The farmer had a total of 10 rows of corn stalks * 9 corn cobs each = <<10*9=90>>90 corn cobs.
He also had a total of 5 rows of potatoes * 30 potatoes each = <<5*30=150>>150 potatoes.
This was a total of 90 corn cobs + 150 potatoes = <<90+150=240>>240 crops.
Because pests destroyed half of his crops, he now has 240 crops * 0.5 = <<240*0.5=120>>120 crops.
#### 120