September Colorado Insect checklist of some common insect-related events – Denver, South and East

 This is a generalized checklist of when some of the more important insect related events tend to occur in the South and East Counties area. Year to year variations are considerable, and this should only be used as a guideline to anticipate and help recognize common insect occurrences.


Early September


Yellow Jackets, Hornets – Nest size and nuisance problems peak. Large paper nests in trees and shrubs attract attention.

Cluster Flies, Box Elder Bugs – Migrates into homes for overwintering.

Spiders, Crickets – Movement into homes accelerates greatly with cool weather.

Large Spiders – “Cat-face” and Garden” spiders become fully grown and attract attention.


Tree/Shrub Insects

Large Caterpillars – Several species of large caterpillars (Cecropia Moth, Polyphemus Moth, Sphinx Moth Larvae) wander about landscapes when fully grown and attract attention.

Peach Tree Borer – Rescue treatments should be applied to your trees before the soil temperature becomes too cool.

Pear Slug – Damage by the second generation occurs during early September.


Garden Insects

Slugs – Damage to gardens increases with the return of cool, wet weather.

Grasshoppers – Migrations to gardens continue.

Bumble Flower Beetles – Beetles feed on flowers and visit bacterial ooze.


Lawn Insects

Nightcrawlers – Tunneling activities during spring can create lumpy lawns.


Late September

Household/Miscellaneous Insects

Millipedes – Moves into homes following wet periods.

Spiders, Crickets, Root weevils, Conifer Seed Bugs – Moves into homes quickly in cool weather.

Yellow Jackets – Nuisance problems with yellow jackets scavenging on sweets persist.


Tree/Shrub Insects

Aphids – High populations of aphids may develop on several tree species (Willow, Oak, Aspen) prior to frost.

Cooley Spruce Gall – Winged stages return to spruces and overwinter on trees.

Yellow Jackets, Bees – Wasps and bees may be seen visiting trees and shrubs where honeydew producing insects (e.g., aphids, soft scales) are present.

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Source: CSU Ext (pdf)


Sep 5, 2019 @ 1:05 pm