April Colorado Insect checklist of some common insect-related events – Denver metro and Boulder, Denver South, and East
This is a generalized checklist of when some of the more important insect-related events tend to occur in our area. Year-to-year variations are considerable – this should be used as a guideline to anticipate and help recognize common insect occurrences.
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DENVER METRO & BOULDER
Early April
Household/Miscellaneous Insects
Boxelder bugs, elm leaf beetles, cluster flies: Overwintered adults become increasingly active in and around homes during warm periods.
Carpet beetles: Early spring is often the period when adult stages are most frequently encountered in homes.
Millipedes: Nuisance movements into homes occurs following wet weather.
Tick season: Tick season usually has started and usually persists until high temperatures occur in early summer.
Ants: Foraging ants in homes are common until temperatures allow them to seek food outdoors.
Trees/Shrub Insects
Ips beetles: Major Ip’s beetle flights are likely to have started by this time and may threaten at-risk spruce and pines.
Aphids on fruit trees: Spray oils on dormant trees to kill overwintered aphid eggs.
Cooley spruce gall: Controls are best applied before the insects make the egg sack in late April.
Zimmerman pine moth: Overwintered larvae remain exposed on the trunk and can be controlled at this time.
Borers: Remove and destroy damaged tree limbs and canes infested with borer larvae before insects emerge.
Honeysuckle witches’ broom aphid: Prune out old, damaged terminals that contain eggs.
European elm bark beetle: Pruned elm wood and logs should be destroyed prior to beetle emergence.
Conifer sawflies: Larvae feed on older growth of various pines.
Lawns
Denver billbug: Overwintered larvae may damage the roots of turfgrass.
Turfgrass mites: Clover mites continue and banks grass mites begin to increase in droughty areas.
Sod webworms, cutworms: Damage to lawns by webworms and cutworms begin at this time.
Nightcrawlers: Tunneling activities and associated lawn lumps continue.
Midges: Non-biting midges emerge from ponds and mating swarms may be observed over lawns.
Late April
Tree/Shrub Insects
Brown-headed ash sawfly: Watch for pin-hole feeding wounds prior to peak feeding damage. Swarms of adult insects may be observed and eggs laid in leaves.
Cooley spruce gall: Insects continue development and usually begin to produce egg sacks in late April.
Zimmerman pine moth: Overwintered larvae will begin to move into the trunk over the next few weeks.
Lilac/ash borer: Flights of adult moths may begin.
Poplar twig gall fly: Adults emerge and begin to lay eggs in emerging aspen shoots.
European elm bark beetle: Preventive sprays should be completed before adults emerge and fly.
Pinyon tip moth: Larvae remain exposed to bark and can be controlled at this time
Spider mites on pines: Oligonychus subnudus populations may increase rapidly on ponderosa and other susceptible pines
Spiny elm caterpillar: Small colonies of these caterpillars may be seen on willow, hackberry, aspen, elm, and other trees.
Douglas-fir beetle: In forested areas, adult emergence, flights, and tree attacks may begin.
Hawthorn mealybug: Overwintered stages on trunk move to twigs and feed.
Walnut twig beetle: Adults move from overwintering sites on the trunk to initiate tunnels in twigs, and branches.
Garden
Spinach leafminer: Egg-laying and tunneling begin in older spinach foliage.
DENVER SOUTH AND EAST
Early April
Household/Miscellaneous Insects
Swallow bugs: Overwintered swallow bugs become active in anticipation of returning migrant birds and bite humans.
Boxelder bugs, elm leaf beetles, cluster flies: Overwintered adults become increasingly active in and around homes during warm periods.
Carpet beetles: Early spring is often the period when adult stages are most frequently encountered in homes.
Tick season: Tick season usually has started and typically persists until high temperatures occur in early summer.
Ants: Foraging by field ants for sweet materials intensifies in homes.
Clover mites: Clover mite migrations into homes occur during warm days
European paper wasp: Overwintered queens start to establish new nests
Trees/Shrub Insects
Ips beetles: Major Ip’s beetle flights are likely to have started by this time and may threaten at-risk spruce and pines.
Poplar twig gall fly: Larvae continue to leave galls and pupate in the soil at the base of trees.
Cooley spruce gall: Controls are best applied before the insects make the egg sack in late April or early May.
Borers: Remove and destroy damaged tree limbs and canes infested with borer larvae before insects emerge.
Honeysuckle witches’ broom aphid: Prune out old, damaged terminals that contain eggs.
Conifer sawflies: Larvae feed on older growth of various pines.
Aphids on fruit trees: Spray oils on dormant trees to kill overwintered aphid eggs.
Late April
Household Insects
Ants: Foraging ants in homes are common until temperatures allow them to seek food outdoors.
Tree/Shrub Insects
Cooley spruce gall: Insects continue development and usually begin to produce egg sacks in late April.
Lilac/ash borer: Flights of adult moths may begin.
Poplar twig gall fly: Adults emerge and begin to lay eggs in emerging aspen shoots.
Spider mites on pines: Populations may increase rapidly on ponderosa and other susceptible pines
Spiny elm caterpillar: Small colonies of these caterpillars may be seen on willow, hackberry, aspen, elm, and other trees.
Douglas-fir beetle: In forested areas, adult emergence, flights, and tree attacks may begin.
Brown-headed ash sawfly: Adults may lay eggs during warm days following bud break.
Zimmerman pine moth: Approximate treatment timing for overwintered larvae.
Lawns
Turfgrass mites: Clover mites continue to feed on lawns and enter homes in nuisance migrations.
Nightcrawlers: Tunneling activities and associated lawn lumps continue.
Midges: Non-biting midges emerge from ponds and mating swarms may be observed over lawns.
Garden
Spinach leafminer: Egg-laying and tunneling begin in older spinach foliage.
