In the book "Life on a Little Known Planet: A Biologist's View of Insects and Their World" [1] the author says that he used to set out insect traps every summer at his home in--if I remember correctly--New England, and every summer he would find species that were unknown to him. He'd then check the literature to identify them and every summer would find that he'd caught some that were unknown to science.
The author's research was on parasitic wasps and he was one of the world's foremost authorities on the subject, yet his summer traps would even trap parasitic wasps that were not yet known to science.
Think about that--this was not in some remote area that humans had barely reached where finding unknown plants and animals is something you'd expect. This was at a professor's house in a part of the planet that has been extensively explored for centuries.
[1] https://www.amazon.com/Life-Little-Known-Planet-Biologists/d...
There are two major categories of ways to kill an insect.
1. Attack it with chemicals that interfere with its biologically processes in such a way that it dies.
2. Physically attack its body causing sufficient damage to kill it, such as by crushing it, or shredding it.
(I'll include things like burning it with fire or acid as physical attacks rather than chemical attacks because they kill by destroying the body. I'll count drowning as a physical attack, too).
Within each of these categories there are simple methods that are effective and cheap, but are also not very discriminatory. In the chemical category, that would be pesticides that attack some widespread biological process that is shared by many insects, or in many cases all animals or even all cellular life.
To use this kind of pesticide safely you have to find ways to confine its application to things you are willing to kill, or you have to rely on it taking a lot less of it to kill the "bad" insects than it takes to harm a human and so with some care you can kill the bugs and not make too many people sick.
There are also within each of those categories ways to kill that will only affect one species.
In the chemical category, the key is hormones. An insect is like a little robot that has several built-in behaviors that normally play out in a specific sequence. The timing of when these behaviors happen is controlled by hormones. They are essentially the clock that drives the program.
So let's say you've got some insect that is bothersome. It hatches in the spring, is a pain in the ass all summer, and lays eggs and dies in the fall. If you study it you will find something like the cooling weather in fall triggers a hormone change, and that hormone invokes the "lay eggs and die" behavior.
Congratulations! You now have a way to make a safe pesticide for that insect! You just need to make a pesticide based on that hormone, and apply it sometime between spring and fall before the insect has matured enough to have viable eggs. The hormone will still trigger "lay eggs and die", but the eggs won't be viable. So not only have you killed the insects that summer, you've also decimated the next generation.
The beauty of a hormone approach is that these hormones are already out there in the ecosystem. Every time some predator eats one of the target insects in the fall, that predator is eating the hormone. The hormone is already spread up the food chain, and so things are already adapted to an environment where they are exposed to it. The extra exposure when we start using it in a pesticide will cause little or no harm to other species.
So why don't we do this more? I'll cover that below, after first talking about the physical category of killing insects.
In the physical category, the key is other insects. For a large variety of insects we consider pests, there exists some other insect that either preys upon it or is a fatal parasite to it or that it depends on in some way.
If you know enough about your target insect, you can often bring in one of its predators or parasites, or attack something it depends on, and indirectly control the target that way. This can be especially good when the target is an invasive species.
In many cases insect predators are very specific in their prey, only preying on one specific species. If you pick such a predator to import to attack your target, that is safe because the predator cannot turn to other insects after wiping out the target. Once the target is gone, the predator dies.
So why don't we do this more?
Same reason we don't do the hormone-based pesticides more. Both of these approaches require a much deeper understanding of the insect and its environment than the broad poison approach does.
A lot of this is covered in the excellent book "Life on a Little Known Planet" by Howard Ensign Evans [1]. In the book he mentions an incident where California was suffering large citrus crop losses from some invasive insect. In Florida that insect was kept under control by a particular predator insect. California officials imported that predator--and nothing happened. It turned out that there was another species related to the predator, but it preyed on something other than the species that was invading California. These two predator species were similar enough that no one knew there were two of them. Even though this predator species played a major role in keeping a major agricultural pest at bay, there was only one or two scientists who actually studied them, and they weren't very well funded. There had simply not been enough research put in to notice that they were dealing with two species.
[1] https://www.amazon.com/Life-Little-Known-Planet-Biologists/d...
There are three effective ways to control insects.
1. Poison them with something that is dangerous to a wide variety of life.
2. Poison them with something that is narrowly targeted to the species you are trying to control and is harmless to other species.
3. Deal with them mechanically, such as by crushing them or tearing them apart.
DDT and similar pesticides take the #1 approach. To do that safely you have to carefully control the spread of your pesticide so that it doesn't spread to species you do not want to harm. You can do this by being very careful in how it is applied, and designing it to break down quickly and not spread beyond the application area. We failed in this aspect with DDT.
The #2 approach is quite feasible. The way it works is that you base your pesticide on hormones of your target species. Insects are essentially biological robots running conceptually simple and very inflexible state machines. Various hormones control this, triggering various programs that run on the state machine. For instance, there might be a particular hormone that activates in the males when the weather changes, triggering in males a flight to find a female, and after that program runs the male dies. A different hormone might trigger female mating behavior.
Entomologists can study that species, and identify the various behavior programs built into the insect state machine, and figure out what hormones active which programs. Then a pesticide can be designed that mimics the hormone that starts the male mating subroutine. Spray that on them when it is not the right time of the year, so the females are not having their mating program trigger, and the males will go do their (now futile) mating flight and die.
The best part about this is that it is safe for other species, because these program control hormones for one species are almost always harmless to other species up the food chain. (To be up the food chain from something, you kind of need to have evolved to not be bothered by its hormones).
Another nice thing about the #2 approach is that the pests do not develop immunity. If an insect develops immunity to, say, the hormone that triggers its mating flight, then it is not going to mate when that trigger fires naturally, and so whatever mutation provided that immunity dies off.
The #3 approach is also quite feasible...and no, I don't mean importing cheap migrant workers to go out with tweezers in the fields and crush insects. :-) I'm talking about finding an insect the preys upon or is a parasite to the insect you want to control, and using it. Entomologists can study a pest species and identify what its enemies and parasites are (and there usually are some), and then you can introduce those to control the pest species. Unlike higher forms of like, like reptiles and birds and mammals, insects tend to be very picky about what they prey upon or what they are parasites to. If you've got a particular species of invasive insect from Florida destroying your California citrus crop, and you import the species of parasitic wasp that kills the pest, when it kills them it does not turn to some other species for hosting services. It dies.
The big problem with #2 and #3 is that there are a lot of insect species, and a lot of predator and parasite species, and not a lot of entomologists. This is discussed in the excellent book "Life on a Little Known Planet" by Howard Ensign Evans [1]. He was one of the worlds leading experts on parasitic wasps. There are hundreds or thousands of species of them, most very small (the size of pinhead or smaller). In the book, he tells the story of that pest destroying the California crop. Officials did try to import the parasitic wasp from Florida that kept the pest in control there.
It failed. Eventually, after the crop was destroyed, they found out why. It turns out that what was thought to be one species of parasitic wasp was actually two, which are almost identical, except one of them was NOT a parasite for the particular species they were trying to control. With hundreds of species of parasitic wasps and very few parasitic wasp experts in the world, no one had looked at this one species close enough to have noticed that there were really two species.
The kind of observation and experimentation is takes to learn enough about the insect state machines to make the hormone mimicking pesticides, or to learn enough about an insect's predators or parasites to effectively and safely control the pest biologically, takes a long time. Industry is not interested in funding that. They want entomologists to work on type #1 poisons, not to spend 20 years in the lab studying one species to identify a key hormone.
Academic entomologists who want to pursue these studies aren't pulling in the big grants, either. Evans tells of having to advise a graduate student to NOT specialize in parasitic wasps, despite the student's interest in and talent at it, because he would not be able to get funding for research in that area.
[1] http://www.amazon.com/Life-Little-Known-Planet-Biologists/dp...
He'd put out traps to catch flying insects at his home over the summer in, if I recall correctly, New England, and often find ones that he didn't recognize. He'd even find parasitic wasps he didn't recognize. Upon further checking he'd find that many of these were unknown to science.
I'd always thought that if you wanted to find insects unknown to science you'd have to go to isolated areas that had not been well explored.
There was a good illustration in the book of how little we know. There was some invasive insect that was causing a lot of damage to California orange crops [2]. The invasive insect was native to Florida. In Florida its population was kept under control by a species of parasitic wasps that laid their eggs in its larvae which became wasp food before they could mature and start attacking the orange crop.
A lot of money was spent to import those wasps into California and release them. This kind of biological pest control is attractive because parasites are often very specific about what species they will use as a host, which was the case with these wasps, so this should be safe as far as species other than the invasive one are concerned. The parasites aren't going to switch to some native host.
They were correct in that the parasites didn't go after anything else, so caused no harm. But they also didn't go after the invasive insect. They just didn't lay eggs and died off.
Eventually someone figured out why it hadn't worked. It turned out that what we had thought was a species of invasive insect turned out to be two very closely related species, call them A and B. The parasitic wasp species that laid eggs in their larvae also turnout out to be two species. One of the wasp species laid eggs in A larvae, and only A larvae, and the in B larvae, and only B larvae.
All the invaders in California were from A, and all the wasps they trapped in Florida and sent to California were from the species that lays eggs in B.
[1] https://www.amazon.com/Life-Little-Known-Planet-Biologists/d...
[2] I read the book a long time ago. I may be misremembering the specific crop, and the specific types of insects involved.