What Are Digger Wasps?


Insect lovers and entomologists alike have long been fascinated by digger wasps. You may have seen digger wasps around but not known what they are. You may have even seen them come out of their underground burrows.

Digger wasps are a group of solitary wasps which excavate burrows and stock them with paralyzed flies and other invertebrates for their larvae to feed on.

If you want to learn more about digger wasps, please read on.

How Do Digger Wasps Build Their Nests?

Burrows are constructed by the female in soil or dirt, usually with many cells. These are then filled with enough food to support the young when they hatch.

In each burrow, the female makes a series of cells, perhaps as many as twenty-five in one season, which she stocks with enough food for her offspring. The female uses a variety of habitats for the burrow, but they are usually dug in rotten wood or soil.

When wasps are mentioned, most people think of social wasps that live in large communal nests with a single fertilized queen. There are also many species of solitary wasps.

These do not live in large colonies but construct individual burrows. While social wasps only have one queen in the nest, several digger wasp females may be found excavating burrows near one another.

The soil-dwelling species are usually called digger wasps. Most of them are wasp-like in appearance with black and yellow bodies. Because they do not enter houses or attack orchards, they are rarely noticed. Another difference from social wasps is that digger wasps have no workers, so the species consists of males and fully fertile females.

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Field Digger Wasp

A common species is the field digger wasp (Mellinus arvensis), found during the summer months. It is fairly large, reaching a length of 11-14mm, (0.5 in,) and has black and yellow coloring. The female excavates a sloping burrow 30-50cm (12-20in) in bare sandy soil, with the entrance usually hidden by a leaf or stone.

She removes the soil with her jaws and front legs, gathering it between her head and forelegs, before backing out of the burrow to deposit the soil a few centimeters away. She excavates up to ten cells leading off from the burrow, making them one after another as each is stored with paralyzed flies.

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Catching Prey

The field digger wasp’s prey consists of medium-sized fly species in the area. The female hunts for flies, normally near dung, although there are usually lots of flies on the leaves of trees. Her method of attack is to stop a short distance from the pile of dung and then crawl towards the stationary fly.

When she arrives within an inch of the victim, she attacks by leaping onto its back, holding it with her legs. She then grasps a wing with her mandibles. Once the wing is trapped, she stings the fly from below by bending her abdomen round. When the fly is paralyzed, she takes it in her jaws before taking the sting out. The female flies back to the nest with the prey and drags it into her burrow backwards.

Each cell of the burrow is filled with up to ten flies, with their heads pointing inwards and occupying about two-thirds of the space. An egg is placed on the last fly in the cell before the passage to the main tunnel is filled with soil obtained from the next cell.

When the egg hatches, the purple-colored larva consumes the paralyzed flies over the next few days. They then spin a soft, yellowish, parchment-like cocoon. It rests in this cocoon in a dormant state until the following spring when it pupates, before emerging as an adult.

Two-girdled Digger Wasp

The two-girdled digger wasp (Argogorytes mystaceus)is a darker insect with the yellow markings on its abdomen reduced to two narrow bands and two short bars. Like the field digger wasp, the female digs a nest in sandy soil, excavating vertically for about 10cm (4in) and then horizontally. She will dig between six and nine cells.

This species preys on the nymphs of froghoppers, which can be found deposited on leaves. The female extracts the nymphs from the gooey liquid. The female carries the nymphs back to the nest one at a time, holding them beneath her body with the middle legs. When bringing back the nymphs, she enters head first, shifting the grip on her prey to her hind legs. Each cell is packed with 20 or more nymphs, an egg is laid, and the cell is closed.

Large-spurred Digger Wasp

The large spurred digger wasp (Nysson spinosus) acts as a nest parasite of the two-girdled digger wasp. It waits patiently by the nest until the owner flies away for more food, before entering it and laying its egg, normally under the wing pad of one of the stored nymphs.

The large spurred digger wasp has also been observed opening up closed burrows to gain access to the cells before closing the burrow afterwards. When the egg hatches into a larva, it first eats the wasp’s egg and then consumes the frog-hopper nymphs.

Photo of digger wasp

Cerceris

Cerceris is the largest genus in the family Crabronidae, consisting of over 1000 species. Digger wasps of the genus Cerceris are called tailed-diggers, as they have particularly strong plates at the ends of their abdomens which are used for pushing soil out of the burrow. They dig vertical shafts in the hard ground with their legs and then use the end of their abdomens to push the soil out so that it stays as a heap on the surface. The entrance is in the center of the mound, allowing the wasp to find its nest again easily.

The sand-tailed-digger wasp is common on sandy heaths. Like others of the same genus, the abdominal segments of this species are wider in the middle than at the ends where they adjoin. The abdomen has alternate bands of yellow and black.

This species specializes in catching weevils as prey, usually selecting just one species in a particular area. Although the weevils are stung between the thorax and the abdomen, where the main nerve centers lie, they can stay alive and fresh for as long as a month.

The nest burrow goes down vertically for about 6cm (2in) and then becomes horizontal, ending in an oval cell. The longer the nest is used, the more cells are added at the sides, with up to six noted.

Other species

The ornate tailed digger wasp (Cerceris rybyensis) feed on solitary bees. It catches the bees while flying, preferring those heavily laden with pollen. The wasp knocks the bee out of the air near its colony and, holding it by the neck, stings it under the thorax.

The wasp grips the bee’s antennae with its jaws and flies off to its burrow, carrying the bee beneath its body. They will sometimes feed on the body fluids of the bee. If the prey is especially heavy, the wasp may transport it in stages. As many as eight bees are packed into each cell.

A common digger wasp found during summer is the slender-bodied digger wasp (Crabro cribrarius). Measuring 10-15mm (3-4in) long, it can be seen flying quickly or sitting near plants. It swoops on these flies before storing them in the cells.

They can be identified by extensive yellow markings on their abdomen and the male’s expanded front tibiae, which are yellow with black markings.

The common spiny digger wasp (Oxybelus uniglumis) is only 5-8mm (1 in) long and digs short burrows in sandy soil, raking out the soil with her legs. The burrow comprises a single cell at the end of the shaft. When the female has finished the burrow, she closes the entrance and flies around it, memorizing the location.

The common spiny digger wasp preys on small flies, but she carries her victim upside down while still impaled on the sting.

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