Catching Ozyptila praticola In Non-lethal Cardboard Traps

3" x 3" flute B corrugated cardboard
stack spider trap in Mahonia sp.

Early in 2017 I decided to augment my cone-tapping efforts by deploying pitfall* traps. In theory, doing so should have made my ongoing search for the introduced European crab spider Ozyptila praticola (Thomisidae) more robust. My plan was to place pitfall traps in areas where fallen conifer cones weren't available, but where I still needed a sample.

I abandoned my pitfall plan after a few months, however, for two reasons. First, when I placed control pitfall traps in an area I knew to be rife with O. praticola, I didn't catch any. This was despite the fact that I'd placed the traps in April and May, months of peak male O. praticola activity (pitfall studies in Europe capture mostly male O. praticola). Secondly, I was getting a troubling amount of bycatch. Even as my pitfall traps failed to capture my target species, they did capture and kill amphibians, bumblebees, slugs and snails. I found this destruction of non-target species to be unethical and unacceptable.

5" x 5" flute B corrugated
cardboard stack spider trap
in Acer macrophyllum

In the mean time I'd been leafing through papers that mentioned O. praticola, and learned that the species is sometimes found on tree trunks in Europe. Among the many gizmos used to collect them on tree trunks, traps made of corrugated cardboard caught my attention. For example, Bogya (1999) found that O. praticola overwintered in or under corrugated cardboard bands placed on the lower trunks of pear and apple trees in an orchard in northeastern Hungary. The traps were placed "in autumn before leaf fall and were collected 2-2.5 months later, after the first frost." Machac & Tuf (2016) reported that in summer in the Czech Republic, pieces of cardboard on oak tree trunks were "inhabited mostly by females with egg sacs, e.g. Clubiona pallidula, Nuctenea umbratica or Ozyptila praticola".

So here was another method of detecting O. praticola that was also non-lethal to other animals - yay! In addition, cardboard is inexpensive and compostable. I decided to conduct a pilot study to see whether cardboard traps were an effective way to capture O. praticola here in western Washington.

Single-face flute A (left) & double-face
flute B (right) cardboard used in traps

The first step was to decide which corrugation flute size to use, since my traps needed to have spaces large enough to accommodate adult female O. praticola. Oddly, almost none of the researchers using cardboard to trap spiders have reported the flute size they used. I decided to try both flute A and flute B. Flute A cardboard has a wavelength of 9 mm and a peak amplitude of 4 mm. Flute B is slightly smaller, having a wavelength of 8 mm and a peak amplitude of 3 mm. Most of the shipping boxes I found in recycling bins were made with smaller flute sizes, but with perseverance and Rod Crawford's help, I managed to find some nice pieces of clean discarded flute B cardboard. Flute A, however, was nowhere to be found. I ended up ordering a 250 foot roll of 3 inch, single-face flute A cardboard from an office supply store.

My plan was to do the following in my O. praticola control plot in Seattle:

• Wrap a band of flute A cardboard around the trunks of ten trees, with the corrugated side facing the tree. Place the band 2 ft off the ground on five trees, and 5 ft off the ground on another five.
• Place one six-layer stack of 3" x 3" flute B squares in the crotch of each of five shrubs. The bottom of the stack should be close to the top surface of the litter layer.
• Place one six-layer stack of 5" x 5" flute B squares in the lowest crotch of each of five trees.

3" single-face flute A
corrugated cardboard band
spider trap on Acer saccharum

When making the cardboard stacks, I alternated the orientation of each square of cardboard so that its corrugated channels were perpendicular to the neighboring pieces. The stacks were held together with cotton string and placed "upright" in the shrub or tree so that half of the channels were vertical and half horizontal. On 25-Sept, while I waited for my roll of flute A to arrive, I made and deployed these flute B stacks. On 1-Oct I put up the tree trunk bands and also added two squares of flute A cardboard to each stack.

On 8-Oct I checked all bands and stacks for inhabitants. Most contained spiders, but none were O. praticola. I didn't check the traps again until the end of December (half on the 20th and the rest on the 30th), and by then both types of traps contained O. praticola. Raccoons had destroyed 5 tree trunk bands, but of the 5 remaining, one contained no O. praticola and the four that did contained a total of 6 females, 0 males, and 20 juveniles.

Close-up of a 3" single-face flute A
corrugated cardboard band
spider trap 

All five 3" x 3" stacks in shrub crotches survived intact, and each contained at least one mature O. praticola. Altogether, they contained 21 female, 1 male, and 125 juvenile O. praticola. I found adult females in the channels of both the flute A and the flute B pieces.

Of the 5" x 5" stacks in tree crotches, one had been removed and thrown to the ground by a raccoon, and contained no O. praticola. Of the four stacks remaining in place, only two contained adult O. praticola. In total, the four surviving stacks contained 3 female, 0 male, and 16 juvenile O. praticola.

This female Ozyptila praticola had been wintering inside
a corrugated cardboard band on a tree trunk

Apparently O. praticola had not yet climbed above the litter in search of overwintering sites when I checked the traps on 8-Oct, but had done so at some point in the following two months. Of the three trap permutations I tried, the 3" x 3" stack in shrub crotch was the clear winner for late fall trapping. Not only did every trap contain adult O. praticola, but the raccoons left them completely untouched. Once the weather warms up in spring and overwintering spiders become active again, however, I can't be certain which type of trap will provide the best odds of capturing O. praticola. Recall that European researchers have found female O. praticola in trunk traps in summer. Perhaps different traps will be ideal in different seasons. I guess I'll find out!

A note on removing spiders from cardboard traps: On 8-Oct, the traps were still fairly dry and so I could prod spiders out of the corrugated channels with a fine wire. However, by December the traps were wet from seasonal rain. This made the wire useless as a prod, since it got stuck in the softened channel walls. Therefore, the only way to remove the spiders was to completely destroy the traps by pulling apart the layers of the cardboard and shaking them into a sweep net. When planning different sampling methods, it could be worth bearing in mind that cardboard traps can be collected and processed even on rainy days. This can't always be said for litter sifting or other methods of collection.

Other spiders common in this set of cardboard traps included juvenile Cheiracanthium sp. and female Clubiona pallidula (both introduced species), female and male Tachygyna ursina and T. vancouverana, juvenile Philodromus sp., juvenile Phrurotimpus sp., and female Pelegrina aeneola and Phanias albeolus.

I still have a lot of work to do to determine which type of trap works best when, and how to raccoon-proof tree bands, but at least this pilot study serves as proof of concept.

*Simon Leather wrote an interesting and entertaining history of the pitfall trap, which can be found here.