Guide to Identifying Field Crop Pests of Minnesota

Guide
Guide
Identifying
totoIdentifying
F ieldCrop
Crop Pests
Pests
Field
Minnesota
ofofMinnesota
Plant Pest Survey Program
Minnesota Department of Agriculture
Prepared by Mark Abrahamson, Survey Coordinator
January, 2001
Table of Contents
Page
Alfalfa
Recognizing and growth staging
Potato leafhopper
Weevils in alfalfa
Alfalfa and tarnished plant bugs
Pea aphid
Spittlebugs
Blister beetles
Caterpillars in alfalfa
Alfalfa blotch leafminer
Incidental captures
4
4
6
8
12
15
16
17
18
19
20
Corn
Growth staging
European corn borer
Corn rootworms
True armyworm
Common stalk borer
Cutworms
Corn earworm
Corn caterpillar recap
Corn flea beetle
Corn leaf aphid
Brown stink bug
Pathogens
21
21
24
28
31
33
35
37
38
39
40
41
42
Small grains
Distinguishing
Growth staging
Aphids
True armyworm
Aster leafhoppers
Cereal leaf beetle
Pathogens
43
43
44
46
48
50
51
52
Page
Soybeans
Growth staging
Bean leaf beetle
Imported longhorn weevil
Caterpillars
Red headed flea beetle
Potato leafhopper
Two-spotted spider mite
Soybean aphid
Soybean cyst nematode
Japanese beetle
Incidentals
53
53
56
58
59
60
61
62
64
65
66
67
Margins
Grasshoppers
Incidentals
68
68
71
Beneficials
Lady beetles
Lacewings
Assassin bugs
Damsel bugs
72
72
75
75
76
Alfalfa
Recognizing and Growth
Staging Alfalfa
Growth staging alfalfa is simple - just estimate the
average height of the stand and record it along with
the number of times the stand has regrown that year.
A stand that has not yet been cut is on its first
growth, a stand that has been cut once is one its
second, etc. Alfalfa stands are rarely pure, often
Figure 1. Alfalfa in bloom - each leaf has 3
leaflets, a plant has multiple stems.
weeds are abundant or the stand is mixed with
clover (Fig. 2 and 3). Try to only sample fields that
are at least 50% alfalfa. When inspecting plants for
insect feeding damage - make sure you are inspecting alfalfa plants.
Figure 2. Clover - note that the leaves are broader
than alfalfa and that they have a light “V”.
Clover
Alfalfa
Figure 3. A mixed stand of alfalfa and clover.
Potato leafhopper
Recognize potato leafhoppers by their small size,
elongate shape and lime-green color. PLH migrates
into the state each June. > 1 generation occurs in
MN, and nymphs and adults may be found in the
same samples. PLH can be confused with the aster
leafhopper which may also be found in alfalfa.
Notice that aster leafhopper adults are grayer,
slightly larger and have black dots on their forehead
(Fig. 9) where PLH has white dots (Fig. 6). Aster
leafhopper nymphs are yellow (see small grains)
rather than green like PLH nymphs (Fig. 7, 8). PLH
feed on plant fluids - damage to the plant is a
yellowing called “hopperburn” (Fig. 5).
Actual adult
size = 3-4mm
Figure 4. Potato leafhopper adults in sweep net
with an alfalfa leaflet.
Figure 5. “Hopperburn”
Figure 6. Adult potato leafhopper.
Figure 7. Potato leafhopper nymph.
Figure 8. PLH
nymph at point
of pocket knife.
Figure 9. Adult aster
leafhopper.
Alfalfa weevil
Alfalfa weevils are most abundant in May and June.
Adults and larvae are likely to be found in the same
samples. Adults are distinguishable from other
weevils found in alfalfa by their size, prominent
snout, and dark dorsal stripe (Fig. 10). Larvae are
instantly distinguishable from caterpillars in that
they do not have legs. AW larvae can also be
distinguished from other weevil larvae by their size,
their black head and their lateral white stripe (Fig.
12). Adults and larvae are defoliators and they
leave behind a characteristic feeding damage called
“skeletonizing” (Fig. 11, 13).
Figure 10. Alfalfa weevil adult.
Figure 11. Weevil
feeding damage
Figure 12. Alfalfa
weevil larva note lack of pink
flecks along
stripe and black
head.
Figure 13. Alfalfa weevil adult, larva and feeding
damage.
Skeletonizing means that they eat the leaf tissue
from between the leaf veins - leaving the veins
intact.
The clover leaf weevil may be confused with the
alfalfa weevil but both adults and larvae can be
distinguised by their larger size (Fig. 14, 15, 16).
Also, CLW adults have much broader dorsal stripes
than AW adults (Fig. 14); and CLW larvae have a
brown head and pink flecks lining their lateral white
stripe. Feeding damage from CLW can’t be distin-
guished from AW.
The clover root curculio is another weevil found in
alfalfa that superficially resembles alfalfa weevil.
However, CRC adults are slightly smaller than AW
and they lack an elongated snout (Fig. 17). CRC
larvae occur in the soil and won’t be found in sweep
net samples. CRC adults feed on leaf edges, rather
than on the interior of leaves like AW and CLW.
Figure 14. Clover leaf weevil (left) and alfalfa
weevil adults.
Figure 15. Clover leaf
weevil adult near
thumbnail.
Figure 16. Actual sizes of weevils in alfalfa:
Adult
Larva
Alfalfa weevil =
Clover leaf weevil =
Clover root curculio =
Figure 17. Clover root curculio adult - note the
short snout.
Plant bugs
Two plant bug species are commonly found in
Minnesota alfalfa: alfalfa plant bug and tarnished
plant bug. Both species produce multiple generations during the summer and it is common to have
nymphs and adults of both species in the same
sample. All life stages of both species feed on
plant fluids - APB tends to be the greater pest in
alfalfa as TPB attack many different crops and
usually aren’t as abundant in alfalfa as APB. Plant
bug feeding tends to cause leaf deformities in
alfalfa.
Plant bugs in general are easy to recognize due to
the characteristic bend in their body and the cells
present in the wings at the bend. The two species
are also easy to recognize - the APB is ~2x’s the
size of TPB. APB adults are green with light
colored wings (Fig. 19), while TPB adults are
brown with dark colored wings (Fig. 20, 21). APB
nymphs are light green (Fig. 22, 23), TPB nymphs
are similar but have very distinct brown mottling.
Smaller plant bug nymphs look superficially like
aphids - however body shape in plant bugs tends to
be more elongate, and plant bugs do not have
cornicles (Fig. 24).
Figure 18. Alfalfa
plant bug adult and
nymph.
Figure 19. Alfalfa plant bug adult.
Figure 20. Tarnished plant bug adult.
Figure 21.
Tarnished
(left) and
alfalfa (right)
plant bug
adults.
Figure 22. Alfalfa plant bug nymph.
Figure 23. Alfalfa
plant bug nymph
(left) and potato
leafhopper adult
(right).
Cornicles
Figure 24.
Pea aphid note cornicles.
Pea aphid
The most common aphid found in Minnesota
alfalfa is the pea aphid. Occassionally other
aphid species may be found but the only one of
significance at this time is the pea aphid. Pea
aphids can be recognized by their size, shape and
cornicles. Aphids feed on plant fluids like plant
bugs and potato leafhoppers, however alfalfa can
tolerate much greater abundances of pea aphid
than potato leafhopper. There are many generations of pea aphids each year in Minnesota, but
there is no need to differentiate between life
stages.
Figure 25. Pea aphid cluster.
Spittlebugs
The name spittle bug comes from the froth that
nymphs excrete over themselves while feeding.
These spittle masses are easy to spot and recognize
in the field. The nymphs may potentially damage
alfalfa if abundant but they’re only present in early
summer (Fig 26). Adults may be found in sweep
net samples later in the summer - they do not damage alfalfa (Fig 27).
Figure 26. Spittlebug nymph minus the spittle.
Figure 27. Adult meadow spittlebug (froghopper).
Blister beetles
Several species of blister beetles may be found in
sweep net samples in alfalfa. Two of the more
common species are black and gray blister beetles.
Blister beetles do not damage alfalfa but they are
toxic to livestock in high abundances. Blister
beetles can be recognized by their body shape and
differentiated from each other by their coloration.
Figure 28. Black blister beetle
Figure 29. Gray blister beetle
Caterpillars
Several caterpillars will be found in alfalfa. Alfalfa
caterpillar is green with a prominent white lateral
stripe flecked with red. They have 4 pairs of abdominal prolegs. Green cloverworm is also green
and has faint white striping. It has 3 pairs of abdominal prolegs. Sawfly larvae are commonly
found in alfalfa - they are actually not caterpillars as
they are in the Order Hymenoptera (not Lepidoptera). Sawfly larvae can be recognized by
always having more than 5 pairs of abdominal
prolegs (Fig. 30). No known economic damage is
caused by sawflies in alfalfa, however they may be
commonly found.
Figure 30. Sawfly larva.
Alfalfa blotch leafminer
ABLM has several generations per year in MN, and
will likely be found throughout most of the summer.
The damage caused by larvae to leaves is characteristic and is due to the larva eating the inner leaf
tissue (Fig. 31). Adults are small flies that chew
small “pinholes” completely through leaves. They
are likely to turn up in sweep net samples where
ablm is abundant.
Figure 31. Alfalfa blotch
leafminer - characteristic
“J” larval mine and pinholes
from adult feeding.
Figure 32. Leaf damage
(called “window paning”)
not from ABLM.
Incidentals
Bean leaf beetle feeds in alfalfa early in the year
before moving to
soybeans - they
may be common in
early summer
sweep net samples.
For more info on
bean leaf beetles see soybeans.
Figure 33. BLB
Common stalk borer and true armyworm larvae
may also found in alfalfa stands that contain a lot
of grass. These insects feed on grass early in the
season until moving into corn later in the summer.
See corn for more information on these insects.
Figure 34. Common stalk borer larva.
Figure 35. True armyworm larva.
Corn
Growth Staging Corn
Corn development is separated into vegetative and
reproductive stages. Vegetative stages are VE, V1,
V2, V3, Vn ..., VT. Reproductive stages are R1,
R2, ... R6. VE equals the time the corn plant first
emerges from the ground (Fig. 36) until the first leaf
collar is visible
(Fig. 37). When
the first leaf collar
is visible the plant
is considered to
be in V1. The
leaf collar is a
light band at the
base of the leaf
blade (Fig. 38).
The plant is then
considered to be
Figure 36. VE
in V1 until the
second leaf
collar is
visible, at
which time
the plant is
considered
V2 (Fig.
40). Each
successive
Figure 37. V1
leaf collar
advances
the stage number of the plant (i.e., 3rd collar = V3,
4th collar = V4, etc.). By about V5-V6,
the lower
leaves have
been shed
due to root
development and
other
factors. To
accurately
stage the
plant, the
stalk should
be split and
the first
Figure 38. Leaf collars.
Figure 39. Close up of
leaf collar.
count up
from this
leaf to
determine the
V-stage.
The last
vegetative stage
is VT,
the plant
is consid
visible
internode
located.
The first
leaf
above
this
internode
should be
the 5th -
Figure 40. V2
ered to have entered this stage when the last tassel is
visible (though tassel emergence begins well before
this). The plant is considered to be in VT until the
silks begin to emerge from the husk(s). Silk emergence is the first reproductive stage - R1. The plant
enters R2 when kernels are first visible on the ears
(this stage is also called the “blister stage” because
the kernels resemble blisters). The R3 stage begins
when milky fluid is present in the kernels (also
called milk stage). R4 begins when the milk has
turned somewhat solid (also called dough stage).
As the ears dry, the kernels begin to dent. When
most of the kernels are dented the plant is considered to be in R5 (also called dent stage). R6 is the
final growth stage, the corn plant enters R6 when all
kernels have reached maximum dryness.
Approximate dates of corn growth stages in southern Minnesota (from data for Arlington, WI - UW
EXT.)
Stage
Date
VE
May 15
V2
May 25
V6
June 10
VT
July 15
R1
July 20
R5
Sept 5
R6
Sept 25
European corn borer
European corn borer is a common and serious
pest in Minnesota corn fields. Eggs are laid on the
undersides of leaves in clusters (Fig. 41 ). There are
5 larval instars which can be distinguished by size
(Fig. 45). The first two instars feed on the leaves
only - they produces window pane feeding (Fig. 43 )
and “shotholes” (Fig. 44). 3rd instars also feed on
leaves but may in addition tunnel in the leaf midrib
(Fig. 46). 4th and 5th instars bore into the plant
stalk - creating long tunnels (Fig. 47).
Figure 41. ECB egg mass.
Figure 42. Newly hatched ECB larvae (1st instar).
Figure 43. ECB
shotholing and
window paning from
early instar larvae.
Figure 44. ECB
shotholing - note
powdery frass.
Figure 45. Actual
sizes of ECB
larval instars
(from left: 5th,
new 5th, 4th, 3rd,
2nd - 1st instar is
pictured on page
at left.
Figure 46. 3rd instar ECB
tunneling in midrib.
Figure 47. 4th
instar larvae
tunneling near
tassle.
Figure 48. 5th instar larvae
Figure 49. ECB
tunnel
Figure 50. ECB pupa.
Figure 51. European corn borer adults (from left:
female with wings spread, male, female).
Corn rootworms
Corn rootworms are beetles that feed on corn roots
as larvae (Fig. 52), and on corn silks and pollen as
adults (Fig. 53). They damage the plants as adults
by interfering with pollination, and as larvae by
damaging the root system - resulting in
“goosenecking” or lodging (Fig. 54). Adults are
present in corn in late July and August when corn is
silking. Three species of corn rootworm occur in
Minnesota. The northern corn rootworm is plain
green - this is the most common rootworm found in
Figure 52. Corn rootworm larvae on brace root.
Figure 53. Corn rootworm adult and clipped silk.
Minnesota (Fig. 55 ). The western corn rootworm
is striped - this species mostly occurs in southeastern MN (Fig. 56 ). The southern corn rootworm
(a.k.a. spotted cucumber beetle) is not regarded as a
significant pest of corn in Minnesota - it is the least
commonly found in corn (Fig. 57).
Figure 54. “Goose-necking”
Figure 55. Northern corn rootworm adult
Figure 56. Western corn rootworm adult
Figure 57. Southern corn rootworm adult
Actual size of an adult corn rootworm
True armyworm
True armyworm larvae are sporadic but potentially
damaging pests of corn and small grains. Moths lay
eggs in grassy areas - these may be small grain
fields, ditches, uncultivated areas, etc. Larvae are
defoliators and feed from the edges of leaves. The
resulting feeding damage is characteristic and easily
distinguishable from that of borers. Armyworm
frass is coarse, as opposed to the powdery frass
produced by borers (Fig. 58). Armyworms are
nocturnal so normally only feeding damage is found
on plants during the day. Larvae are generally
found on the ground during the day hiding under
some kind of debris. The larvae are recognizable by
the yellow and black lateral stripes (Fig. 59, 60).
Figure 58. Armyworm feeding damage and frass.
Figure 59.
Armyworm
larva and frass.
Figure 60. Armyworm larva - note striping.
Figure 61. Extremely high abundance of armyworm
larva on ground in corn.
Figure 62. Severe damage to corn from armyworm.
Common stalk borer
Like armyworm, stalk borer eggs are laid in grassy
areas. After feeding and developing to a certain size
in grasses, stalk borers move into corn to feed.
Damage from stalk borer is usually limited to field
edges (Fig. 63) - but may be throughout a field if the
field is weedy or abundances are high. Stalk borers
defoliate corn when they first move into corn.
Damage is typically a line of holes (Fig.65) This is
because they are feeding on the whorls before they
open. Feeding may also cause some growth deformities. Stalk borer frass is finely grained. Eventually, a larva will bore into a stalk (Fig. 66, 67).
Larvae are striped with a dark section near the
middle of their body (Fig. 64).
Figure 63. Inspecting for stalk borer damage.
Figure 64. Stalk borer larva.
Figure 65. Damage from stalk borer larva.
Figure 66. Stalk borer larva near tassle.
Figure 67. Stalk borer
larva and tunnel.
Cutworms
Several species of cutworms damage corn in
Minnesota. Cutworm larvae can be recognized by
the readily visible tubercles all species have. Larvae tend to hide in debris on the ground during the
day and are hard to find. Usually, just their damage
is found. Young larvae defoliate plants - damaged
plants have ragged leaves - this feeding occurs early
in the year (May/June). Older larvae bore into the
base of stalks and may cut plants completely (Fig.
71).
Figure 68. Cutworm larva
Figure. Hop vine borer.
Figure 69. Hop vine borer.
Figure 70. Cutworm larva.
Figure 71. Cutworm and boring.
Corn earworm
Corn earworm larvae are often found in corn ears in
the later part of the summer. The larvae are similar
to armyworm larvae (Fig. 72) and could be confused
except for the differences in their biologies. Corn
earworm larvae are typically found later in the
summer when armyworms are less abundant.
Moreover, corn earworm larvae feed on the ears of
corn (Fig. 73), as opposed to armyworms which
feed on the leaves.
Figure 72. Corn earworm larva.
Figure 73. Corn earworm larval feeding damage.
Corn caterpillar recap
European corn borer
- young larvae shothole leaves
- older larvae bore into stalk
- frass is always powdery
Common stalk borer
- young larval feeding produces lines of holes
- older larvae bore into stalk
- frass is always powdery
True armyworm
- all larval stages feed on leaf edges
- frass is coarse
- larvae hide on ground during day
Cutworms
- young larval feeding produces
ragged leaves on young plants
- older larvae bore through
base of stalk
Corn earworm
- larvae feed on corn ears
- larvae have tubercles and spines
Corn flea beetle
Corn flea beetles feed on corn throughout the
summer. They can be particularly damaging to
seedling corn if they are abundant. Flea beetles are
very small and also quick. Note the difference in
size and shape between corn flea beetles and redheaded flea beetles (in soybeans) which could also
be found in corn. Adults are hard to spot and count
on corn plants (Fig. 74). However, they leave
behind a characteristic “window pane” feeding
damage as streaks on the leaves (Fig. 75).
Figure 74.
Corn flea
beetle.
Actual size
~ 2mm
Figure 75. Corn flea beetle feeding damage.
Corn leaf aphid
Corn leaf aphids are most commonly found once
the plants begin tasseling. CLA are also commonly
found in small grains (see small grains for more on
aphids).
Figure 76. Corn leaf aphids and cast skins.
Figure 77. Corn leaf aphid
Brown stink bug
Brown stink bugs may be found in corn as well as
other crops. They overwinter as adults and may be
damaging to seedling corn if they are abundant. 1 2 additional generations of adults will occur in
Minnesota during the course of the summer.
Nymphs are similar to adults but there are no wing
covers over their abdomens so they appear to be
more soft-bodied.
Figure 78. Brown stink bug adult.
Pathogens
Some pathogens are commonly seen when inspecting cornfields in Minnesota. Common smut is
easily recognized - it is a white mass protruding
from the ear in the picture below (Fig. 79). Rusts
(common and southern) may also be common.
They are recognizable by the red pustules that form
on leaves (Fig. 80).
Figure 79. Common
smut on corn ear.
Figure 80.
Rust on corn
leaf.
Small Grains
Distinguishing Small Grains
To distinguish small grains prior to seed development you need to look at the structure of the leaves
where they attach to the stem. The auricles on oat
are absent, on wheat they are short, and on barley
they are very long.
Figure 81. Location of
auricles (left) and difference between barley
(top), wheat (middle) and
oats (bottom).
Growth Staging Small Grains
Specific developmental stages of small grains can
be classed into broader groups (approximate cumulative days spent in each stage in parentheses - from
UMN EXT).
Pre-tiller (0-14)
After the plant germinates, it grows and produces
increasing numbers of leaves, these stages can
generally be called pre-tiller.
Tillering (14-28)
As the plant continues to grow it produces additional stems - these are called tillers. How many
tillers are produced vary with the plant and the
environment.
Jointing (28-45)
Eventually the main stem begins to elongate significantly - this stage is called stem elongation or
jointing. Generally, four internodes form during
this time, the leaf at the top of the fourth internode
is called the flag leaf.
Boot (45-50)
When the base of the flag leaf begins to swell, the
plant is said to have entered the boot stage (the
swelling is called the boot).
Head (50-60)
The head of the plant will emerge from the boot, as
soon as the head is visible the plant is considered to
be in the head stage.
Flower
When flowers begin to form on the head, the plant
is considered to be in the flowering stage.
Milk
When seeds filled with milky fluid have formed
from the flowers, the plant is considered to be in the
milk stage.
Dough
As the seeds dry to a pasty consistency the plants
are considered to be in the dough stage.
Ripening (90 days)
Once the seeds have hardened the plants are considered to be ripening.
Aphids
Four species of aphids are commonly found in small
grains (English grain, oat birdcherry, greenbug,
and corn leaf aphids). Aphids will be found in
sweep net samples and also on the developing parts
of plants when inspected. Aphids are distinguised
as being aphids by their size, shape and the presence
of cornicles. The four species are distinguished by
their body shape, cornicle size, and coloration on
and around the cornicles. The Russian wheat
aphid is not known to occur in Minnesota at this
time (Fig. 84).
Characteristics of wingless forms:
Oat bird-cherry aphid
- body is dark greenish/purple
- posterior of abdomen and base of cornicles
red
- cornicles tipped with black
English grain aphid
- body is light green/tan
- cornicles are long and black
Greenbug
- body is light green
- darker stripe runs down back
- cornicles are pale green
Corn leaf aphid
- body is blue/gray/green
- short, black cornicles with black areas at
base
Russian wheat aphid
- body is light green
- cornicles are short (barely visible)
- has a tail-like appendage
Figure 82. Corn leaf aphid.
Figure 83. Oat
birdcherry aphids
(faded from ETOH).
Figure 84. Russian wheat aphid - cornicles are very
short (specimen damaged from ETOH).
True armyworm
True armyworm is a potential pest of small grains
as well as corn. Larvae are nocturnal and generally
spend the day hiding on the ground. If larvae are
found in sweep net samples or if feeding damage is
observed, then the ground needs to be inspected to
determine the # of larvae/sq foot. Low level larval
feeding results in damage to leaves. Greater feeding
results in the leaves being completely stripped from
the stems. Older larvae will feed through the stem,
clipping the heads off.
Figure 85. True armyworm feeding damage.
Figure 86. True armyworm feeding damage.
Figure 87. Multiple armyworm larvae.
Figure 88.
Armyworm
larva at base of
plant.
Figure 89. Wheat-headed armyworm larva - this
caterpillar may be found in small grains, but is
easily distinguished from armyworm by the bristles.
Aster leafhopper
Aster leafhopper adults and nymphs are commonly
found in sweep net samples in small grains (and
alfalfa and vegetables). Don’t confuse them with
potato leafhoppers which are also found in alfalfa
(see alfalfa for more on potato leafhoppers).
Figure 90. Aster leafhopper adult.
Figure 91.
Aster leafhopper nymph.
Cereal leaf beetle
Cereal leaf beetle is known to be distributed
through at least the southeastern portion of MN.
Adults are primarily active during the spring and
may not be conspicuous during the summer (Fig.
92). Larvae may be found during the summer.
Feeding damage is a distinctive streaking (Fig. 93).
Leaves with heavy damage may appear brown (Fig.
94).
Figure 92.
Cereal leaf
beetle adult.
Figure 93.
Leaf with
feeding
damage.
Figure 94.
Heavily
damaged
plants.
Pathogens
Fusarium head blight is one of the many pathogens that infect small grains. The heads on infected
plants turn an orangish color as the disease
progresses - the greater the area of the head colored
orange, the more severe the infection.
Figure 95.
Fusarium
head
blight on
wheat
head.
Soybeans
Growth Staging Soybeans
In this
Figure 96. V0
picture, the
soybeans
have recently
emerged
and only the
cotlydons
are present this stage
would be
V0. The
first true leaves are beginning to emerge as well.
The first
true leaves
are unifoliFigure 97. V1
ate (all
leaves
afterwards
are trifoliate). When
only the
first true
leaves are
present, the
plant is in
V1. The V
stage is
increased by
1 with each
trifoliate leaf that unfolds (the leaflets cannot be
touching each other for the leaf to be considered
unfolded).
A plant with
1 trifoliate
leaf is V2
(Fig. 98). A
plant with 2
trifoliate
leaves is V3
(Fig. 99),
etc. When
the first
flower has
opened, a
plant is
Figure 98. V2
considered
to be in R1
(Fig. 100).
R2 = most flowers opened.
R3 = pods begin forming.
R4 = most pods have formed.
R5 = beginning of seed formation (Fig. 104).
R6 = most pods have seeds fully formed.
R7 = beginning maturity, a few pods are brown.
R8 = full maturity = most pods are brown and dry.
Figure
99. V3
Figure 100.
Soybean flowers ready to
open (R1
doesn’t begin
until at least 1
flower has
opened).
Figure 101.
Beginning of
seed formation (R5).
Bean leaf beetle
Bean leaf beetle is a common pest of soybeans.
Only adults are found on the foliage as the larvae
feed on the roots below ground. Adults overwinter
and can be found in alfalfa in the spring before
beans emerge. As soon as beans begin emerging the
beetles will switch to feeding on them. By late June
the overwintered adults will have mated and died
and their offspring will begin emerging from the
soil as mature adults. You may find adults on plants
or in row foot samples taken with a sweep net. The
yellow phase (Fig. 102) is the most commonly
found but there is a red phase also (Fig. 103). Bean
leaf beetles can always be confidently identified by
the triangle present on the elytra at the base of the
protonoum. Feeding damage by bean leaf beetles is
characteristically holes in the middle of the leaf
(Fig. 104). Beetles will also feed on seed pods in
late summer - this is noticeable by white patches on
the pod.
Triangle
Figure 102. Bean leaf beetle adult (yellow phase).
Figure 103. Bean leaf beetle adult (red phase). The
triangle is hard to see in this picture, but it is
present.
Figure 104. Bean leaf beetle feeding damage.
Imported longhorn weevil
Imported longhorn weevil may be found on
soybeans or in sweep net samples. It is one of the
few weevils that may be found in soybeans (clover
leaf weevils may occasionally turn up in soybeans).
Feeding damage is very distinctive (Fig. 106)
Figure 105. Imported longhorn weevil
at point of knife.
Figure 106. Weevil feeding damage.
Caterpillars
Several caterpillars may be found in soybeans. Two
of the more commonly found species are the soybean looper and the green cloverworm. The soybean looper is recognizable by its green color,
white lateral stripe, and it’s two pairs of abdominal
prolegs (Fig. 107). The green cloverworm is also
green and has a faint white stripe, however it has 3
pairs of abdominal prolegs.
Figure 107. Soybean looper larva.
Red headed flea beetle
The red headed flea beetle is found in soybeans.
This is a different flea beetle species than the corn
flea beetle and it is roughly twice as large (Fig.
108). Their feeding leaves streaks on the leaves
similar to that seen on corn leaves (Fig. 109).
Figure 108. Red headed flea beetle adult
Actual adult size:
Figure 109. Flea beetle feeding damage on soybean.
Potato leafhopper
Potato leafhoppers are also found in soybeans - see
alfalfa for more information.
Figure 110. Potato leafhopper adult.
Figure 111. Potato leafhopper nymph
Two-spotted spider mite
Two-spotted spider mites only become a problem
under drought conditions. Most of the time the
mites are found on grasses and other weeds in the
margin, but when conditions are right they will
begin moving into fields from the margins inwards.
The mites are tiny (the size of the period at the end
of this sentence), but their damage is readily visible
as leaf stippling (Fig.112). However, remember that
other things can cause stippling as well so the
presence of stippling doesn’t necessarily mean
spider mites are also present. To check mites, look
on the undersides of the leaves - if you look closely
they will appear as black specks from the two spots
(Fig. 113). Clusters of mites will produce webbing
and eggs will also be seen (laid singly - Fig.115). A
hand lens will help to verify spider mite presence.
Figure 112. Leaf stippling from spider mites.
Figure 113. Two
spotted spider
mite adult.
Figure 114. Spider mite cluster.
Figure 115. Two spotted
spider mite egg.
Soybean aphid
Soybean aphid was first discovered in MN in 2000.
It is the only aphid species that is likely to be found
on soybeans in Minnesota. The aphids can be found
anywhere on the plant but are most likely to be on
the undersides of leaves - they are much larger than
spider mites and can easily be seen with the naked
eye. The aphids are light, lime green and have long
cornicles that are distinctly dark if viewed with a
hand lens. The aphids could be confused with
nymphal plant bugs so it is important to check for
cornicles (plant bugs don’t have cornicles).
Figure 116. A soybean aphid mummy (it was parasitized successfully by a wasp).
Soybean cyst nematode
Chlorosis of soybeans may be symptoms of soybean cyst nematode (Fig. 117). However, many
other things can also cause chlorosis, so its presence
does not necessarily mean that SCN is present. If
SCN is present, cysts will be present on the roots of
affected plants (Fig. 118). The cysts are small, but
can be viewed with the naked eye. The cysts are
white and round, and consequently can be distinguished from root nodules and soil particles.
Figure 117. Chlorosis of soybeans.
Figure 118.
Cysts of
soybean cyst
nematode.
Japanese beetle
Japanese beetles have an extremely broad host
range and may turn up in any crop - however,
soybeans may be the most likely place to find them.
At the present time (December 2000), Japanese
beetle is not known to be established in MN outside
of the Twin Cities Metro Region. Larvae (grubs)
feed on roots in the soil and won’t be seen without
digging. Adults are defoliaters and have a greenishred iridescent sheen. There is a diagnositc patch of
hair between each abdominal segment (Fig. 119,
120). The false Japanese beetle is commonly
found. It is similar in appearance to the Japanese
beetle, but it doesn’t have the same sheen and the
tufts of hair are not present (Fig. 121)
Figure 119. Japanese beetle - note
iridescent sheen.
Figure 120. Japanese
beetle - note tufts of
hair.
tufts
Figure 121.
False Japanese
beetle.
Incidentals
Several insects that are primary pests of other crops
may also turn up in soybeans including clover leaf
weevil, tarnished plant bug and western corn
rootworm.
Figure 122. Clover leaf
weevil.
Figure 123. Tarnished plant bug.
Figure 124.
Western corn
rootworm.
Margins
Grasshoppers
Five grasshopper species are considered crop pests
in Minnesota: redlegged, twostriped, differential,
migratory, and clearwinged grasshoppers. Of these
five, the first three are the most commonly found.
Grasshoppers will be found in every crop and in the
margins along the crops. Distinguishing species is
possible but may be difficult as several cues may
need to be checked. Grasshoppers overwinter as
eggs and nymphs will be present for much of the
summer until later in the year when adults develop.
Redlegged grasshopper nymphs are generally easy
to distinguish because they have a very distinct
lateral white stripe on their face and thorax (Fig.
127). Nymphs of the two striped grasshopper are
usually much greener than the other species (Fig.
129). Differential grasshopper nymphs will not
have either of these characteristics (Fig. 131).
Figure 125. Grasshopper in net.
Figure 126. Redlegged grasshopper adult.
Figure 127. Redlegged grasshopper nymph.
Figure 128. Two striped grasshopper adult.
Figure 129. Two
striped grasshopper
nymph.
Figure 130. Differential grasshopper adult.
Figure 131.
Differential
grasshopper
nymph.
Figure 132. Migratory grasshopper adult.
Figure 133. Migratory grasshopper nymph.
Figure 134. Clearwinged grasshopper adult.
Incidentals
The “margin leafhopper” is commonly found in
sweep net
samples
from the
margin. It
has no
economic
significance. Note
that even
though it is
similar to an Figure 135. “Margin leafhopper”.
aster leafhopper - it
has no black spots on its head. Common stalk
borers
will
often
be
found
in
sweep
net
Figure 136. Common stalk borer larva.
samples
from margins - this is important because it indicates
areas with potential problems. The same is true for
true armyworms which will also turn up in margin
sweeps.
Figure 137. True armyworm larva.
Beneficials
There are seven lady beetle species that are likely
to be found while sampling crops in Minnesota.
Adults of the 7 species can be distinguished by
markings on their pronotum and elytra(Fig.’s 139145). Larvae may also be present at any time during
the summer, but the species are difficult to distinguish (Fig. 138). Other natural enemies that are
commonly found include green lacewings (Fig.
146). Brown lacewings may also be found - they
are similar to green lacewings except they are
brown. Assassin bug adults and nymphs (Fig.
147,148), and damsel bugs are also frequently
found (Fig. 149). In general, predacious insects are
recognized by their raptorial
forelegs (for grabbing prey).
Figure 138. Lady beetle
larva on wheat head.
Figure 139. 12 spotted lady beetle.
Figure 140.
7 spotted
lady beetle.
Figure 141. Convergent lady beetle.
Figure
142.
Japanese
lady
beetle.
Figure 143. Parenthesis lady beetle.
Figure 144. Two spotted lady beetle.
Figure 145. 13 spotted lady beetle.
Figure 146. Green lacewing adult.
Figure 147. Assassin bug adult.
Figure 148. Assassin bug nymph.
Figure 149. Damsel bug adult.
All pictures in this booklet were taken by the Minnesota Department of Agriculture’s Plant Pest
Survey and Biological Control Program. Permission/acknowledgement required.