A 20-year-old Alexandria man scheduled to go on trial today on a murder charge in the fatal shooting of a Federal Trade Commission economist pleaded guilty to seven other offenses yesterday in Alexandria Circuit Court.

Boggess Steven Davis, of 421 N. Payne St., pleaded guilty to malicious wounding, two counts of robbery, burglary while armed and three weapons charges in the Jan. 2 incident. Economist Steven Marston was slain and Marston's wife, Martha F. Riche, was wounded in the incident at their Parkfairfax town house.

Steven Rideout, an attorney for Davis, said his client made the last-minute pleas because "he is guilty" and because the pleas may help establish his credibility at his murder trial. If convicted of capital murder, Davis could be sentenced to death.

"To be believable to a jury , he had to admit to his involvement," Rideout said, adding: "There was no prearrangement" with the prosecutor's office. "There was no plea bargaining."

Two other defendants, Bartel E. Davis (Boggess Davis' brother) and Jeffrey D. Harris, both 17, are scheduled to stand trial as adults in circuit court next month on murder and other charges in the Marston slaying.

Riche, who is expected to be a key prosecution witness at Boggess Davis's trial, testified at a preliminary hearing that he shot her and her husband.

The couple, making preparations to move to a house in Arlington, was shot at about 9 p.m. on Jan. 2 when three persons entered their house at 3603 Greenway Pl. through an open door, according to police.