In The Case of the Poisoned Crops, General de Groot (or so he styles himself) was an engineer who ran the experimental station at Klookerstein in North Central Africa developing methods for dealing with locust swarms. De Groot had apparently developed a power driven spray which he expected would be successful and earn him a lot of money.
De Groot had stolen four drums of a powerful insecticide called "Vegicide" and had been using it to destroy crops, probably with a view to inciting a revolt by starving the farmers. When one of his pilots, Felix Harley, stumbled onto the plot and attempted to escape in one of de Groot's two aircraft, de Groot or de Groot's men had him shot. Harley died but not before sending out a distress signal asking for someone to look in at Klookerstein.
De Groot gave Biggles an unfriendly reception when he visited Klookerstein to investigate the crop poisonings and also the disappearance of Felix Harley. De Groot did not volunteer much information but Ginger managed to spot the stolen drums of "Vegicide". They had been overpainted with black paint but their original red colour showed up through scratches. Biggles, finding the atmosphere becoming ever more tense, decided to leave.
Later, while Biggles and co. were on the ground not far from Klookerstein where they had found the body of Felix Harley and the aircraft he had escape in, they spotted what must have been de Groot flying towards them in his De Havilland Puss Moth and starting to release an oily black spray, the poisonous Vegicide, probably in an attempt to kill them. Biggles and Ginger took off in time to avoid the spray. Algy was on a spot but he had a loaded verey pistol which had just used to signal to Biggles. He fired the flare to make de Groot swerve but the flare ignited the stream of insecticide, causing the aircraft to catch fire and crash.
Whether de Groot was in the Puss Moth or not was not mentioned, but as the gang's last remaining transport had been destroyed, it may be presumed that the rest of the gang at Klookerstein was easily rounded up, including de Groot if he was not in the plane. At any rate, the crop poisonings were stopped and justice meted out for the murder of Harley.