The bug catching contest is held on Tuesday's, Thursday's and Saturday's in the national park, which is a small area just above of goldenrod city. ( bear in mind, this is based on my g/s knowledge ) in it, you catch Pokemon for twenty minutes and the winner is determined by who has the strongest Pokemon. In gen 2 the top prize is a sun stone, which you can use on a sunkern you can find there or any Pokemon of your choice. The other prizes are assortments of berries. The Pokemon you find are Pokemon like pinsir, scyther, caterpie/weedle, kakuna/metapod and beedrill/butterfree ( not actually sure about the butterfree part ) and maybe sunkern ( check serebii ). The bug catching contest is only in gold/silver/crystal and heartgold/soulsilver. It should be noted that the prizes differ in gen 4 than gen 2.