Grasshopper Monitoring and Control in British Columbia
Introduction
Of the estimated 60 species of grasshoppers found in British Columbia, only
two species are of economic importance in most outbreak situations. These
are the clear-winged grasshopper, Camnula pellucida, and the migratory
grasshopper, Melanoplus sanguinipes. Other species, such as the two-striped
grasshopper, Melanoplus bivittatus, and the redlegged grasshopper,
Melanoplus femurrubrum, have also been recorded as causing problems in B.C.
Rangelands are under constant threat from grasshopper outbreaks during
which grasshoppers compete with livestock for available forage. It is
important that grasshopper outbreaks be detected early and any control
actions be taken at the correct time and on an economic basis.
Biology of Grasshoppers
Economic species of grasshoppers overwinter in pods of 20 to 30 eggs laid
in the soil the previous year. These pods are very resistant to cold and
drying. Depending on site and weather, the eggs hatch during May and June
and the young hoppers (nymphs) begin feeding and growing. Over the next 4
to 6 weeks they will pass through five stages (called instars) at which
times they shed their skins in order to grow. Fully winged adult hoppers
emerge from the final instar and begin mating about a week later. Only
adult hoppers are able to fly and reproduce. During outbreaks when suitable
forage is scarce, adults will swarm over large areas in search of food.
Swarming will cease when females begin laying eggs. Each female can lay two
to three egg pods per week until she dies due to freezing temperatures.
Clear-winged grasshoppers tend to lay their eggs in concentrations or beds
around sloughs and draws in rangelands. Egg beds will also be found in
unbroken sod along roadsides, fences and pastures.
Migratory grasshoppers and other Melanoplus species tend to scatter their
egg pods on drier hillsides and in weedy areas, especially where the sod
has been disturbed and vegetation is sparse, such as stubble fields, road
allowances, over-grazed and weedy pastures.
Weather conditions play a very important role in the survival of hoppers
and development and intensity of outbreaks. Outbreaks are usually preceded
by 2 to 3 years of above average temperatures during the summers and falls.
Open falls allow hoppers more time to feed and lay eggs and allow more
complete egg development for faster and more even hatching the next spring.
A late spring and cool summer delays nymphal development so that fewer
adults are available to lay eggs. Cool, wet conditions during hatch will
increase nymphal mortality; in August and September, such conditions will
slow down egg laying. An early fall will cause many females to die before
laying their full complement of eggs.
Damage and Action Thresholds
Grasshoppers feed on a wide variety of grassy and broadleafed plants, and
if preferred hosts are lacking, will attack trees and shrubs. The amount of
damage or crop loss is directly related to the number of grasshoppers
present. Clear-winged grasshoppers prefer sedges and grasses, including
cereals. Migratory grasshoppers and related species feed on both grassy and
broadleafed plants, hence their preference for weedy areas.
Research has found that over the season, 12 to 24 grasshoppers per square
metre in bluegrass pasture eat as much forage as one cow per acre. A
population of one grasshopper per square metre destroys about 11 kg of
forage per hectare per month (10 lbs/acre/month). When weather conditions
promote adequate forage production to compensate for grasshopper feeding,
they are not a serious threat and numbers can increase unnoticed. However,
when conditions adversely affect forage production, grasshoppers become
serious competitors to livestock for food. Loss of desirable forage species
has also been reported, especially if stocking rates are not adjusted to
accommodate grasshopper feeding. Infestations can also spill over into
adjacent cultivated forage and cereal crops causing extensive losses if
left uncontrolled.
Table 1 lists grasshopper infestation ratings as indicated by nymph and
adult counts and the possible action required.
Table 1. Nymphs and Adults per Square Metre
Rating |
Field |
Roadside |
Control Action |
Normal |
0 - 3 |
0 - 6 |
not required |
Light |
4 - 6 |
7 - 12 |
usually not required |
Moderate |
7 - 12 |
13 - 24 |
may be required |
Severe |
13+ |
25+ |
required |
For rangeland, use the values under Roadside to establish infestation
rating. The quality and quantity of the forage (AUM value), cost of
alternative or supplemental forage sources and cost of control (product and
application) should be considered when deciding whether control is
justified.
Monitoring Grasshopper Populations
It is important to locate the egg beds or infestations as early as possible
so that grasshopper development, abundance and spread can be closely
monitored. As part of the process to decide whether or not to take action
against a grasshopper infestation, essential information to be considered
is 1) grasshopper abundance, 2) predominant species of grasshoppers
present, 3) stage of grasshopper development (size), and 4) range condition.
1) Grasshopper abundance
By observing where grasshoppers are laying eggs in the late summer and
fall, time will be saved the following spring in locating hatching nymphs
to begin monitoring the population. A sweep net (a mesh or cloth bag
attached to a hoop with a long handle) is useful in collecting the small (4
to 5 mm long) nymphs. It is important that infested sites be monitored
weekly for the first 3 weeks (longer if hatching is delayed), then every
other day. Inspections should be made during the warm part of the day when
the hoppers are most active and easier to spot.
Inspections involve walking through the pasture in a set pattern (Figure 1)
and counting the number of grasshoppers in each of 20 one-square-foot areas
spaced 15 to 20 paces apart. Select a square foot area well ahead of you
and, as you approach and step down, count the number of grasshoppers within
or jumping out of that square foot. Walk into the field, not just along the
margin where the hoppers may be more abundant due to movement in from
adjacent areas. Total the number of hoppers counted and divide by 2 to get
the number of hoppers per square metre.
This sampling procedure should be repeated at enough locations in the
pasture to give a good indication of the grasshopper situation throughout
the pasture and the location of hot spots.
2) Grasshopper identification
Newly hatched clear-winged grasshopper nymphs are black in color with a
distinctive white band around the thorax (body section right behind head).
Adults are yellowish to brownish in colour, about 21 to 32 mm long, with
clear wings mottled with rounded brown spots. They have two stripes
beginning at the thorax and converging at the tip of the wings.
Migratory grasshopper nymphs have black bands on the top of their thorax.
Adults are grayish to reddish-brown on top, yellow beneath, and a little
larger than clear-winged adults. The front wings have no stripes but have
small black square patches rather than the rounded brown spots of
clear-winged adults. The thorax has a pale stripe along each upper edge and
black bands on the sides.
Redlegged grasshoppers are gray-green to yellow brown above and yellow
beneath. The last segment of the hind leg is red.
Two-striped grasshopper nymphs are green to yellowish-brown in color.
Adults are much larger than the preceding species (up to 40 mm long), and
are distinguished by having two pale stripes extending back from the eyes
to the tip of the wings.
3) Stage of development
As previously mentioned, grasshoppers grow by shedding their skins
periodically, and their current stage of development (instar) can be
roughly determined by their size (length). By monitoring the growth of the
hoppers, application of chemicals can be properly timed against the most
susceptible stages and before significant damage has occurred. Grasshoppers
are best controlled when most are about 6 to 10 mm long. At this time most
eggs will have hatched. However when egg hatch is prolonged by cool
weather, additional treatments may be necessary if numbers warrant. As
grasshoppers get larger they require more chemical to control and forage
loss increases.
4) Range condition
Grasshoppers should be controlled when they are so numerous that the cost
of controlling them is less than or equal to the expected value of the crop
or forage losses. Therefore it is important to assess the condition
(quality and quantity) of the plant stand as part of deciding whether or
not it is economic to apply a chemical or other treatment. Under hot, dry
conditions and poor plant growth, fewer hoppers will cause economic losses
compared to conditions that favor plant growth. However if the forage is
worth little, chemical control may not be justified because the cost is
greater than the value of the crop saved.
Pest Management
Various options are available to manage grasshopper populations and
minimize crop losses. Use of any one or combination of options (integrated
pest management) is dependent on compatibility with the crop and production
system. 1) Natural control
As previously mentioned, weather plays a major role in grasshopper
population increase and decline. There is little one can do to influence
weather, however knowledge of its relationship to grasshopper populations
is useful in anticipating potential changes in grasshopper abundance.
Grasshoppers are also attacked by a number of natural enemies such as other
insects and several disease organisms, as well as many birds and rodents.
Next to weather, natural enemies probably have the greatest influence on
localized grasshopper abundance. Two diseases, Nosema locustae and
Entomophaga grylli, are commonly observed in B.C. during outbreaks. Dead
Nosema-infected grasshoppers turn brown and are fed upon by other
grasshoppers, helping to spread the disease. Grasshoppers killed by
Entomophaga are easily seen clinging to the stems of grasses and other
plants. Another disease organism, Beauvaria bassiana, also attacks
grasshoppers. Research is ongoing towards developing biological control
products containing one or more of these disease organisms. 2) Cultural control
Cultural control methods involve the application of crop management
practices that discourage or delay the development of grasshopper
populations or facilitate the chemical control of outbreaks. For pastures,
research has shown that natural grass stands have fewer grasshopper
outbreaks than tame grass pastures because of greater plant diversity and
fewer economic species of grasshoppers. By not over grazing, pastures will
have less broadleafed plants to attract Melanoplus species. Cultural
methods used to control grasshoppers in cultivated crops include seeding as
early as possible (crop gets a jump on the hoppers), crop rotation (seeding
less favored crops in infested fields), tillage (eliminate food plants in
spring and fall) and seeding traps strips (concentrate hoppers for more
economical chemical control).
3) Chemical control
When natural and cultural controls fail to prevent outbreaks, the only
option left to protect crops is to apply an appropriate chemical control
product when and where necessary. By following the monitoring methods
previously described, areas requiring treatment can be identified and
treated in a timely fashion.
Before applying any chemicals, carefully read the label for mixing and
application instructions, and any precautions such as interval before
grazing or harvesting. The latest edition of the FIELD CROP GUIDE to Weed,
Disease, Insect, Bird and Rodent Control for Commercial Growers contains
the insecticides currently registered for grasshopper control. A copy can
be obtained from your nearest Ministry of Agriculture & Food office.
Compiled by
Hugh Philip, Entomologist
Plant Industry Branch, Kelowna
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