1. Oklahoma Master Gardeners
Who Dunnit? Diagnosing Plant Damage Due to Insects
Eric J. Rebek
Dept. of Entomology and Plant Pathology

2. Where Arthropods Attack Plants
Insects and their relatives can attack all portions of a plant, both below- and aboveground plant parts. The style of feeding on the same plant tissue also varies among arthropod species (e.g., leaf miner vs. defoliator).
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3. Signs and Symptoms of Insect Damage

4. Signs vs. Symptoms Signs Symptoms
Waste products; honeydew, frass (excrement)
Webbing
Cast skin (exuvium)
Insects themselves
Symptoms
Discoloration or distortion of leaves, blossoms or twigs
Chewing damage
Cracked bark
Dieback of plant parts
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5. Signs, Signs, Everywhere Are Signs…
Frass
Insects
Webbing
Lacebugs will leave obvious signs such as black specks of fecal matter and dark, oval eggs on the undersides of leaves. You may also see the insects themselves. Spider mites produce a fine webbing. You can also use the paper trick to try and detect mites.
Eggs
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6. Look for Signs of Causal Agent
So we have ruled out abiotic factors, the damage we see is isolated to just a branch or single shoot. Now how do we differentiate between insects and diseases?
Look for signs of the damaging pest: Insects often leave very obvious sings of their presence, such as excrement.
Disease
Insect
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7. Look for Signs of Causal Agent
So we have ruled out abiotic factors, the damage we see is isolated to just a branch or single shoot. Now how do we differentiate between insects and diseases?
Look for signs of the damaging pest: Insects often leave very obvious sings of their presence, such as excrement.
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8. Symptoms: Discoloration Due to Insect Feeding
Small, spotty patches of discolored leaf tissue
Can often coalesce into larger blotches
Leaves discolored by insect feeding have smaller, spotty patches of discoloration.
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9. What Caused This Leaf Discoloration?
The yellow portions of this euonymus variety (Euonymus fortunei ‘Harlequin’) are normal. You must know what your plants look like when they are healthy to be able to diagnose problems as they arise.
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10. What Caused This Leaf Discoloration?
Nutrient deficiency caused this chlorosis or yellowing. How can we differentiate between disease, insect, and nutrient problems?
Pattern of abnormality – nutrient deficiencies are expressed throughout the entire plant; insect and disease problems are more isolated.
There are four general approaches to iron treatments: 1) lowering the soil’s pH, 2) soil iron treatments, 3) foliar sprays, and 4) tree injections.  Each has advantages and disadvantages.  Each procedure gives variable results depending on plant species and soil conditions.
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11. What Caused This?
Wilting can be tricky – disease, under/over-watering, and insects can cause wilting. So what should we do? Look for signs of agent - Rose aphid feeding damage on apple.
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12. What Caused This?
Insect: I can’t think of any diseases that cause holes like these in leaf tissue. This is the work of a chewer, in this case the imported cabbage worm.
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13. What Caused This?
But not all chewing-type damage is caused by insects. Deer browsed tulips. What other critters cause chewing damage? – slugs, mice…
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14. What Caused These?
Many people ask about holes like those on the right. Do you know who made those holes?
Linear pattern associated with sap sucker (on right)
Random holes are wood boring insects – usually beetle or moth larvae (on left)
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15. What Caused This? Tire tracks in turf. Oklahoma State University
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16. Types of Feeding Damage
Knowing that an insect caused the damage is not enough. If you want to control the insect, you need to know what it is. One useful way to narrow our list of culprits is to examine the type of feeding damage present.

17. Sucking Damage
Sucking insects remove chlorophyll and leave white or yellow stippled marks.
Both spider mites and lacebugs can cause these white stipples.
How do we know which we have?
What do we do to determine which agent is causing the discoloration?
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18. Sucking Damage Discoloration Wilting Distorted Growth
Damage from aphids (l), scales (m), and lacebugs (r).
Distorted Growth
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19. Who Sucks? Aphids Scales Mealybugs Thrips Plant bugs
A number of insects damage plants by sucking sap. When you see damage caused by a sucking insect, one of these agents may be the culprit. How do we determine who dunnit? Look for signs of the damaging agent.
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20. Piercing-Sucking Mouthparts
antennae
eyes
leg 1
leg 2
Piercing-sucking mouthparts are also very common. There are intermediate types of mouthparts such as rasping-sucking, as found in thrips; and chewing-lapping, as found in honeybees, and siphoning types, as seen in butterflies and moths. House flies have sponging mouthparts. Insects are classified in part based upon the type of mouthparts they have.
leg 3
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21. Chewing Damage
Chewing damage is much more obvious. But there are also different types of chewing that can be associated with different insects. The circular holes on the right are characteristic of leaf cutter bees. The more jagged holes on the apple are caused by the Eastern tent caterpillar.
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22. Chewing Damage Internal External
Pine shoot borer (l), damage from leafcutter bees (r).
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23. Who Chews? Caterpillars Beetles Grasshoppers and crickets Sawflies
larvae & adults
Grasshoppers and crickets
nymphs & adults
Sawflies
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24. Caterpillars vs. Sawflies8 5 7 4 1 3 2 1 6 2 3
≤ 5 pr. prolegs
crochets present 5 4
≥ 6 pr. prolegs
crochets absent
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25. Chewing Mouthparts
Piercing-sucking mouthparts are also very common. There are intermediate types of mouthparts such as rasping-sucking, as found in thrips; and chewing-lapping, as found in honeybees, and siphoning types, as seen in butterflies and moths. House flies have sponging mouthparts. Insects are classified in part based upon the type of mouthparts they have.
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27. Other Types of Insect Injury
Wood borer
Gall insect
Many of the worst pests feed within plant tissues during a part or all of their destructive stages. The chief groups of internal feeders are borers in wood or pith; worms or weevils in fruits, nuts or seeds; leaf miners; and gall insects. Nearly all of them live inside the plant during only a part of its life, sooner or later emerging, usually as an adult.
Leaf miner
Apple maggot
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28. Other Types of Insect Injury
Root feeders
Oviposition damage
Many insects attack plants below the soil surface. Subterranean insects include chewers, sap suckers, root borers, and gall insects. Other insect damage is caused during oviposition, as commonly seen on apples. Some insects can cause damage by removing plant tissue to build nests.
Leaf cutters
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29. What kind of damage is this?
Stippling: Specks of leaf tissue discolored
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30. What kind of damage is this?
Imported cabbage worm
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31. What kind of damage is this?
Stunted new growth is another type of damage caused by sucking insects.
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32. What kind of damage is this?
Thrips damage to flower and leaves.
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33. What kind of damage is this?
Spruce spider mite (left) and two spotted spider mite injury on strawberry plant (right).
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34. Insect Friend or Foe?

35. The Good & The Bad
It is important to remember that most insects are beneficial rather than destructive. They help with pollination or act as predators of more harmful species. Of course, there are also many pest insects that damage plants by defoliation, boring into the wood, or transmitting disease agents.
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36. Beneficial Insects Parasitoids Predators Weed feeders Pollinators
Fortunately, most insects are either beneficial or harmless. Some are predators, some insects eat weeds, others are parasitic (for example, some wasps). Parasitic insects kill other insects, often pests, by laying eggs on or in their victim’s bodies or eggs. Still others, such as honeybees, produce honey and pollinate fruits, vegetables, and flowers. And many insects are responsible for decomposition of plant and animal matter.
Pollinators
Decomposers
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37. Is This a Pest?
This is only a pest when it is in your house. Otherwise these lady beetles eat aphids in your landscape and garden.
Use this example to generate a discussion about pests.
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38. Where Is the Pest? Which one is the predator?
The one on the right, a spined soldier bug.
It’s bigger, has legs to run, and a beak to skewer its prey.
On the left is the Mexican bean beetle larva.
How does the spined soldier bug kill?
The predator stabs its prey and is injecting it with digestive enzymes to dissolve the inner body cavity.
Then, it will suck out the contents and toss the “outer shell” away.
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39. Stink Bugs
Rule of Thumb: The brown bugs tend to be predators and the green ones tend to be the pest.
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40. NO! Are These Pests? Lacewing larva Lady beetle larva
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41. Where is the pest (or predator)? (bigger, fewer)
Lady beetle adult feeding on aphids…
White exoskeletons of aphids can be called aphid ghosts.
Tell tale signs that the beetle is a predator.
One beetle many aphids
Aphid in its mouth.
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42. Large grasping jaws Oklahoma State University
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43. Is This a Pest? Yes…spotted cucumber beetle Oklahoma State University
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44. Is This a Pest?
This adult minute pirate bug also kills by jabbing its beak into its prey (in this case an egg) and sucking out the contents.
These insects are critical for controlling many soft bodied insects, like mites and aphids in gardens and landscapes.
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45. Is This a Pest?
The yellow jacket wasp is carrying off a caterpillar that it will consume or carry into the nest to consume. It has some competition from an ant that has the same idea in mind. Both these nuisance pests are meat eaters most of the year. They can have a place in the garden if the nest is not in an area where it can bother people.
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46. Oklahoma State University Oklahoma Master Gardeners
Which is the predator? The big spider is eating the fly… (bigger= predator)
Spiders are helpful, they eat flies---- and anything else that comes into their field of vision.
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47. What happened to this insect?
This tomato hornworm has not been spit upon. Rather, it is covered with pupae of a parasitic wasp.
The caterpillar is about 1/2 the size it ought to be.
The pupae will hatch out adult wasp.
If you see a parasitized hornworm, you should probably leave it in your garden so the emerged wasp can attack other caterpillars. The wasp will not provide immediate control but will reduce population of hornworms.
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48. Braconid wasp parasitizing a gypsy moth caterpillar

49. Is This a Pest?
Flower fly adult : this is a fly not a yellow jacket wasp. It has only two wings and is found hovering around flowers to eat pollen.
Larva is a maggot that feeds on soft bodied insects.
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51. Is This a Pest?
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52. Master Gardener Diplomacy: (Insect Management)
Before making recommendations:
Find out “WHY” your client gardens
Create an atmosphere of enjoyment and learning
Ask lots of questions about the situation (DETAILS)
Try not to treat insect management as a ‘CHORE’ but as a necessary challenge
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