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Montana Field Guide

Montana Field Guides

Grass Shieldback - Steiroxys trilineatus

Native Species

Global Rank: G5
State Rank: SNR

Agency Status


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General Description
The following comes from Caudell (1904), Hebard (1928), and Vickery and Kevan (1985). The head is small and deeply inserted into the pronotum (thorax) with small eyes. The pronotum is elongated posteriorly with the lateral lobes nearly as deep as long, with a curved margin, and distinct dorsal lateral and central carina (ridges), the former slightly diverging posteriorly, the latter straight. The lateral lobes of the pronotum are commonly bordered by yellowish, border with a black marking. Body colors are green or gray-brown (this being the most common). Both color forms often occur within the same population areas. Abdomen is plump, usually with a row of dark V-shaped black markings down the dorsal surface.

Note on the genera Steiroxys and Pediodectes
These two genera have never been studied in detail. Some of the other eight known species presently recognized may not be valid and others may still be awaiting discovery. This lack of study remains as current today as it did at the turn of the 20th century. Much of what is described here is very open to uncertainties, and in need of future diagnosis and discovery.

Calling song description
To date, no description nor sonograms have been reported.

To date, no studies or observations have been found within the published literature, but speculated to be similar to other Shieldbacks in their adult stage.

Diagnostic Characteristics
The following is taken from Caudell (1904). The male pronotum length is 5.5-6 mm, and female is 4.5-6 mm. Hind femur length for males is 18-19 mm, and females 15.5-19 mm. The ovipositor length is 16-24 mm. The cerci of males (see illustration above) is somewhat flattened and tapering posteriorly to an inward curved point and possessing inside with an upward pointing tooth near the middle.

The Grass Shieldback can be confused and misidentified with Stevenson’s Sheildback (Pediodectes stevensonii), but unlikely with other Shieldbacked species, such as Anabrus and Peranabrus.

Species Range
Montana Range Range Descriptions


Range Comments
Occurs along the eastern front of the Rocky Mountain Range from south Alberta, south to northern Utah. It is mapped as a finger-like pattern from the eastern half of Idaho, eastward to western Montana. There are two isolated occurrences reported for north-central Oregon and western Nevada. In Montana, it has been reported in 3 counties (Walker SINA website 2020).

Observations in Montana Natural Heritage Program Database
Number of Observations: 9

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

This species prefers open grassy areas of sagebrush steppe and mountain meadows, favoring dry sites with sparse forbs and grasses growing on loose soils. It seems to favor sites ranging from 4,400 to 7,600 feet in elevation. The Grass Shieldback is not found commonly over its range, only as localized populations where individual numbers can be relatively high. This species is diurnal and best observed or collected during daylight hours (Hebard 1928, and Vickery and Kevan 1985).

Food Habits
To date, no studies or observations have been found within the published literature.

Reproductive Characteristics
To date, no studies or observations have been found within the published literature.

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Citation for data on this website:
Grass Shieldback — Steiroxys trilineatus.  Montana Field Guide.  .  Retrieved on , from