Aerial Assassins

Robber Flies are also called Assassin Flies, and for good reason!

What is that powerfully built, fly-like creature that waits in ambush to catch their prey in flight? It’s a robber fly, otherwise known as an assassin fly, which feeds almost exclusively on other insects, including other flies, beetles, butterflies, moths, bees, ants, dragonflies, damselflies, wasps, grasshoppers, and even some spiders.

Robber flies have stout, spiny legs, short antennae, three simple eyes on the top of their head between two large compound eyes, bristly bodies, and a short, stout proboscis that encloses a sharp, sucking mouthpart called a hypopharynx. They also have a dense moustache of bristles on the face called the mystax, which appears to offer some protection from their struggling prey. These flies attack their prey by stabbing it with the proboscis and injecting it with a saliva that contains both a neurotoxin and proteolytic enzymes.  This paralyzes the prey and liquifies the insides, allowing the robber fly to suck the material up through their proboscis.

In spring and summer in our area, some of the more common robber flies include the Bee-like Robber Fly (Laphria macquarti), Prairie Robber Fly (Diogmites angustipennis), and the Maroon-legged Robber Fly (Promachus hinei). 

The Bee-like Robber Fly is large (up to 1.25” long), hairy, mostly black with a yellow thorax, basal abdominal segments, and some yellow on the legs.  It mimics the American Bumblebee, and while it prefers to feed on small beetles, it will take a wide range of prey, often perching on tree stumps in the open.

Bee-like Robber Fly

The Prairie Robber Fly, also called the Slender-winged Hanging Thief, is about 1” long, with greenish-red eyes, tan to reddish-brown body, thorax with darker brown longitudinal stripes, and abdomen with darker lateral stripes.  It is common in both woodland and meadows, hunting bees and other insects, and typically hangs from vegetation by its forelegs while it consumes its prey.

Prairie Robber Fly

Another large robber fly is the Maroon-legged Robber Fly, up to 1.5” in length.  It has dark eyes, light-colored bristles around the face, a brown thorax, a cream-colored abdomen with thick black bands and a black tip, and hairy reddish-brown legs. It prefers hunting bees and wasps, and can be found flying around a variety of habitats, perching on the ground or in vegetation.

Mating Maroon-legged Robber Flies

Female robber flies deposit tiny, whitish eggs on low-lying plants and grasses, or in cervices in wood or bark, or in soil. After hatching, the larvae live in this detritus, feeding on other soft-bodied insects. Robber flies overwinter as larvae, pupate in the soil, and eventually emerge as adults.  This developmental cycle can take one to three years, depending on the species and environmental conditions. Robber flies generally occur in relatively open habitats with scattered vegetation, or in forested margins and woodland edges.  

Robber flies are generally considered beneficial insects due to their predatory nature, as they primarily feed on other insects, many of which are considered pests. These aerial assassins help to naturally regulate insect populations thereby contributing to a more balanced ecosystem!

A Parade of Pollinators

Fall-blooming plants like Goldenrod attract a variety of insect pollinators.

Much has been written lately on the importance of pollinators, as they are vital to creating and maintaining the habitats and ecosystems that many animals (and humans!) rely on for food and shelter.  In fact, more than one in three bites of food we eat or beverages we drink are directly dependent on the success of pollinators. While most people think of bees as the primary pollinators, or even charismatic groups such as butterflies and hummingbirds, pollinators also come in the form of wasps and flies.

This time of year, you can find many of these bees, wasps, and flies nectaring on and pollinating our fall-blooming plants.  Aside from the well-known but non-native European Honey Bee, other native bee species that are still around this time of year include the Metallic Epauletted-Sweat Bee (Augochloropsis metallica), American Bumblebee (Bombus pensylvanicus), Southern Carpenter Bee (Xylocopa micans), Parkinsonia Carpenter Bee (Xylocopa tabaniformis parkinsoniae), and Texas Carpenter Bee (Xylocopa virginica texana).

Metallic Epauletted-Sweat Bee

The uncommon Metallic Epauletted-Sweat Bee is a small bee, to 0.5 inches, overall metallic green with distinctive scale-like coverings at the base of each amber-colored veined wing.  It nests in soft ground, and is especially attracted to asters, grapes, and legumes.  

American Bumblebee

The American Bumblebee is common and robust, to 1 inch, with a thorax yellow in front and black in back, an abdomen with the first three segments yellow and the rest black, and mostly black veined wings. Once abundant, populations have declined significantly in recent years, and it is listed in Texas as a species of ‘greatest conservation need.’

Carpenter bees are typically separated from bumble bees by their big, shiny abdomens.  They bore into the surface of wood to build their nests, often leaving a small pile of sawdust underneath. Due to their large size and heaviness, carpenter bee species often perform what is called ‘nectar robbing’, using their mouthparts to bite through the base of tubular flowers to access nectar rather than entering the flower directly.

Southern Carpenter Bee

The uncommon Southern Carpenter Bee is large, to 1 inch, with the female being shiny blackish overall and the male more of a glossy dark greenish-blue, both with smoky veined wings.  

Parkinsonia Carpenter Bee, nectar robbing

The more common and endemic Parkinsonia Carpenter Bee is a bit smaller at 0.8 inches, overall blackish, with males having gray-blue eyes and females with dark blue eyes, both with a black abdomen with four distinct yellow bands on each side that do not meet in the middle, and smoky veined wings. 

Texas Carpenter Bee, nectar robbing

Also common is the Texas Carpenter Bee, to 1 inch, with a thorax covered in yellow hairs except for a round black spot on top, a shiny black abdomen, and smoky veined wings. 

Blue-winged Scoliid Wasp

Wasps differ from bees in that they are smooth, shiny, and often slender or narrow-waisted. The Blue-winged Scoliid Wasp (Scolia dubia) is an uncommon native wasp, to 1 inch, overall black with a mostly reddish abdomen (sometimes with a large yellow spot on each side), and dark blue veined wings. Adults provision their nests with beetle larvae, commonly of Green June Beetles. 

Fraternal Potter Wasp

The Fraternal Potter Wasp (Eumenes fraternus) is an uncommon native, to 0.8 inches, overall black with an elongated but swollen waist, a bulbous tapering abdomen, with an ivory stripe behind the head and some before the waist, two ivory side spots and a stripe near the abdomen’s tip, and dark amber-brown veined wings. Females fashion unique, marble-sized urns of mud as nests, one for each egg, and provisions them with small caterpillars.

Potter wasp nest

While many view flies as pests, flies are actually second in importance to bees as pollinating insects.  Flies pollinate more than 100 cultivated crops such as cocoa, strawberries, apples, blackberries, peaches, onions, parsley, and carrots.  A large number of wild native plant species, including many medicinal plants, are aided from fly pollination as well.  In our area, two interesting fly species are the Oblique Stripetail Fly (Allograpta obliqua) and the Four-speckled Hover Fly (Dioprosopa). Both are native but uncommon, most often seen through November on a wide variety of flowering plants with or near aphid colonies.

Oblique Stripetail Fly

The Oblique Stripetail is a small fly, to 0.3 inches, with reddish-brown compound eyes, transverse bands on a dark abdomen with four longitudinal yellow stripes near the tip, and clear veined wings. It is a member of the hover fly family, having the rare ability to hover or fly backward.  

Four-speckled Hover Fly

The Four-speckled Hover Fly is a bit larger, to 0.5 inches, with dark brown compound eyes, a black abdomen with two pairs of whitish speckles or dashes, and a pair of clear veined wings with a brown leading edge.  Its larvae eat aphids, and this fly is considered to be a wasp mimic, due to its thin, ‘wasp-waisted’ abdomen.

The species shown above are only some of the many distinctive species of bees, wasps, and flies that are important pollinators. This fall, take a closer look at the insects that are busy nectaring on your plants, and pollinating them in the process. Make a difference to their populations by adding a wide variety of native plant species in your yard that bloom from spring to fall, and be sure that you avoid using pesticides.  Your actions alone can help promote a healthy parade of pollinators for many months of the year, that in the end benefit us all!