Natural Egg Sac Clutch Size of the Brown Widow Spider,Latrodectus geometricus(Araneae: Theridiidae) in Southern California

Abstract

The brown widow spider, Latrodectus geometricus C. L. Koch, 1841, is a non-native arachnid that became established in peninsular Florida in the 1930s (Pearson 1936). In the first decade of the 21 century, it rapidly spread through the southeastern United States and southern California (Brown et al. 2008; Vincent et al. 2008; Vetter et al. 2012a) and has become a ubiquitous urban pest species in the Los Angeles Basin and San Diego area (Vetter et al. 2012b). One possible reason for its success lies in its fecundity, as it can produce an egg sac every four days during its earliest ovipositions and averages 22 egg sacs in a lifetime (Bouillon and Lekie 1961). We were interested in documenting the seasonal field egg production of brown widows from June to October to determine whether there is a specific peak and subsequent dropoff in productivity or whether it is fairly consistent throughout the summer season. This provided basic biology information that benefitted research involving insecticide bioassays on brown widow egg sacs (Vetter et al, in prep); knowing the phenology of seasonal clutch size helped determine whether lab-produced egg sacs would be temporally comparable with natural oviposition and, hence, whether pesticides tested on egg sacs would have real world relevance. To determine monthly egg clutch size, brown widow egg sacs were collected during the day from several cities in Orange and Riverside counties, California from June through October 2012. Collections were restricted to urban sites such as around homes, in parks, under playground equipment, etc. as documented favored habitats for this spider (Vetter et al. 2012b). We attempted to collect egg sacs primarily from sites that were occupied by female brown widows as this presumedly increased the chances of collecting sacs with freshly laid eggs. The study was terminated at the end of October because temperatures cool considerably in November in southern California. We did not collect egg sacs for the first seven days of any month in order to more accurately assign sacs with eggs to the correct month of oviposition. At 29uC, brown widow eggs hatch in 9 days. Considering that daily high temperatures in many of our collection locales are typically 35 to 40uC, we felt that this was an acceptable estimate to separate clutch size into monthly cohorts. We also received some sacs in the mail from citizen scientists who participated in previous brown widow studies (Vetter et al. 2012a,b); again, no data from sacs collected and received during the first week of the month were used.