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University of California Cooperative Extension Black
Widow
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There are five species of black widow spiders in the United States. The western black widow Latrodectus hesperus is the most common widow spider found in California. While rarely encountered in most table grape vineyards, recent experience in the field has shown a dramatic increase in the densities of black widow spiders reaching up to one black widow for every five vines sampled. Although grapes can be treated with CO2 and SO2 as post-harvest treatments, some spiders manage to survive. A number of black widow spiders have been found inside grape bunches on exported table grapes within the U.S. and abroad. The movement of ANY black widow spiders from the vineyard to the market place is an immediate concern to the table grape industry.
Black widow spiders are primarily nocturnal, non-aggressive, and avoid contact with most things they don't perceive as food. The black widow's importance is directly related to its reputation as an poisonous spider, however, bites are uncommon and serious long-term complications or death is rare. It is not the danger but the perception of danger that has created a near zero tolerance for black widows in grape bunches.
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General
Identification Females The adult female black widow is densely covered with microscopic hairs but appears smooth, shiny and black. On the underside of the spider's abdomen there is a characteristic red hour glass shape and a red spot just above the spinnerets. The female’s abdomen is relatively large, high, and globose and the legs are moderately long. The body of the full grown female is about half inch, with a spread of one and a half inches. Males Males are much smaller than the females, have relatively large legs, and their abdomen is narrower and lower. Males are not covered in black pigmentation like the females; instead they are brightly colored displaying yellow or red spots and white bands on their back side. Mature males often resemble a fifth instars female, but it can be distinguished by the knoblike appendages (Pedipalps) it carries in front of his head Immature The immature black widow look nothing like the adult spider and can be mistaken for beneficial spiders like the Theridion Immature black widows are smaller than the adults and do not have the black color typical of the adults. Immature black widows range in color from yellowish to a light brown with a pattern of white and brown bands and spots on the dorsal side. The hour glass mark is not red at this stage; it will appear white or be completely missing. As the immature molts, it gains more and more of the black pigment until reaches adulthood and acquires its typical black color. It is very difficult to distinguish male from female at this stage since they appear identical. Spiderlings The newly hatched spiderlings undergo their first molt inside the egg sack. They emerge from the egg sac about 30 days after oviposition. The newly hatched spiders are pale reddish brown, with light and dark stripes and two rows of dark spots on dorsal side of the abdomen. Egg Sacs The egg sac in Latrodectus hesperus is tan, pear-shaped, and it has a papery texture. Females are able to produce multiple eggs throughout their lives. There may be 4 to 9 egg sacs produced per season at a the typical climate in the central valley. The number of eggs deposited in an egg sac is variable, most often ranging from 100 to 400 eggs. Latrodectus species are cannibalistic therefore spiderlings disperse soon after hatching by ballooning, a process in which spiders raise their abdomen and spin a web until the air blows them off.
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We have developed sampling methods to assist us in our research. Vineyard managers may want a different type of sampling method faster and more efficient at covering a larger area. Below I provide some suggestions for black widow sampling, but first and most important is recognizing the black widow.
To sample for black widows you have to look at hundreds of vine per block. This can be done quickly if you look for the web rather than the spider. Start at the base because that's were most webs start. We count 200 vines per block each month. This is easier than it sounds. Simply walk about 10 vine rows per block and look for webs. Favorite hiding spots are in irrigation pipes, milk cartons, trellis systems, trellis stakes (especially those close to the trunk), cracks in the vine, and even on the ground in clumps of leaves (away from the trunk and usually on the berm).
Because the black widow is not aggressive and will hide, if a web is seen use a pencil or stick and pull back the web and look for the live spider. Often the web is empty. In our sampling, when the spider is found it is categorized by gender (male female) and developmental stage (mature, immature, spiderling, egg sac). The location in the vine is recorded as: canopy, cluster, arms, cordon, mid-trunk, base of trunk, or in the berm. For farm managers, a simple presence or absence bay be all that is needed.
Just before harvest, focus sampling efforts on the upper canopy. Typically, even when spiders are in the clusters there will be a web found near the trunk. In the clusters, black widows show up easily on green grapes but are more easily missed on darker grapes. For this reason, train the crew to look for spiders during the harvest process (photo).
For our harvest samples, we divided each field into seven blacks of equal number of rows. Within each block four rows were randomly selected for sampling. In each row, 10 vines were selected throughout the row and from each vine, five clusters were inspected for the presence of black widows. A total of 1,400 clusters were evaluated throughout the field. Again, the harvest crew will see far more but this earlier cluster check by the PCA will provide time to apply a pesticide.
One final note: Because black widows can have overlapping generations, the small brown colored immature spiders can also be found near harvest time, however, these smaller spiders typically do not web in the clusters. |
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Black widow spiders are known to prefer dark sheltered places to build their webs. In nature, females construct their webs in hallow logs, rodent holes, under loose bark, stones, and shrubs. In vineyards, spiders have been found under the bark, inside cartons, stakes, grape clusters, canopy, under debris in the berm, irrigation pipes, and rodent holes. Black widows mate and spin egg sacks mostly during the warmer months. For Latrodectus hesperus mating occurs in late spring and early summer with the exception of areas of mild climates in which mating pairs may also be seen in late summer and early fall. Mature spiders may be present throughout the year, but are most abundant in the late summer and fall. The complete cycle from egg to mature requires at least four months. Black widows are able to over-winter as immature and adult spiders. Males live an average of 120 days and females can have a longevity in excess of one year. Under favorable conditions, females are able to survive up to 3 years. Females can produce an average of 9 egg sacs throughout their lifespan . Males mature much faster than females molting only five times, while females can molt up to nine times.
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1. Evaluate chemical control tactics for the black widow.
2. Study the field biology and ecology of the black widow in table grape vineyards.
3 Determine life-history traits in the San Joaquin Valley.
Return to top Return to Biocontrol page
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