Minnesota Grasshopper Management

Ian MacRae, Ken Ostlie, & Bruce Potter - UMN Dept. of Entomology

Contents:

Introduction
Grasshopper Species
Life History
Grasshopper Damage
Grasshopper Scouting
Grasshopper Management Strategies
Integrated Pest Management

Introduction

Introduction – Grasshopper populations are heavily influenced by climate. Long, warm autumns, followed by warm, dry springs contribute to the building of grasshopper populations. A long, warm autumn favors egg-laying by grasshoppers well into September and even October in some Minnesota locations. In addition, if economically impacting populations of grasshoppers occurred the previous year in a variety of cropping systems, autumn populations will likely be high. In a warm, dry spring, many areas in the state that had elevated populations the previous year may face localized outbreaks. However, with early scouting and carefully applied management when necessary, grasshopper populations can be controlled and economic damage to cropping systems can be kept to a minimum.

Economically Important Grasshopper Species in Minnesota

Although there are 75-100 grasshopper species on the Northern Great Plains, only 5 are likely to become economically damaging pests of crops in Minnesota. It is therefore necessary to recognize which species of grasshoppers are present. All 5 have features in common that make them potential pests of a variety cropping systems. All 5 species:

1. have young which readily move from areas in which they were hatched to neighboring cropping systems,

2. will accept many different types of plants as food sources, and

3. have adults which are strong fliers that can disperse to find and exploit new food resources (such as crops)

It is important to note that although the following species are the most economically important grasshoppers in Minnesota, other species can occasionally become problems in cropping systems. Early monitoring of crops will provide the data necessary to make sound control decisions, regardless of grasshopper species identification. They are presented in their order of seasonal appearance.

Twostriped Grasshopper (Melanoplus bivittatus)– This is a very common grasshopper throughout the US and Canada. It is found in a wide variety of habitats and can be a major pest of not only agricultural crops but also in urban gardens. This grasshopper is polyphagous, eating a wide variety of different plants. It causes extensive damage in small grains, alfalfa, soybeans and corn. It is a wasteful feeder, clipping plant material, much of which simply drops to the ground uneaten. In outbreak situations may completely destroy the crop. Adults are grayish or brownish green with two distinct yellow strips extending from the head to the wing tips. They are relatively large grasshoppers (adults are 1¼" – 2" long), and have a distinct black band on the top of the femur of the jumping leg. It is normally the 1^st grasshopper to hatch in Minnesota. Nymphs will begin to hatch in early May and will be present through early July.

Migratory Grasshopper (Melanoplus sanguinipes)– The migratory grasshopper has a very broad distribution and causes more damage to crops than any other species of grasshopper in N. America. It prefers open grasslands in areas with compact soils and short grasses. Although it prefers to feed on weeds and forbes (short, leafy plants) it will become a pest in small grains, alfalfa, clover, corn, and vegetables. Very heavy populations will also start to feed on bushes and trees, eating fruit, leaves and even bark. Their preference for feeding on weeds may draw them into weedy fields even in years when populations are not high. Smaller than the twostripped, the adult migratory grasshopper is about 1" long, brown to gray with a distinctive black mark behind its eye. It is a strong flier and disperses readily. It is sometimes confused with the Redlegged Grasshopper but has a slight hump behind the spine on its underside, between the middle pair of legs.

Clearwinged Grasshopper (Camnula Pellucida)– Common in the western and northeastern states and southern Canada, the clearwinged grasshopper prefers meadowlands with short, thick grasses. It feeds mainly on grasses and so is particularly damaging in the early season to spring wheat. Swarms will invade vegetable crops and feed preferentially on onions, lettuce, cabbage, and peas. Smaller than either the Migratory or the Redlegged, the Clearwinged Grasshopper adult is about ¾" long. It is very light tan to brownish and has clear wings, distinctly marked with large brown spots.

Redlegged Grasshopper (Melanoplus femurrubrum)– Another widely distributed species, the redlegged grasshopper prefers dense stands of weeds and grasses. It has very wide food preferences, in one study, this species fed on 8 different types of grass, 3 sedges and 28 forbs. During outbreaks it may severely damage alfalfa, clover, soybeans, small grains, various legumes, corn, vegetables and tobacco. Like the twostriped grasshopper, it is a wasteful feeder, leaving as much as 75% of the plant clipped but unconsumed. It is a moderately sized grasshopper, ¾" – 1" long and is brownish red. It has a pinkish-red tibia on the jumping leg, but these may also be bluish. It also has a line of black spines on the hind margin of the tibia. Although they will oviposit in alfalfa, Redlegged grasshoppers cannot complete development on a diet entirely composed of alfalfa. CRP is a prime breeding and production habitat for Redlegged grasshoppers.

Differential Grasshopper (Melanoplus differentialis)– Found more frequently in the west, the differential grasshopper is found in variuous habitats of mixed vegetation. It is a mixed feeder, preferring forbes. Nymphs will become a pest in small grains, alfalfa and other hay crops. After they become adults and have destroyed these crops, the adults will fly into corn soybeans, and occasionally sunflowers. A dense swarm will destroy a young cornfield in 3-4 days. A large grasshopper, adults are 1 ½" – 1 ¾", yellowish or greenish gray. The femur of the jumping leg is distinctly marked with black chevrons. They are the last economically important grasshopper to hatch and develop in Minnesota and are more abundant in the central and southern part of the state.

More excellent grasshopper images are available at the Iowa State University Entomology Insect Image Gallery

Life History

Female grasshoppers lay eggs in the soil usually in a small burrow. They prefer firm, undisturbed (untilled) soil such as CRP, roadsides, pastures, etc. These are called production areas. Eggs are cemented together and the burrow is closed with a plug constructed of cement and soil particles. Egg masses are buried 1/2 to 2 inches below the soil surface. A female can lay up to 25 egg masses. All of the five most economically important species in Minnesota overwinter as eggs. Eggs are the stage in which grasshopper suffer the most mortality. If soil moisture content gets too high, many of the eggs will not hatch. Normal levels of rainfall are important to control grasshopper populations. Dry springs favor the increasing grasshopper populations; this explains why grasshopper outbreaks generally occur during dry years.

Different species of grasshopper develop at different rates. The earliest species (the Twostriped grasshopper) begins to hatch in late April to early May with other species beginning to hatch out at approximately 3-4 week intervals. This means that young grasshoppers of different species are going to be present throughout most of the summer. This also means you must scout for different species at different times through the growing season (see Scouting for Grasshoppers below).

Young grasshoppers are called nymphs; young insects called nymphs closely resemble the adult stages except they lack wings. Like all insects, grasshopper nymphs must molt to grow. Grasshopper nymphs usually go through 5 molts before they reach adulthood. This process takes is controlled by temperature but usually takes about 60 days. Nymphal stages of most insects are generally easier to control as they are more susceptible to pesticides. Heavy rains can also cause mortality among grasshopper nymphs. This is another reason dry years can result in heavy grasshopper populations.

Grasshopper Damage

Damage includes not only defoliation but direct feeding on pods and/or seeds. Grasshoppers prefer to lay eggs outside the cropping system in production areas. When populations get high, food in the production areas is depleted and grasshoppers disperse to neighboring cropping systems. For this reason, the first areas in crop fields to show grasshopper damage are the field borders. The heaviest damage to crops tends to be in these locations. In outbreak years, however, grasshopper populations are so high they disperse and damage crops throughout the field.

Adult grasshoppers are much more damaging than younger nymphs. Adults are much more mobile, readily moving into neighboring cropping systems to utilize new food sources. In addition, adults are less susceptible to pesticides, making control even more difficult. Early detection of potential grasshopper problems is, therefore, essential to avoid heavy crop damage.

Scouting for Grasshoppers

There are two methods of scouting for grasshoppers. The first involves egg counts. Grasshopper eggs are laid in the top 0.5" - 2.5" of soil. Samples of soil from this layer can be gathered from areas in which grasshoppers laid eggs the previous season. This soil is sifted through a 0.25" sieve and the egg masses recovered and counted. This is a time consuming procedure and depends on sampling soil from areas wherein grasshoppers have laid eggs. A much simpler and more accurate method of estimating grasshopper populations is to conduct adult and nymph counts in the spring and summer.

Adult and nymph counts are conducted by walking through the region to be sampled (in the early season, this would be either the production area or the field border) and visualizing 1 sq.ft. areas along your path. As you come to the 1 sq.ft area, count the number of grasshoppers within it (this is usually best done by counting the number of grasshoppers that hop out of the area or are moving within it). Do this a minimum of 20 time through the region to be sampled. Divide the total number of grasshopper counted by the number of 1 sq.ft. areas sampled to arrive at the average number of grasshoppers per sq.ft. in the field. Multiply this number by 9 to calculate the average number of grasshoppers per sq.yd. in the field. Grasshopper treatment thresholds are based on numbers per sq.yd. The 1 sq.ft. sample areas are used because it would be too difficult to count the number of grasshoppers in a sq.yd. when populations are very high.

Scouting should start in early May to catch the first hatch of twostriped grasshoppers and continue through until late June or early July to monitor for the later hatching species (click here for the seasonal distribution of the 5 important grasshopper species). Because grasshoppers lay eggs in production areas and the young nymphs and adults are moving into the cropping system to find food, the edges of fields are the first part of the field damaged by grasshoppers and frequently suffer the greatest damage. Scout the production areas and the field borders first. Move scouting activities deeper into the field as the season continues. Pay special attention to the number of nymphs (the small grasshoppers with no wings). Nymph counts will indicate what the grasshopper population will be later in the season.

No control action should be undertaken unless the number of grasshopper in the field surpasses the action threshold. Occasionally, these thresholds may rarely need to be tempered if economic stand loss is observed in sensitive crops including seedling alfala and sugarbeets. Thresholds are based not only on the stage of grasshoppers being sampled but also on where in the field the samples are taken. Because more damage results from older grasshoppers, adults have lower thresholds than do nymphs. Likewise, because the edges of the field are the first hit, high numbers of grasshoppers within the field mean there must be even higher numbers at the edge of the field. Consequently, there are lower thresholds for grasshoppers sampled within the field than at the margins of the field.

Table 1. Grasshopper action thresholds

Rating	Nymphs per square yard		Adults per square yard
	Margin	Within Field	Margin	Within Field
Light	25-35	15-25	10-20	3-7
Threatening	50-75	30-45	21-40	8-14
Severe	100-150	60-90	41-80	15-28
Very Severe	200+	120+	80+	29+

Grasshopper populations do not just arrive out of thin air. Regions that had high grasshopper populations last year ('hot spots') will likely have high populations this year. Grasshopper populations build over years, with outbreak populations taking upward of 3-5 years to develop. Late, warm autumns followed by warm, dry springs generally favor developing grasshopper populations. The autumn of 1997 was exceptionally long and warm with grasshopper egg laying observed in some areas of the state as late as October. This indicates the number of grasshopper eggs in the soil is quite high and there is the potential for very high grasshopper populations in several areas of Minnesota.

Grasshopper Management Strategies

The control strategies suggested here follow the philosophy of Integrated Pest Management; maximize pest control and crop protection while minimizing cost and environmental impact. This basically amounts to treating only when and where necessary. This means only applying when the above action thresholds have been exceeded. Often, grasshopper management involves treating areas outside of cropping systems, raising concerns over external costs such as human risk, wildlife hazard, risk to water quality, and risk to other non-target organisms. Recommended strategies depend on not only the grasshopper population but also the time of year and location of the population.

Early Season Control Strategies

· When populations are high and causing damage to cropping systems, such as small grains and sugarbeet, protective applications can be made to the borders of crops (late April, early May)

· If populations or damage continue to increase, more extensive treatments to the crop itself can be made

· If nymphal populations should rise to 150+ per square yard, treatment should be moved to the grasshopper production site (roadsides, ditches, pastures, CRP, etc) from which the population is originating

· Treatment of nymphs should be delayed until nymphs are in the 4-5th stage (wing pads visible). Most egg hatch should be complete by that time. Treating earlier than this means unhatched unhatched grasshoppers will survive.

· Roadsides and other grasshopper production sites should not be treated until most of the nymphs are 5th stage

· Crops adjacent to grasshopper production sites may require earlier treatment because stand reduction or defoliation may reach levels causing yield loss.

· Some sensitive crops including alfalfa and sugarbeets will be killed if feeding cuts the plant below the cotyledon. These crops may need to be treated before 4-5th stage nymphs are present to avoid stand loss.

· If timed correctly, a single treatment of the grasshopper production sites can give good seasonal control

Late Season Control Strategies

· Treat grasshoppers before adults show up (mid-June) to prevent them mating and laying eggs for next season. More importantly, once they become winged, grasshoppers can disperse to neighboring cropping systems much more easily, increasing control problems. At this stage, a treatment to prevent immigration of grasshoppers will only last the residual period of the pesticide and then the field will require another application

· Cutting CRP for hay will cause grasshoppers to leave the cut field and move into neighboring crops. Leaving 10-20 yard uncut strips will keep grasshoppers in the field.

· Likewise, cutting alfalfa for hay will have the same result. If leaving uncut strips is impossible, it may be possible to apply an insecticide with a short pre-harvest interval 1-2 weeks before cutting to prevent movement of grasshopper into neighboring fields. This strategy depends on availability of an appropriate registered insecticide, the value of crop to be protected, and the grasshopper population.

In general, it is advisable to border treat cropland early when nymphs are small and nymph numbers are moderate (50-75 square yard). If numbers are high early in the season, it may be necessary to border treat both the crop and the grasshopper production sites. As the nymphs become larger, move the treatment to the grasshopper production site and enlarge the treated portion to handle the grasshopper population. When grasshopper numbers are high in the grasshopper production site (100+ per square yard), it is environmentally safer and less expensive to treat the grasshopper production site than make repeated treatments in the cropping system. A number of insecticides are registered for use against grasshopper in a variety of cropping systems.

Integrated Pest Management

Natural Enemies
There are a number of organisms which feed on grasshoppers and some can be used as biological control organisms.

· Protozoans - The best known is Nosema locusta, a sporazoan. This is a disease-causing organism which occurs naturally but usually does not cause the kind of epidemics in the wild that would control grasshopper populations. When N. locusta is applied to bran baits and targeted against early occurring nymphs, the rates of infection can be dramatically increased. In addition, because grasshoppers will cannibalize when populations get high enough, even higher infection rates are brought about by healthy older stages consuming younger nymphs dying of the disease. N. locusta can also be transmitted through the egg to he nymphs, impacting next year’s population. Unfortunately, like any other disease, N. locusta takes time to control populations and crop protection does not start until a sufficient proportion of the population has been infected. It is therefore only suitable in low-value crops and its expense had even made commercial preparations difficult to establish in this market.

· Fungi - There are a number of disease-causing fungi that affect grasshoppers. These entomopathogenic fungi infect and feed on living grasshoppers, eventually killing them. Infected grasshoppers generally crawl up a plant just prior to dying and the body fills with fungal spores. As the grasshopper body disintegrates, the spores are released throughout the area to infect other grasshoppers. Beauveria bassiana is another naturally occurring disease-causing organism and is available as a commercial product. It infects grasshoppers, penetrating their external skeleton and feeding on the liquid inside the grasshopper’s body. It can be applied to baits to increase the efficacy and because it has no mammalian toxicity, it can be applied in any cropping system. Unfortunately it shares the same shortfalls as Nosema locusta; it is slow to act, expensive and requires specific conditions to become established. Consequently, crop protection is non-existent until a sufficient percentage of the grasshoppers have become infected with the fungus. Other species of fungi have been researched but none have been successfully released as commercial products.

· Parasites - Parasites which have been noted to affect grasshopper populations include nematodes and parasitic flies. Although the natural rates of parasitism in grasshopper populations for both of these natural enemies can be high, actual population control is rare and there are no commercially reared parasites available specifically for grasshopper control.

· Predators - Considerable mortality occurs to grasshoppers through predation by birds, small mammals and even other insects. Grasshopper eggs are eaten by a variety of ground-dwelling predators including ground beetles, crickets, bee flies, and the larvae of blister beetles. Nymphal and adult grasshoppers are heavily preyed upon by spiders, wasps, robber flies, rodents, and birds. Unfortunately, this mortality is not sufficient to control grasshopper outbreaks as it is a natural part of grasshopper population cycles.

Cultural Control
The five species of grasshopper which become economic problems in Minnesota all prefer to lay their eggs in untilled, hard packed ground with minimal rooting structures. Sites such as roadsides, ditches, CRP, and pasture are prime grasshopper production areas. Egg laying in soybeans, untilled stubble or fallow and occasionally in alfalfa will also sometimes occur. Roadsides and ditches cannot be tilled, and it is not possible to till many row crops early enough to prevent egg laying by grasshopper. Weedy fallow and grain stubble, however, are ideal grasshopper egg deposition sites and they can be successfully managed to decrease next year's grasshopper populations. Even light tilling of grain stubble will decrease grasshopper egg laying to almost nothing. Weeds in fallow should be controlled because they attract grasshoppers and provide excellent egg laying habitat. If the fallow is disturbed (especially if this removes weeds)grasshopper egg laying in the site will be almost non-existent. Tilling after egg laying is almost completely ineffective. It is difficult to kill enough grasshopper eggs, even with deep plowing, to impact the following year's population.

· Tillage in small grains or weedy fallow must occur before egg laying to be effective in grasshopper control. This will reduce or even stop grasshoppers from laying eggs in these sites.

· Tillage after egg laying will not sufficiently reduce grasshopper eggs to control outbreaks the following year.

· Other justifications for tillage methods (erosion control, moisture retention, etc) are far more important considerations and may outweigh the usefulness of tillage as a management tool for grasshoppers.

BIBLIOGRAPHY

Capinnera, J.L. & T.S. Sechrist. 1982. Grasshoppers (Acrididae) of Colorado - Identification, Biology, and Management. Colorado State University Experiment Station, Ft. Collins, Bull. No. 584S

Noetzel, D.M. 1990. Grasshopper Management. Minn. Extension Service, University of Minnesota Agriculture, AG-FO-3936.

Phadt, R.E. 1988. Field Guide to the Common Western Grasshoppers. USDA APHIS & Wyoming Agric. Expt. Stat. Bull. 912.