One reason that the Axis convoys to North Africa were safer was that the British had lost three submarines. Two U-class submarines, the famous Upholder and the Urge were both lost in April 1942. Lt-Cdr Wanklyn's luck had run out when the Upholder was sunk by an Italian torpedo boat on 14 April. The Urge was lost without a trace after sailing from Malta on 27 April. The Official History suggests that she was mined. The third submarine, the Olympus, was mined a short distance from Malta on 8 May.
The Germans lost three submarines during May. A Lockheed Hudson from No.233 Squadron attacked U.573 and forced her into internment in Spain. A Consolidated Catalina from No.202 Squadron and the destroyers Wishart and Wrestler sank U.74 a day later. A Short Sunderland from the same squadron damaged a submarine late in May. U.568 was sunk by the destroyers Eridge, Hero, and Hurworth northeast of Tobruk. The air connection, in this case, was that a Blenheim from No.203 Squadron had sighted the submarine and alerted the destroyers.
This is based on the account in Vol.III of the Official History.