Pr. William, Loudoun Gypsy Moth Plans Staff members of Prince William and Loudoun counties recently presented to their boards of supervisors plans they hope will fight the spread of gypsy moths this year.

Gypsy moth larvae feed on the leaves of several types of trees, particularly white oak. Unchecked infestations that last for a period of more than three years kills trees, the staff members said.

Prince William County gypsy moth coordinator Kim Largen said a combination of chemical spray and parasite release will be used in the mostly rural western end of the county near the Blue Ridge Mountains where infestation is heaviest. The chemical that will be used is Dimilin, a growth inhibitor. In addition, 50,000 wasps whose natural prey is gypsy moth larvae will be released. The county and state will equally share the cost of the nearly $20,000 project, Largen said. Residents who wish to do their own spraying are advised to call Largen at 335-6285.

The Loudoun County campaign against the pests will be a three-pronged approach, according to coordinator Terrie Laycock. In addition to the spray and the parasite wasp, a biological virus called "Bt" will be used on infested areas near water. About 100,000 acres will be treated in Loudoun, most of them in the rural western end of the county. The state will also pick up half of Loudoun's $80,000 tab for its project. Spraying is expected to begin after trees have 30 percent of their foilage, Laycock said. 115 Apply to Be Manassas City Manager

When Manassas officials counted applications last week for the job of city manager, there were 115, according to Vice Mayor Stewart Vetter. The Virginia Beach consulting company of Municipal Advisers Inc. will screen the applicants and present the City Council with the top 10 candidates for the post.

In other business, bid proposals for a consultant on a proposed regional incinerator will be sent out this month, Vetter said. Four jurisdictions, Loudoun and Prince William counties, Manassas and Manassas Park, have agreed that such an incinerator will be a necessity when the landfills in the two counties are filled. All four jurisdictions will share the consultant's cost, as well as the cost for the incinerator -- if it is built. According to spokesmen for the counties, the Loudoun County landfill is expected to be full in six years, Prince William's in 10. A site for the incinerator has not been chosen, Vetter said. Areas being considered are in the Dulles district in Loudoun, Manassas or an undetermined area in Prince William. The land may be furnished by an investor, Vetter said. Pr. William Symphony Raffle Slated

The Prince William County Symphony Orchestra Guild will hold its second annual fund-raising car raffle April 26 at Potomac High School during intermission of that evening's concert performance. According to guild spokeswoman Rebecca Randolph, 125 tickets at $100 apiece will be sold for a chance to win a 1986 Honda Civic with AM-FM radio and air conditioning. The winner may choose $5,000 instead, Randolph said. Second through fifth prizes are $100 each. The raffle raised $4,400 last year for the symphony's general fund, Randolph said. For information, call 670-8086, 670-2325 or 361-4614, or send a check to the Prince William County Symphony Orchestra, P.O. Box 2153, Manassas, Va. 22110. Home address and work and home phone numbers should be included. Scholarship Finalists Named in Loudoun

Five Loudoun County High School students have been chosen as National Merit scholarship finalists. They are Eric Whitaker, Loudoun Valley High School; Ray Paul Lebeau, Broad Run High School; Andrew Lewis, Thomas Jefferson Regional High School in Fairfax; Claudia Radel, Loudoun County High School and Steve Sincavage, Park View High School. In addition, Broad Run senior Karen Saunders is a finalist for the 1986 Achievement Scholarship Award for Outstanding Negro Students. Horse Races in Upperville, Middleburg

Point-to-point races will be held in Upperville and Middleburg the next two weekends, according to Loudoun tourism chief Hugh Harmon. The Piedmont point-to-point post time in Upperville on March 22 is 1:30 p.m. Admission is $10. For information, call 592-3304. The Glenwood Park point-to-point in Middleburg on March 30 will begin at 10 a.m. There will be 10 races, including the Middleburg Bowl Open Timber. Admission is $4. For information, call 777-0519. Interim Library Requested for Purcellville

Kamstra, Dickerson and Associates, architects for the Purcellville Library addition, have recommended that library materials be moved out of the building for one year while construction is going on, according to town manager William Dennis. The new wing of the library is the first phase of a $6.5 million building program approved by voters in November 1984.

On March 26, the Town Council will hold a workshop to discuss several bids it has received for cable television service. Dennis said the council wants to hire a consultant to review the bids; cost for the consultant is estimated at $3,000 to $4,000.