Western Conifer Seed Bug

Summary

Western conifer seed bugs (WCSB) are herbivorous insects that feed on conifers, including various pines, spruces, hemlocks, and Douglas fir. They are native to western North America but spread eastward beginning in the 1950’s. Feeding by western conifer seed bugs does not impact the health of trees but they can be annoying fall home invaders.

Classification

Common name: Western conifer seed bug, WCSB

Scientific name: Leptoglossus occidentalis Heidemann, 1910

Family: Coreidae (Leaf-footed bugs)

Order: Hemiptera (True bugs)

Description

Adults are 3/4 inch (19 mm) long and reddish-brown to dark brown (Figure 1). The membranous halves of the forewings are dark brown and form a diamond at the rear (posterior). There is often a thin, jagged while line along the back, although this may be darker in color or even absent. The upper (dorsal) side of the abdomen is visible along the edge of the wings and has alternating patches of yellow/light orange and black. The rest of the abdomen is yellow/orange and black pattern, but is only seen during flight or when the wings are otherwise open (Figure 2). Flying adults produce a loud buzz and, combined with the bright coloration of the abdomen, this may mimic bees and serve as defense against birds.

Figure 1. Adult western conifer seed bug. Photograph by Brandon Woo via BugGuide, used with permission.

Figure 2. Adult western conifer seed bug with the wings open, revealing the black and yellow/orange pattern of the abdomen underneath. Leptoglossus species can be distinguished based on this dorsal abdominal pattern. Photograph by Peter Cristofono via BugGuide, used with permission.

The young nymphs are orange and become reddish brown as they grow larger (Figures 3, 4).

Figure 3. Early instar nymphs on a pine cone. Photograph by Sandy Kegley via Forestry Images, used under a CC BY-NC 3.0 US license.

Figure 4. Older nymph. Note the darker color compared to younger nymphs. The wing bugs are just starting to form and become obvious. Photograph by André Miquet via iNaturalist, used under a CC BY-NC 4.0 license.

The eggs are laid in chains on conifer needles and are measure about 0.08 inches (2 mm) long.

Range

Western conifer seed bugs are native to temperate areas of the Pacific Coast in western North America. They began spreading eastward in the 1950’s and were first detected in Pennsylvania in 1992. Western conifer seed bugs are now found throughout the United States and southern Canada but are most common and abundant in the Northeast. They have since spread to many parts of the world through international trade, including Mexico and Central America, Europe, Japan and southeast Asia, Chile and central South America, South Africa, Australia, and some Pacific Islands.

Life History

In North America, western conifer seed bugs have a single generation per year, while in more tropical areas, such as Mexico, they can have up to three generations per year. In temperate regions, including northeastern North America, adults emerge from overwintering sites in late May to early June and feed on one-year cones and inflorescences. Eggs are laid on conifer needles and hatch in approximately 10 days. First instar nymphs feed on the needles and tender tissue of cone scales. Older nymphs feed on developing seeds. Nymphs in all five stages of development and newly molted adults can be observed feeding on the same group of cones by mid-August, at which time the nymphs begin to reach adulthood. Adults feed on ripening seeds until early fall and then seek overwintering sites in protected areas, such as under bark, in bird and rodent nests, under building siding, and in homes.

Damage and Medical Importance

Consumption of conifer seeds can damage up to 80% of a seed crop, so western conifer seed bugs are a serious pest in conifer seed orchards and may impact forest regeneration in natural stands. Because they feed primarily on developing cones and seeds and do not affect the health of the host tree, western conifer seed bugs are unlikely to be a major tree pest for homeowners with ornamental conifers.

Western conifer seed bugs do not generally bite people and are not considered a medical risk. A single bite has recorded worldwide. This was likely the result of the bug probing in a mistaken attempt to feed. The bite resulted in a red, itchy area about 1 inch (2.5 cm) in diameter. The itching subsided after 48 hours and the red area went away without intervention after 4 weeks.

Because adult western conifer seed bugs overwinter in protected areas, they often become fall home invaders similar to brown marmorated stink bugs, cluster flies, and multicolored Asian lady beetles. Throughout the winter, low numbers of individuals may emerge indoors, while on warm fall and spring days, large numbers may emerge and fly around. This can be cause for concern for homeowners, but beyond being annoying the bugs are not a threat.

Control

Western conifer seed bugs frequently congregate on the outside of buildings in late summer and early fall in northeastern North America and can gain access through small cracks and gaps in siding, around windows and doors, and in soffit and facia. The best method of control is to prevent them from entering a building in the first place through mechanical exclusion. This may include replacing loosely fitting screens, windows, and doors; caulking gaps around door frames, window frames, and soffits; caulking cracks behind chimneys and underneath the wood fascia; and screening fireplace chimneys and attic and wall vents.

Similar species

Five other Leptoglossus species occur broadly across eastern North America may be confused for western conifer seed bugs (four additional species occur along the Gulf Coast states or just Florida and are not covered here). Leaf-footed pine seed bugs (L. corculus) tend to be dark brown (rather than mid- to light brown), the abdomen is black (rather than black and yellow), and the outer edge of the “leaf” expansion on the hind leg is unequal in length to the inside expansion (rather than the outside and inside expansion are similar in length) (Figure 5). Magnolia leaf-footed bugs (Leptoglossus fulvicornis) are plain brown without patterning and have prominent “shoulders” (dentate pronotal margins), which WCSB lack, and are specialists on magnolia seed pods (Figure 6). Leptoglossus oppositus (no common name) are darker in color, have the white line across the back reduced to small spots, and the “leaves” on the hind legs are larger and more deeply scalloped; they feed on a wide range of plant hosts but tend to feed on fruit and reproductive structures and can be abundant on catalpa pods (Figure 7). Eastern leaf-footed bugs (L. phyllopus) are darker in color, the white lines across the back are thicker, and the “leaves” on the hind legs are larger and more deeply scalloped (Figure 8). Western leaf-footed bugs (L. clypealis) have a thicker white line across the back is thicker and a spine at the front of the head that WCSB, and other Leptoglossus species, lack; they most frequently feed on junipers but have been recorded on other hosts and are occasional pests of pistachio and almond (Figure 9). Other coreids that might be confused for WCSB include squash bugs (Anasa tristis), which are pests of squashes and lack flared margins on the hind legs (Figure 10).

Figure 5–10. Leaf-footed bugs that might be confused with western conifer seed bug. 5) Leaf-footed pine seed bug (L. corculus). Photograph by Mary Spolyar via iNaturalist, used under a CC BY-NC 4.0 license. 6) Magnolia leaf-footed bug (L. fulvicornis). Photograph by Sharon Moorman via BugGuide. 7) Leptoglossus oppositus (no common name). Photograph by Patrick Coin via BugGuide, used under a CC BY-ND-NC 1.0 license. 8) Eastern leaf-footed bugs (L. phyllopus). Photograph by Sharon Samford via BugGuide used with permission. 9) Western leaf-footed bugs (L. clypealis), note the distinctive spine on the head. Photograph by Margarethe Brummermann, Ph.D. via BugGuide used with permission. 10) Squah bug (Anasa tristis). Photograph by Tom Murray via BugGuide, used under a CC BY-ND-NC 1.0 license.

Authored by Steve Jacobs, 2002; reviewed 2013; revised by Michael Skvarla, 2023.

References

Fent, M., and P. Kment. 2011. First record of the invasive western conifer seed bug Leptoglossus occidentalis (Heteroptera: Coreidae) in Turkey. North-western Journal of Zoology 7(1): 72–80.

Hornok, S., and J. Kontschán. 2017. The western conifer seed bug (Hemiptera: Coreidae) has the potential to bite humans. 54(4): 1073–1075. doi: 10.1093/jme/tjx040

Lesieur, V., A. Yart, S. Guilbon, P. Lorme, M.-A. Auger-Rozenberg, and A. Roques. 2014. The invasive Leptoglossus seed bug, a threat for commercial seed crops, but for conifer diversity? Biological Invasions 16: 1833–1849.

Mitchell, P. L., and A. G. Wheeler, Jr. 2008. Host plants of Leptoglossus oppositus (Say) (Hemiptera: Coreidae). Proceedings of the Entomological Society of Washington 110(4): 1034–1041.

Wheeler, A.G. Jr. 2018. Leptoglossus clypealis Heidemann (Hemiptera: Coreidae): Eastward spread in North America, new host records, and evaluation of host range. Proceedings of the Entomological Society of Washington 120: 196–210.