One military has developed insects with strong jaws capable of chewing through the weaker parts of CBRN (Chemical, biological, radiological and nuclear defense) gear and then injecting neurotoxins into their enemies' bodies. These insects would be deployed through cluster munitions, IED (improvised explosive device) like devices left behind while retreating, and airburst artillery (assume modern technology on both sides) and are resistant but not completely immune to insecticides. How would you prevent these from decimating your forces?
These sort of cat and mouse games are hard to speak to on this kind of site. The answer is always the same "the military will identify a weakness in the attacker's attack, and exploit it."
I would expect the MOPP suits worn in CBRN strikes to very very rapidly be improved to support use in CBRNI strikes (Chemical, Biologial, Radiological, Nuclear, and Insect). The jaws of the insects chosen will be analyzed at a very rapid pace and new suits which are designed to not be chewed through will be distributed.
While these new suits are being developed, I would expect soldiers to knock the insects off of the suits, and crush them. MOPP suits aren't that easy to chew through, so it will take a large insect a reasonable amount of time to get through and deliver a killing blow. Insects are not a match for a capable soldier. Soldiers will kill the insects en masse. A 2000 pound airburst round may deliver a lot of insects on the scene, but you may be astonished at how enthusiastic a team of Marines can get when squishing bugs that threaten their lives.
I would expect a lot of research will go into understanding why these insects attack the soldiers. A soldier in protective gear is not typical prey for an insect. Most insects avoid attacking mammals two orders of magnitude larger than them. My first instinct is that the humans who engineered these insects developed these insects to try to lay eggs in humans, akin to the blowfly. Mating and propagation of the next generation is strong enough to cause these insects to get close to animals they otherwise would avoid. I would expect great interest in developing a lure which mimics whatever these insects seek, permitting squishing of them in great quantities with less effort and risk.
You're going to need a bigger bug.
A lot of genetic engineering went into these battle bugs. Pesticide resistance alone is no mean feat, plus they have fast knockdown neurotoxins plus they chew thru suits and attack.
I am taking away that you think readers / players in your world will dig superbugs. So have more! Your anti bug bug is not pesticide resistant , no neurotoxins, just bad of ass with a hatred for those dang neurotoxin bugs and a desire to bite them in half. Which is what this Japanese Giant Hornet is going to do to this bee.
Your marines would release their anti bug bugs when they came under attack. By those bad bugs, I should be clear. Antibug bugs stay inside for nonbug attacks. But if there bad bugs come, each antibug bug will fly out and rapidly and summarily deal with dozens or even hundreds of those chewy poison bugs. Maybe each marine would carry a few in a pouch, and on release the anti bug bugs will crawl and buzz around their soldier, taking out any GMO bug with the temerity to try something.
Then the anti bug bugs will return and the marines will sing the antibug-bug song and feed them sweets and ground meat and puff marijuana smoke into the hive.
I think the easiest method to counter this would be to keep your infantry 'buttoned up' in armored personnel carries and IFVs until the area was definitely cleared. Vehicles like the Bradley are designed to resist NBC attacks and I imagine the insects would have a hard time actually working they way in to the vehicles, if they even take notice of the vehicles at all.
Having your troops only dismount when they've engaged the enemy would make it a lot harder for the enemy to make full use of their bioweapons as once they are within a few hundred metres, the enemy isn't going to want to call in bug artillery on their position. Even if they beat back the assault, they now have a ton of bugs that are liable to attack them (assuming the bugs can't tell between friend and foe).
Incendiaries like flamethrowers and white phosphorous would probably also be rushed back in to service for infantry units in order to counter any bugs they come across. I think the biggest threat these bugs would pose would probably not be in IEDs but artillery and air support on troop concentrations, especially against non-combat units in the rear. The main way to counter this would just be to massively boost your counter battery and air defence capabilities.
Fire, Its The Only Way To Be Sure
Many creatures are naturally afraid of fire, perhaps due to the light, the heat, the noise of a combination of all three. Igniting the insects would not only kill them but it may cause a rout, sending the insects back towards whoever sent them against you. This could cause the insects to instead attack their handlers “the enemy of my enemy is my friend”. Flaming pigs have historically been used by the Romans to cause elephants to rout so we could assume a similar principle will apply here (though with giant insects and flamethrowers).
Fire could also be used defensively, even if the insects are unafraid of it. If you are being charged at by the insects, igniting the first few rows will ensure the destruction of the entire army. The first few rows will be directly killed by the flamethrowers, their ignited corpses would litter the battlefield. The insects behind them would have to climb over these flaming bodies and, in turn, be killed by the fire. This would lead to a mountain of charred insect corpses with little effort on the part of the defenders.
Gallons of Pesticide
In a similar vein, you could spray the insects with toxic chemicals, causing their death. Your own troops would be safe inside their CBRN suits. Whilst actual pesticide may be a ridiculous suggestion depending on their size, you could instead use a plane to mist them with chlorine gas or hydrochloric acid.
Capture the Young or Queen
I recently rewatched Nausicaä, Valley of the Wind so i can not help but to reference it in this answer. Towards the end of the movie a young Ohmu (a large cattle-sized pillbug-like insect) is captured and is being used as a weapon. Whilst this young creature has no offensive capabilities, it is instead calling to its herd. The adult Ohmu are the size of 3 or 4 storey buildings and are easily capable of levelling a city when enraged. The young Ohmu is being used as bait by a military, attracting the adults to a city to destroy it.
Your military could employ a similar thing, capturing the queen or the young and drawing the insect army away from your troops and towards your enemy.
If It Bleeds, We Can Kill It
A simple yet effective way is to just fire bullets into them until they stop moving. The Starship Troopers movies are a good example to use here, the marines are armed with “pulse rifles” (essentially just sci-fi machine guns) which they use to kill the Bugs (6-8ft creatures with huge manibles capable of slicing a man in two). In the movies you see creative uses of grenades, plasma walls, Zerg tactics from the insects, it seems to be what you’re after. However, i must warn you these movies are somewhat gory, not for the feint of heart nor weak of stomach.
Use this ecological menace against their creator
Use armored vehicles to prevent immediate exposure.
Determine the chemical that attracts the insects.
Use armor and air to lay down a trail of attractant leading the insects back into the home territory of their creators, thereby rendering much of their CBRN defenses ineffective.
Then gas the creators mercilessly. Karma.
modify the suits or if they can control the meeting place have the grass and plant matter that's genetically modified can do it for them.
If its the suits they could be self healing or maybe have a layer inside that will fill up with light weight liquid that will harden quickly to help protect the wearer longer while they either spray the air or else the layer has the counter agent inside that once it makes contact with any digestive substances it explodes or sends a shock to it so the bugs might get a 10,000 volt jolt to their tiny frames that would kill them.
If it's plants I heard they want to make corn that will change color to signify poisons in the air or tell earthquakes are incoming so the idea is similar enough the plants are gmoed ether via spraying the war zones or via making and panting the items in hostile zones they look just a tiny bit different then the actual item it is based off of your solider would be trained to spot this but its largest tale beyond the subtle non activated from would be color change or illumination or harmonics that link to the suit's inner comms that squawk only to those with the programs to tell them what it sees, hears, or knows about the area like explosive detected or traffic came in stopped for 5 minuets then left.
Another method would be to have animals or pet robots to try and suss out such particulars. If these bugs are alive and moving inside the small mine then the computer could sense the tiny tremors located in one spot but it could also be somewhat faulty if you are in a country with regular tremors it could give false positives for a wide area that's for if you wanted to not make it too easy for your group to move through an area.
Essentially, what will happen is that the suits will be adapted VERY quickly to deal with this threat. These suits are already heavy duty, so why not make them more so. There is a variety of solutions to this actually, feel free to choose.
Make it stronger. Basically, toughen up the materials with whatever science/ hand waviness you wish. For example you could put a mesh of wire in that fouls the bug's jaws, or use an entirely different material.
Kill the damn thing. For this one, you can incorporate it in many ways as well. You could put electricity (props to Mio) etc. Alternatively, you already have the suit, why not use that to your advantage. You said not completely immune to insecticide, so I am assuming that you mean you need something pretty heavy to kill them. So just put that in the actual CRBN that you are being protected against.
- Counter it. Props to Willk for this one. The anti bug bugs are a great and funny idea that would be good to incorporate into your story.
- Eliminate it. Final idea, spread a disease, blight, infertility ray that is used against these to wipe them out entirely.
Essentially, any of these alone or in combination will work, pick and choose as you wish. Good luck!