Sweepin’ for Bugs

On our daily hikes the use of the sweep net has become an essential tool while stationed on the central coast of California. All but the hardiest flowers are dormant or “toast-ed”, king tides haven’t arrived yet, and fungi are awaiting the right conditions to fruit. Being a naturalist is all about adapting to the surroundings, and getting creative in order to find lifer species at the “lowest” part of the year for nature.

So what is a sweeping, and what is a sweep net?

Think a wind sock meets a butterfly net. You can see in the photos it’s a pole at one end and a hoop with a long sock-like attached, which is where the insects, arachnids, and anyone else fall into. The dimensions can vary but generally are include: a pole two to three feet long, a hoop 10 to 30 inches in diameter, and the sock is roughly 20 inches deep. Most popular is the “American” net.

Trevor showing off his long looping swings in a dry swamp in BC.

Once outdoors, a sweep net is moved back and forth across a small area in long semicircular swings, creating a sort of endless infinity-shaped loop or a long winding snake (see the above video). Doing so will catch any insects that get alarmed and spring or fly out of the way of the net, but end up getting caught in the long extended canvas sock. After sweeping vegetation, the sock is flipped inside out and the contents are dumped on the ground, preferably on a sheet where the contents and critters can be observed and documented. A second method and the style we do the majority of the time is holding the opening of the net underneath foliage and tapping the leaves and/or branches with a stick so as to startle the insects, which then fall, and therefore temporarily catch the insects in the net (video below).

Trevor tapping a Santa Lucia Fir, Abies bracteata in CA

As you may have gathered, some insects’ escape tactic from predators is to drop off the foliage they were just perched on. A “leap of faith” one might call it.

In 1935 Geoffrey Beal wrote the Study of Arthropod Populations by the Method of Sweeping. In an Illinois woodland he investigated how well sweep nets capture an area’s insect population, and what’s the best sweeping method to capture said insects. “Accordingly, nine sweeps should contain the animals of a square meter. Somewhere between six and nine of the single 100 inch strokes used indicate, as well as anything, the population of one square meter. A more representative population sample is obtained by two strokes over the same vegetation since the first stroke rouses the insects and the second stroke takes them.”

We of course go to areas with a high majority of native plants, but this method can be used for non-native insects as well. A study in 1977 with soybeans, and about seven soybean pests. Rudd and Jensen found “the sweep net method is economically more efficient than the ground cloth method for sampling pest species.” By ground cloth, I believe they mean beating the soybeans while a cloth underneath exposes the fallen insects. The whole paper was behind a pay-wall… But I read that soybeans are often sampled for pests using a vertical beat sheet, where the insects fall into a container.

A 2010 study by Doxon et al. looked at the difference between sweep netting and vacuum sampling (it’s what it sounds like, a vacuum for insects). While the orders of families differed between the two methods, “the mean size of invertebrates collected and overall invertebrate biomass were greater for sweep-netting than vacuum sampling. Vacuum sampling was more effective at collecting small (e.g., <5 cm) invertebrates, whereas sweep-netting captured large (>5 cm) Orthopteran and Lepidopteran larvae at higher rates. Thus, our results indicate that neither sampling method effectively sampled all invertebrate families and investigators should be aware of the potential biases of different sampling techniques and be certain that the technique selected will allow study objectives to be met.”

Net Ideas and Options:

Make a homemade sweep net!

Or with a capture pouch at the bottom

Our nets are from BioQuip

Once we’ve swept enough in one habitat type, before moving too far along the trail we dump our contents onto a thrifted sheet and see what we got! Out comes the cameras, and we get to work documenting who we’ve swept!

Take a look at some of the insects we’ve found recently:

Scriptured Leaf Beetle, Genus Pachybrachis

Tanks! Artfully patterned Tanks! That’s what these beetles are! How big or small do you think it is?

Well, this cookies and cream beetle is about3-4mm, which is smaller than a short-sized grain of rice. Tiny!

Unlike some other insects in this post, this one has a pretty wide distribution, largely holartic.

Larvae of these beetles are case-bearers, members of the Case-bearing Leaf Beetle subfamily Cryptocephalinae. To protect themselves against the scary and dangerous world larvae build protective cases from anything within reach. As young pupa, they attach themselves to plant stems or leaves, but not in a cacoon! Naked little beasts!

A few of the fall colored bugs we’ve found:

Top Left: Genus Sinea, a member of the Assassin bug Family and ranges from Canada to Colombia. I hope to find some in our travels down south this year and following trips because they are simply stunning. Their body shape is so distict, the prominent spines on their front tibiae (“leg”) just add to their fascinating morphology.

Top Right: Hedge Nettle Stink Bug Cosmopepla conspicillari.The black and orange coloring was very fitting, as we swept this one on Halloween. We’ve observed this species in Santa Barbara and Monterey County about a week apart. As the name implies they prefer Hedge Nettles (Genus Stachys). This point illustrates how sweeping a diversity of plants is essential to observing an array of species because some insects prefer a very narrow range of plants. That’s why supporting native plants, and a diversity of native plants is essential for a healthy and therefore robust ecosystem.

What makes this one different from other Cosmopepla due to the small white half circle on the scutellum, as well as the number of orange markings on the dorsal thorax, think “shoulders”.

Bottom left: Scentless Plant Bugs Family Rhopalidae. This bright beauty was swept during our Halloween camping trip in Los Padres National Forest. Scentless plant bugs get their name because they lack”well-developed scent glands.” Other features that separate them from similar-looking Coreidae (Leaf-footed bugs) include our bug here has raised ocelli (primitive eyes), and the front wings have many veins. All of these features are obviously very small, illustrating the difficulty in insect ids.

Bottom right: Pacific Madrone Psyllid Neophyllura arbuti. This psylloid was swept from an area with madrone, coyote brush, manzanita, coast live oak. Id help came from @alica_abela on iNaturalist. They’ve been observed on iNat along the west coast of the US, but I imagine they could be anywhere madrones live. Due to their name, I’m assuming that’s their specific host and making a note of what shrubs were swept probably aided in getting an identification.

Oh boy I love these funny-looking planthoppers, who go by Genus Orgerius. Their upright stance, their head that looks like a giant nose (to me), and how abundant they were when sweeping the vegetation in our recent trip to Los Padres NF just make them great insects to observe and marvel over.

Most species exist within California, but the 15 species within the genera can be found across the western US.

I did find a key, so I might give it a go, and figure out it’s species.

What a hairy little happy man! This Monoxia was swept from coastal scrub vegetation in Morro Bay, CA. Members of this genus start out their lives as leafminers!

I wanted to include at least one example of an insect that can’t be identified down to species with only photos. That’s the reality for a variety of insect taxa. Usually what’s required is killing the insect, putting it under a microscope, and looking at their reproductive junk. For this genera, looking at the male’s reproductive organs, aka aedeagus is necessary. Currently, we don’t have the setup for this detailed work, but hey everyone’s got their odd hobbies in retirement.

Wow-what a wonderful speckled pattern. Even though this barklouse is quite the looker, this species is quite overlooked. We are only one of eight observations on iNat, even though it’s widespread over the western US.

These little guys apparently prefer ponderosa pine and juniper, which makes sense because we swept this guy while in Los Padres NF which has the occasional ponderosa pine.

To wrap things up:

Of course, there will be some degree of bias, with this method. We won’t find all insects chilling in shrubbery by sweeping, but it is highly effective for certain families of insects. If your goal is to find a greater diversity of insects then give sweep netting a go. Sweeping is easy, light and cheap, but is biased towards insects that hang out on the tips of leaves, larger insects, the more active critters, and ones that don’t escape the net (Dixon et al. 2010)

If you made it the whole way through this post, here’s a short video of Trevor showing off his mad net skills

References