Savings and loan association customers withdrew nearly $9 billion more than they deposited in the first half of this year, and the associations' net worth also declined, but the losses were slightly smaller than for the previous six months.

The Federal Home Loan Bank Board released monthly figures yesterday showing a $2.9 billion drop in net worth since the end of December. It was slightly less than the $3.1 billion decrease in the latter half of last year but substantially more than the $1.7 billion reported a year ago at this time. In the past 18 months, the net worth of S&Ls has dropped from $33.3 billion to $25.4 billion.

Net worth equals assets minus liabilities and is often considered the equivalent of corporate net income. Net worth for June actually showed an increase of $32 million because of a change in accounting methods to include for the first time income capital certificates, the effects of "push" accounting in mergers, and some stock sales.

Without the change, savings and loans' net worth would have shown a further drop of $200 million to $300 million, according to Richard Pickering, deputy director of the bank board's office of economic research and statistics.

Customers continued to withdraw more money than they deposited, attracted by interest rates on other investments that are higher than savings institutions are permitted to pay. Net new deposits declined by $8.7 billion during the first half of 1982. Last year's drain amounted to $25.4 billion. For June, withdrawals exceeded deposits by $3.1 billion, compared with withdrawals of $5.8 billion for the same period a year ago. With interest credited, however, June deposit balances rose by $3.5 billion at the country's 3,500 S&Ls.

The 400 mutual savings banks were harder hit proportionately, according to figures issued yesterday. Their June outflows amounted to $1.2 billion. For the year to date, net new money has fallen by $5.8 billion.

Savings and loans closed $4.8 billion of mortgage loans in June. That was a $1.1 billion increase over May but still 18 percent below June 1981. Total mortgage loans made in the first half of this year were $21.6 billion, down 27 percent.

In related news, the bank board announced two more supervisory mergers, bringing the total for the year to 19. Two New Jersey S&Ls were merged at an estimated cost of $10.5 million to the Federal Savings and Loan Insurance Corp., the government agency that insures savings and loans. Two others in Puerto Rico were merged at a cost of $7 million.