgALFALFA SNOUT BEETLE
Description
The alfalfa snout beetle, which has been damaging alfalfa since the early 1930s, has now spread to additional counties. The pest occurs nowhere else in the northwestern hemisphere except Oswego, Jefferson, Cayuga, Wayne, Lewis, St. Lawrence, Clinton, Essex, and Franklin Counties of New York and in Ontario, Canada, bordering the St. Lawrence River. Further spread is considered to be a serious threat to alfalfa growers because the alfalfa snout beetle makes growing alfalfa very difficult and expensive. Some growers have been forced to grow only grass hay because of the destructive damage by this insect to alfalfa and clover.

The adults are mottled gray and humpbacked, do not fly, are about 1/2 inch long, and are all females. Beetles are transported in gravel, hay, farm equipment, and water and disperse by walking. Circumstantial evidence also indicates the adults are transported with the movement of beehives. Suspected new infestations should be brought to the attention of the local Cornell Cooperative Extension educator and alfalfa fields with uncharacteristic amounts of winter kill should be investigated for the presence of this insect.

Management
Control is best achieved by a three-year rotation of alfalfa with row crops. The use of an insecticide for the control of this insect has not been shown to be effective. Current research efforts focused on persistent entomopathogenic nematodes as biological control agents show great promise. In addition, a research effort to breed a snout-beetle-resistent alfalfa has begin to show positive results.

Related Publications: Research shows promise for controlling destructive alfalfa snout beetle, Cornell Chronicle Online, September 2011

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