An LD50 is a ratio of  in milligrams (mg) of pesticide per kilogram (kg) of body weight that kills 50 percent of test animals - mice, bees, frogs, whatever. So it literally is Lethal Dose of 50 percent of the critter.

Because it is a ratio, that means low is bad. 

Vitamin D is all the latest craze among supplement salespeople, for example, supposedly curing everything from autism to AIDS. It has an LD50 of 10 mg/kg. Table salt has an LD50 of 3,000 so if a James Bond or Mission Impossible villain ever gets you the option of choosing which chemical to drink, take the salt rather than the vitamin.

But you get the point. The difference between nutrient and poison is just dose. If someone claims their organic pesticide, like copper sulfate, is non-toxic, they are not just stupid, they are lying. There is no such thing as a non-toxic pesticide, the feel-good fallacy label placed on it, whether it is older or more natural, is irrelevant. The most dangerous substance in the world, Botulinum, could kill everyone with 2 ounces, but in tiny amounts wealth people get Botox treatments. 

The dose makes the poison and when it comes to organic pesticides, the doses need to be far higher than with modern science. That is bad for everyone.