Information, including identification, symptoms and control, related to most common pests that plague rose growers in Western Missouri and Eastern Kansas. 
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Insects that chew and suck can be especially harmful to roses. Not only do they disfigure and weaken the plant, they can also spread disease pathogens.

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Aphids  
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Description:
  • Small green or black insects, with or without wings, about 1/8 inch long.
  • Appear throughout the spring and fall.
  • Suck the plant juices from tender parts of the plant, such as, new foliage and buds.
  • Excrete honeydew, a sweet sticky substance that attracts ants and favors the growth of sooty mold.
  • Prolific breeders and multiply rapidly in warm weather.

Symptoms
  • Leaves curl upward.
  • Buds are deformed.
  • Buds may fall before opening.
Control
  • Easily controlled, washing the plant with a stream of water will knock the aphids off the plant, and since most cannot fly, they will not be able to return to the plant. 
  • Most insecticides will control aphids.
  • Natural enemies that feed on aphids such as lady beetles, lacewing larvae and parasitic aphid wasps.

Cane Borers

Description:
  • Several wasps and bees lay eggs in the pith of cut rose canes.
  • Larvae bore a hole down the middle of the pith to make their nests.
  • Active boring from spring through early fall.

Symptoms
  • Damage is easily seen, a hole in the center of a cut cane.
  • Hole may only be a few inches deep or down to the base of the plant.
Control
  • The most common cane borer wasps are predators, so keeping aphids under control will ​remove their food source.
  • Damaged canes should be cut below the larvae to prevent generation of cane borer wasps.
  • Borer damage can be prevented by sealing each cut cane after pruning with waterproof wood glue.
      
Japanese Beetles
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Description:
  •  3/8 inch long metallic green beetle with copper-brown wing covers.
  • Adults emerge in June and feed on plants.

Symptoms:
  • Feed individually or in large groups.
  • Totally consumed flowers and flower buds.
  • Skeletonized leaves starting from the top of the plant.
 
Control:
  • Can be removed by hand and placed in a container of soapy water.
  • Many insecticides are available for control.
  • Don’t use Japanese beetle traps because they attract more beetles from the surrounding area.
References:

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Leaf Cutter Bees

Description:
  • Rarely seen and are similar to honeybees.
  • Excellent pollinators of alfalfa and other crops.

Symptoms
  • Perfectly cut semi-circles on leaf edges.
  • Little other damage. 
  • Mostly an aesthetic problem.
  • Often prefer the leaves of a particular rose.

Control
  • Control by insecticides is questionable due to their value as pollinators.

Reference: Missouri Botanical Garden, Leaf Cutter Bees  

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Midge
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Description:
  • A tiny fly lays its eggs in the flower buds and new shoots.
  • Fly larva’s rasping mouth parts cause damage to the soft rose tissue.

Symptoms:
  • Bent and misshapen flower buds and withering shoots.
  • Black flower buds and shoots.
  • Flower buds and shoot tips fall off.
  • Lack of flowers.

Control:
  • Prune out affected buds and shoots.
  • Treat soil with imidacloprid drench or granules
  • Spray plants with lambda-cyhalothrin or other pesticide listed for midge.
Rose Slug
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​Description:
  • Caused by the yellow-green larval stage of several different sawflies.
  • Feed on the soft tissue of rose leaves.

Symptoms:
  • Soft tissue of leaves is missing, while tough veins are left intact.
  • Papery, translucent leaf surface which crumbles to leave holes.
  • Holes appear as tissue crumbles.

Control:
  • Remove affected leaves at first sign of damage
  • Use a forceful spray of water on both sides of the leaves to knock-off larvae. 
  • Minimize use of pesticides to allow beneficial predators to control.
  • Many organic and chemical controls are available for control if needed. 
Two-spotted Spider Mites
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Description:
  • Tiny mite with two red spots.
  • Mouth-parts adapted to suck the juices from plant tissue, especially leaves.
  • Feed on most landscape plants as summer temperatures heat up.  

Symptoms:
  • Leaves become spotted or stippled, dull, and lose substance, then become yellow or bronze, and then fall off.
  • Telltale webbing on the underside of the leaves.  
  • Major infestations can lead to death of parts of the bush or the entire bush.  
  • Presence of spider mites can be confirmed by shaking a branch of the plant over a white sheet of paper.  Tiny, dark specks will be seen moving across the paper and will leave a red streak if squashed with a fingertip.

Control:
  • Controlled at first sign of spider mite activity by spraying the underside of the affected leaves with a forceful spray of water every three days.  
  • Organic controls include insecticidal soap and horticultural oils.  
  • Spider mites are not insects, but are related to spiders, so many insecticides are not effective for control.
  • Pesticide must be labelled for mite control.  
Reference: K-State Extension, Spider Mites  PDF

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Thrips 
  
Description:  
  • Tiny, slender brownish yellow winged insects.
  • Just visible with the naked eye hiding at the base of the flowers.
  • Hide inside buds and blooms and damage the petals with their rasping mouth-parts.
  • Prefer lighter colored blooms and attack the bloom when it is still a bud making control difficult as they hide inside an unopened bloom.

Symptoms:
  • Browning and brown streak of the flower petals.
  • Flowers deformed and petals drop quickly.

Control: 
  • The link below has a long list of possible control options.
  • Because thrips lay their eggs in grasses and weeds, keep the garden weed-free and control grasses.

Reference: U of California, Pest Notes