Quick Facts...
- Hornworms are among the largest caterpillars found in Colorado.
- Although the “tomato hornworm” damages garden plants, most
hornworm species cause insignificant plant injury.
- Adult stages of hornworms are known as sphinx, hawk, or “hummingbird”
moths.
Hornworms are among the largest of all caterpillars found in Colorado,
some reaching lengths of three inches or more. Characteristically they
sport a flexible spine ("horn") on the hind end, although in some species
this is lost and replaced with an eyespot marking. The most widely recognized
hornworms are those that feed on tomatoes - the tomato hornworm and the
tobacco hornworm. Although these two insects are considered garden pests,
the majority of the more than 30 hornworm species found in Colorado are
rarely observed and do not cause significant injury to plants.
Full-grown hornworm larvae migrate from their host plant and dig in loose
soil where they pupate. Pupation occurs a few inches below the soil surface
in a small chamber of packed earth. Pupae are typically brown, two inches
or more in length, and many have a pronounced "snout" off the head end.
Adult stages of hornworms are heavy-bodied, strong flying insects known
as sphinx or hawk moths. However, some local species are popularly known
as "hummingbird moths". These moths have a superficial resemblance to
hummingbirds in flight while they similarly feed from deep-lobed flowers.
The whitelined sphinx is the species most commonly observed in this habit
and is usually most active during late afternoon and dusk. The great ash
and twinspot sphinx are two other species commonly observed in "hummingbird
moth" behavior. Adults of most hornworms (including the "tomato" hornworms)
fly after dusk and are rarely observed except occasionally at porch lights.
(Moths of North America, a Web site maintained by the USGS Northern
Prairie Wildlife Research Center, documents the known distribution of
various moths, including sphinx moths. It's available at: www.npwrc.usgs.gov
)
Life History and Habits
Whitelined Sphinx
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| Hornworm pupa. |
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| Whitelined sphinx, a common "hummingbird" moth. |
The whitelined sphinx (Hyles lineata) is the most common hornworm of Colorado
and, by far, the most commonly encountered "hummingbird moth". Larvae
develop on a variety of plants but seldom do they significantly damage
those plants considered economically important. Portulaca, primrose, and
wild grape are among the most common larval hosts. Rarely, you may see
large outbreaks of caterpillars that sometimes result in very visible
migrations across roadways when food plants are exhausted.
Hornworms of the whitelined sphinx can vary in color. Most are predominately
green, with some yellow, white, and/or black markings. Less commonly you'll
find predominately black forms, with yellow markings. Adults have a prominent
white band on the upper forewing.
“Poplar Sphinx Moths”
The largest common sphinx moth in Colorado is the big poplar sphinx (Pachysphinx occidentalis); a closely related species, the modest sphinx (Pachysphinx modesta), can be found in higher elevations. Caterpillars are generally green with light striping. Both species develop on aspen, cottonwood, poplar, and willow, but they are never abundant enough to cause injury to the tree.
Achemon Sphinx (“Hornless Hornworm”)
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| Achemon sphinx larva. |
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| Achemon sphinx adult. |
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| Tomato hornworm sphinx full-grown larva. |
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| Tobacco hornworm full-grown larva. |
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| Hornworm egg. |
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| Whitelined sphinx larva. |
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| Tomato hornworm adult. |
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| Tobacco hornworm dult. |
Larvae of the achemon sphinx (Eumorpha achemon) are unusual in that they lose the terminal "horn" after the first molt. Instead subsequent larval stages are marked by having a prominent "eyespot" marking at the hind end. The caterpillars develop on Virginia creeper, grape, and related vines and are dark brown with purplish tones. Apparently there is one generation produced per year, with full-grown caterpillars being observed in late August and early September.
“Tomato Hornworms”
The most familiar hornworms are those associated with tomatoes and related
plants. This is a complex of two species, the tomato hornworn (Manduca
quinquemaculata), and the tobacco hornworm (Manduca sexta). Both species
are pests in vegetable gardens. The caterpillars chew leaves, and plants
can be rapidly defoliated. Fruits may also be chewed. Tomatoes are particularly
susceptible to injury; but other related plants, such as peppers and potatoes,
are occasionally infested.
The tobacco hornworm is usually much more common in regional vegetable
gardens than the tomato hornworm, but both may be found together as they
generally have similar habits. The tomato hornworm has a dark green "horn"
with black sides, while that of the tobacco hornworm is red. The caterpillars
also are differentiated by the white striping along their sides. These
form a series of V's with tomato hornworms, while there are diagonal dashes
on the tobacco hornworm. Occasional dark forms of the tomato hornworm
larvae occur. The caterpillars of these are much darker than the normal
green, although the adult moths differ little in appearance.
Many of the tomato and tobacco hornworms overwinter as pupae in the soil
in the vicinity of previously infested gardens, typically emerging in
late May and June. However, the adult moths are strong fliers that may
travel long distances, with some apparently migrating into Colorado in
late spring from more southerly areas. Female moths lay large pearl-colored
eggs on the upper surface of leaves. The young caterpillars hatch and
feed on the plant for a month or more. They have tremendous appetites
and consume large amounts of leaves as they grow older and larger. After
feeding, they wander away from the plant and pupate in the soil. In southern
areas of the region, a second generation frequently occurs with caterpillars
present in late July and August and during warm summers.
Adult moths are identified by examining the hind wing which has two separate
wavy bands next to the border on the tomato hornworm that are fused with
the tobacco hornworm. The white marks on the abdomen of the tomato hornworm
are also more angularly marked.
Control in Gardens
"Tomato" hornworms are easily controlled by garden insecticides (e.g.,
carbaryl, permethrin, spinosad). A biological control organism that is
also highly effective and sold commonly is Bacillus thuringiensis (e.g.,
Dipel, Thuricide). Hornworm larvae can also be hand-picked although they
are surprisingly difficult to detect because of their cryptic coloration.
Larvae tend to feed on the exterior parts of plants during shadier periods,
near dusk and dawn, when they may also be more readily observed and destroyed.
| Table 1. Some Common Hornworms Found in Colorado
|
| Common Name |
Scientific Name |
Host Plants; Notes |
| Whitelined sphinx |
Hyles lineata |
Very wide host range that includes primrose, portulaca,
apple, grape, four o’clock, and peonies. The adult is marked
with strong white bands on the wing and is by far the most commonly
encountered “hummingbird moth”. |
| Achemon sphinx |
Eumorpha achemon |
Grape, Virginia creeper, woodbine. These purplish-brown
caterpillars lose the characteristic “horn” and have a dark
eyespot in its place. |
| Tobacco hornworm/Carolina sphinx |
Manduca sexta |
Tomato, eggplant, and other nightshade family plants.
The most common of the “tomato hornworms” east of the Rockies. |
| Tomato hornworm/Five-spotted hawkmoth |
Manduca quinquemaculata |
Tomato, eggplant, and other nightshade family plants. |
| Waved sphinx |
Ceratomia undulosa |
Ash, privet, oak, hawthorn. |
| Elm sphinx |
Ceratomia amyntor |
Elm. Birch, basswood, and cherry are occasional hosts. |
| Great ash sphinx |
Sphinx chersis |
Ash, lilac, privet. |
| Vashti sphinx |
Sphinx vashti |
Snowberry, other Symphoricarpos spp. Primarily west
of the Rockies. |
| Big poplar sphinx |
Pachysphinx occidentalis |
Poplars, cottonwood, willow. This is the largest species
found in much of the state. |
| Modest sphinx |
Pachysphinx modesta |
Poplars, aspen, willow. A large species but uncommon
and confined to higher elevations. |
| Twinspot sphinx |
Smerinthus jamaicensis |
Poplar, birch, elm, willow, ash, apple. |
| One-eyed sphinx |
Smerinthus cerisyi |
Poplar, willow. |
| Snowberry clearwing |
Hemaris diffinis |
Honeysuckle, viburnum, hawthorn, snowberry,
cherry, plum. Adults have clear wings and resemble oversized bumblebees. |
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