Illinois police say they have already harvested and destroyed almost twice as much wild-growing marijuana this year as they did in all of 1991.

A team of 15 state troopers and local officers from rural police departments had slashed, burned and buried more than 38 million ''ditch weed'' plants as of Monday, said Illinois State Police Sgt. Jim Hinkle, coordinator of a state eradication program called Operation Cash Crop.

Hinkle said in 1991 law enforcement officials harvested about 20 million naturally growing pot plants. In 1990, they destroyed just 2.8 million wild plants, which are remnants of a nationwide effort by farmers to grow hemp for rope during World War II.

State officials say the increased harvests are the result of an increase in federal grants for marijuana eradication and reflect a higher emphasis within state and federal goverment on eliminating the drug threat.

The state received $406,000 this year and $541,000 last year from the U.S. Drug Enforcement Administration for its marijuana program. Before that, the state received about $50,000 annually.

''It`s a big priority,'' said Franz Hirzy, a DEA spokesman in the Chicago field office. ''We`ve made some tremendous gains here, to the point that there is a shortage of quality marijuana in the Midwest.''

Most of the police harvesting has been in rural north central and northwestern Illinois, where the history of the naturally growing cannabis weed can be traced to pre-settlement days, when Native Americans also used it to make rope.

But since smoking dope became popular in the 1960s, local officials have complained about pot pickers and farmers infiltrating the region. In the continuing efforts to eradicate the nuisance, one county`s officials recently gave their sheriff the extra title of ''noxious weed superintendent.''

The wild weed found along roadways is very low-grade and would sell for a fraction of the up to $2,500 per pound that premium marijuana attracts on the street. But officials say the native plant is a public threat because dope dealers use it as filler, which they mix with the potent stuff to increase their profits.

Meanwhile, police have not had as successful a year eliminating illicit marijuana growing operations around the state. They blame that partially on this summer`s wet and cool climate that`s been favorable to the growers.

As of Monday, only 18,400 cultivated plants had been destroyed in Illinois, compared with a total of 347,700 in 1991, Hinkle said. But he said this year`s tally is bound to rise dramatically as the growing season ends and dealers move in to harvest.

''The weather has been so good that the guys-the bad guys-have not had to go out to tend their plants,'' Hinkle said. ''When they come out to tend them, boom, we`ve got them!''