Every year, Neil Engelman carefully collects his data, stands before his company's board of directors and is asked the same question: What caused more outages? The lightning or the squirrels?

Four of the past five years, the answer has been the squirrels, says Engelman, vice president of operations for the Lincoln Electric System in Nebraska. Nebraska is not alone. Many states are grappling with a big increase in the number of power outages caused by squirrel electrocutions.

Squirrels that fry themselves on power lines and transformers cause tens of thousands of blackouts every year.

Some states have seen a massive jump in recent years in the number of such outages. In Georgia, squirrel-related outages more than tripled from 5,273 in 2005 to 16,750 in 2006.

While the outages are usually smaller than ones caused by weather, they are costly. Georgia Power officials estimate the rodents cost them $2 million last year. Stopping the squirrels is costing utilities millions more dollars.

"It's serious when it causes power outages to 50-60,000 people," said Cathy Engel, a spokeswoman for PECO, which provides electricity to the Philadelphia area.

It appears that the problem may in part be due to acorns.

Acorns from oak trees are a squirrel's main diet, says Peter Smallwood, a squirrel expert and biology professor at the University of Richmond in Virginia. When oaks produce more acorns, you get more squirrels - and more outages.

Smallwood, who has studied squirrels for more than 20 years, said their affinity for power lines and fighting through manmade barriers is in their nature.

"They're naturally curious, and they are also determined," Smallwood said.

Squirrels are not electrocuted when they run across power lines. It's when their body makes contact with both the wire and either the ground or a transformer that they become a conduit for electricity to flow through.

"That completes the circuit and bammo!" said Ed Bettinger of the Public Service Company of Oklahoma.

Among recent outages:

- A squirrel caused a power outage in October that shut down Merced College, southeast of San Francisco, for half the day.

- In January, a squirrel cut power to 4,500 customers in Amarillo, Tex.

- Hundreds of gallons of raw sewage poured into Mobile Bay in Alabama after a squirrel cut power to a sewage lift station there.

Stopping the squirrels is not easy.

"Those guys are awfully clever," said Tim Fox with Ameren, which provides electricity to St. Louis-area homes and businesses.

"When they want to get into something, they do," Fox said.

In Lincoln, dubbed a Tree City USA by the National Arbor Day Foundation, "squirrel guards" have been placed on all 19,391 transformers.

The guards vary. Some are plastic or silicone caps that protect the point where the power line and the transformer meet. The "Critter Guard" features a flat disk that spins around whenever a squirrel tries to climb past. Others deliver a minor shock to the squirrel to scare it off.

PECO, which powers Philadelphia and its surrounding counties, spends $1 million a year on squirrel guards to stop outages from "those rascally little varmints," Engel said.

The utilities say they're seeing some success. PECO has seen its squirrel-related outages tumble from 11,605 in 2003 to 1,345 in 2006.

But squirrels adapt to the technology, forcing the utilities to switch to different forms of what's known in the business as "wildlife abatement technology."

"Whenever we think we've got them figured out, they try something else," Engelman said.