Dr. Stellwagen has used several properties of these droplets to characterize them and measure their performance. Each droplet has a core of glycoprotein, a protein with sugar attachments, and an outer aqueous coating (Figure 2).  She uses measures such as the size of the droplets as well the ratio of glycoprotein to aqueous coating. She also measures their toughness through the angle of thread deflection created when sticking a probe to the droplet and pulling the silk (Figure 3). The angle is then correlated to tension, which can be used to calculate toughness.  Using these characters and the knowledge that humidity affects the droplets, Dr. Stellwagen began to wonder how other environmental conditions such as temperature, and ultraviolet B (UVB) radiation could change the glue droplets’ performance.

Stellwagen and colleagues (2014) knew that temperature and humidity fluctuate on a daily basis. Temperature tends to be the lowest at night when humidity is high and the highest in the afternoon when humidity is low. The inherent hygroscopic quality (ability to attract and hold water) of the glue means that it swells with increased humidity, which makes it stretch farther but reduces the adhesion or stickiness, depending on the species and its habitat. Lowering temperature causes the viscoelastic protein to become stiffer, similar to a jar of refrigerated honey. She hypothesized that the effects of temperature and humidity are working counteractively to each other to stabilize droplet function. Dr. Stellwagen measured droplet toughness in Argiope aurantia, an orb weaving spider, under three different temperature and humidity conditions and found that indeed temperature was acting as moderator to changes in humidity, and vice versa. Afternoon conditions of high heat and low humidity resulted in the best glue. Interestingly, afternoon is when these spiders capture the bulk of their prey, which corresponds well with the glue being optimally sticky.

Many orb web building species prefer open, sunlit areas, so in addition to temperature and humidity, their webs are exposed to ultraviolet radiation, which can cause molecular damage. This inspired Dr. Stellwagen and colleagues to examine the impact of UVB radiation on the glycoprotein glue of orb-weaving spider capture thread (Stellwagen et al. 2015). They hypothesized that species which spin webs in sunny habitats will produce glue droplets that are less susceptible to UVB damage than those that spin webs in shaded forest habitats or those by nocturnal species. To test this idea, they collected thread samples built by five different species that are representative of five distinct habitats with varying degrees of sunlight (full sun, partial shade, mostly shade, full shade and nocturnal). Fresh threads were exposed to two treatments: UVB energy equivalent to a full day of summer sun and three times that amount. Droplets were also placed in a dark chamber allowing evaluation of post-production changes and aging (Figure 4).

The authors found that some material properties of glycoprotein web glue, such as the ability to absorb moisture from the air, were unaltered by the UVB treatments. However, for full shade species, the amount of energy the droplets could dissipate was reduced after UVB exposure. Thus as predicted, species that spin webs in sunlit habitats produce glue droplets that are better adapted to cope with UVB radiation and are therefore less vulnerable to its damaging effects.

Dr. Gundersen-Rindal’s team is currently working on a sustainable pest management method using a next-generation molecular technique, the silencing of genes through RNA interference (RNAi). The RNAi technique would, ideally, create a custom management tool specific to a particular species, which would be based off of a readily degrading biological molecule, reducing issues of persistence. This technique takes advantage of a virus-fighting mechanism found in most organisms. As DNA is transcribed into proteins, an RNA copy of it is made from an original template strand. This strand is “checked” at certain points by proteins within cells. If these mechanisms detect foreign sequences such as those inserted by a virus, RNAi intervenes and “chops” the RNA sequence before it can become a protein. RNAi technology creates custom double stranded RNA (dsRNA) that are ‘chopped’ into small interfering RNA strands which can target and silence specific genes. This concept can be better understood through this short video created by the TED online lecture series. RNAi was developed in the 1990s and is now being studied as a tool for arthropod pest management, by targeting genes that are essential for development and survival. A commercial pest management program based on RNAi would have many advantages. It can be made specific down to the species level, avoiding harming species other than our targeted pest and the likelihood of resistance is extremely low, especially if multiple genes are targeted simultaneously. The dsRNA could be delivered to pests through transgenic plants, similar to Bt crops, reducing environmental effects. Even if applied in a manner similar to conventional insecticides like a spray or powder, it would only persist in the environment for a short time. However, there are still challenges with delivering a potentially fragile biological molecule into an insect through conventional means such as sprays and baits.