Although spiders can sometimes get a bad rap, when we are talking about beneficials and crop health, the eight-legged creatures actually can go a long way in protecting plants from hungry pests.
For this episode of the Pests & Predators podcast, brought to you by Field Heroes powered by the Western Grains Research Foundation, host Shaun Haney is joined by Dr. Carol Frost, assistant professor in the department of renewable resources at the University of Alberta, as they discuss the number of benefits that spiders can provide crops.
Spiders may have coined the term, power in numbers, as they are literally everywhere, all around the globe in varying species. An estimate that was published in the 1970s states there are, on average 131 spiders per square metre and this number increases to 152 spiders per square metre in grasslands. That’s a lot of spiders.
Even though each spider only eats about 10 per cent of its bodyweight. with over 50,000 different species of spiders around the globe and with the numbers per square metre listed above, collectively, they consume between 400 and 800 million tons of prey per year. To put that number into perspective, humans consume approximately 400 million tons of meat and fish per year and whales consume between 280-500 million tons of prey per year.
One of the large benefits of spiders is they aren’t picky eaters, says Frost. They are making up those hundreds of millions of tons per year through various food sources including flies, ants, bees, small soil critters, beetles, grasshoppers and even other spiders. This generalization of food preferences bodes well for producers as they are likely to eat whatever pest is in high concentration.
“The one thing that makes them great natural enemies is that if there’s a kind of insect in the environment that becomes really abundant, like a pest, spiders are great at switching and focusing on that species. So they’re really great at taking advantage of any insect because it’s really abundant in the environment,” says Frost.
The good news is, there’s a lot of spiders to go around and they’ll pretty much eat anything – the bad news – they don’t love calling crops home as there are number of disturbances, including the, from the spiders perspective, wreckage that happens every harvest.
Frost shares that the concentration of spiders in grasslands is almost 10 times higher than what we see in crops.
“A global estimate for the standing biomass of spiders in annual crops that get harvested every year is 17 milligrams per meter squared. If you compare that to the estimated biomass of spiders and grasslands, the estimate for crops was 17, the estimate for grasslands is 160 milligrams per meter squared,” explains Frost.
If farmers are able to make their crops a hospitable environment for spiders, Frost says they could see up to a 20 per cent reduction in herbivory.
“As of 2012, only, only three per cent of toxicology papers on natural enemies had actually even studied effect on spiders,” says Frost. “What we do know is that spiders are mainly affected by the neurotoxic insecticides, more than other pesticides like herbicides, or fungicides. And so most synthetic insecticides and herbicides, are neurotoxic. Those are the ones that affect spiders most.”