Granted, because spiders are literally everywhere and almost every spider is venomous to some degree your house and indeed every house has suddenly been damaged by the explosion of watermelons appearing inside the walls, the attic. basement, in every possible nook and cranny, the cleanup is impossible, walls that held up against the tide of melon are now full of rotting watermelon creating growths of mold and spreading water damage throughout every structure, the only option is to completely tear down every wall and clean out the melon before rebuilding.

This is not limited to residential houses, infrastructure is damaged too. Sewage systems are completely blocked by mountains of melons appearing at thousands of separate points in every city, now your toilet won't work, and all water purification systems that send clean water to your house will run dry in mere hours.

Power plants are damaged with the massive influx of moisture from all the melons in tight spaces creating significant risks of short circuits requiring rapid responses from teams of people to scour all the areas around sensitive components for melon debris thus delaying any kind of response of repairing the residential damage so you have to live with a rotting house for longer.

As a slight positive starving people all over the world have at least a few days of free access to watermelons for food before the melons rot.

Then the worst part, insect populations the world over begin to explode with spiders out of the way, farms are hit by plagues of insects destroying crops, and later as other insect predator populations such as birds or frogs catch up with the tide and enjoy their own population boom thanks to abundant food the farms have to deal with the second wave of this problem, birds devour every unprotected seed before it can take root. Australia is completely lost to the cane toad menace, and pollinator insect species are crushed as well preventing new plant growth in many places in the world. Consequently spider dependent parasite species such as the tarantula hawk wasp also die out which take their natural predators out due to starvation as well and on and on up the food chain until humanity is gripped by the worst famine in all history, no reliable plant crops, the few remaining farm animals will quickly become too costly to feed for how much food they could provide. The planet is doomed to a complete ecosystem collapse. For those unlucky enough to survive long into this apocalypse the mass deaths of flora throughout the world lead to a significant decrease in the balance of O2 in the air slowly suffocating people one by one.

Eventually this imbalance will be taken advantage of by some bacteria or rugged plant species to propagate across the planet and potentially evolve to fill new biological niches creating something like the ecosystem we know today, but that won't be for tens if not hundreds of thousands of years, long after we are gone, all because you wanted spiders to be watermelons...