Pestology Blog

There is an old adage says that mother knows best, but it appears that an invasive species is proving that to be true. Brown Widow spiders, or Latrodectus geometricus, are relatively recent invaders. Transported to different locales through human trade, the Brown Widow features higher reproductive abilities and dispersal abilities compared to their native cousins, the Black Widow.  These traits mean that they often outcompete their Black Widow cousins in the same environment. But are they hiding other abilities that give them that extra edge? 

Spiders, much like many other arthropods, are subject to parasitism. Wasps are common culprits of parasitism of spiders. The adult female wasps will insert their ovipositors into a spider egg sac and will lay their own eggs. From there, the wasp eggs will hatch, and the wasp larvae will consume the spider eggs before they’re able to hatch. The wasps will eventually emerge from the spider egg sac as adults. However, the researchers noticed out in the field that the Brown Widow spider egg sacs were less susceptible to being parasitized by a wasp compared to Black Widow egg sacs collected in the same area. 

To unravel the mechanisms behind what makes these Brown Widow egg sacs less susceptible to wasp parasitism, the researchers turned to the Octo-Moms in the room- the Brown Widow females. Using Latrodectus pallidus, or the White Widow, as another Widow species as comparison, the researchers exposed egg sacs to Philolema latrodecti, a species of parasitoid wasp.  

The first experiment examined the defensive behavior of the two spider species in response to the presence of the wasp and then analyzed the number of spiderlings that hatched from those same egg masses. Both spider species were more active and more likely to circle the egg sac in the presence of wasps. However, White Widows were more likely to induce a “body shaking behavior” that created vibrations through the web, while Brown Widows were more likely to tap the egg sac with their first and second pair of legs as an inspection of the egg mass. But one of the main behavioral distinctions of Brown Widows was to capture the wasps and wrap them in silk in the webs. Unsurprisingly, White Widow egg sacs experienced a higher rate of parasitism at 64% compared to the Brown Widows, which only experienced 19% parasitism. 

For the second experiment, the researchers focused on the physical characteristics of the egg sacs. Brown Widow egg sacs can be easily distinguished as they feature silk spikes all over the egg sac, while the other widows feature a smooth egg sac. To determine whether this is a physical defense against parasitism, the researchers gave the egg sacs a haircut on one half of the egg sac. These freshly-buzzed egg sacs were then exposed to a wasp, and the number of eggs laid by the wasps was recorded. Wasps were more likely to lay eggs on the shorn side compared to the spiky side, meaning that these spikes are acting as a protective layer. 

The third and final experiment examined whether exposure to the parasitic wasps changed the way that Brown Widows laid their egg sacs. Females were allowed to lay one egg mass, and the number of spikes on that egg mass were counted. Females were then allowed to lay a second egg mass and then were exposed to wasps. This allowed females to potentially add more spikes to the egg mass if they chose to do so. The spikes on the second egg mass were counted. The females were then allowed to lay a third and final egg mass, and the spikes on this egg mass were also counted. The researchers then compared the number of spikes on all egg masses and found that Brown Widows increase the number of spikes on their egg masses following exposure to the wasps. Therefore, Brown Widow females will react accordingly to protect to their egg masses from wasp exposure.  

Understanding the way that a species interacts with its environment can provide context and clues for which management strategies are worth pursuing, particularly in the case of an invasive species like the Brown Widow. In this study, Brown Widow moms clearly show adaptive behaviors in response to a biological control threat that allows them to protect their offspring. Therefore, scientifically proving that mother does know best.