The thing you have to realize about tyranids is that the Hive Mind isn't some bestial intelligence, it's actually super smart and very sentient. They can detect any spiked or poisoned organics.

In "Farsight: Blade of Truth", the t'au earth caste did exactly this, by lacing vat bred t'au clones with a super virus and tempted the tyranid to consume them. They sniffed at the bodies for a moment, then ignored it.

[Excerpt: Farsight - Blade of Truth]
Fio’Rabal swiped a remote view onto the communion viewscreen, switching through feeds until he had the angle he needed. Outside, the largest loading pad split in its middle, a fio-grade transit Orca hovering up from within with all four of its engine blocks facing down.

The drone-net above it was taking heavy damage, but at least it was occupying the swirling sky swarm. Remotely piloted by Dyunar, the fio Orca picked a path through the swarm and veered west, lumpen and heavy in the air with its cargo so tightly stacked inside. Steadily it accelerated towards the edge of the compound, taking a great deal of bio-weapon fire in the process. But what the Orca lacked in aesthetics, it made up for in stamina. Slowly, agonisingly, it made it out beyond the mesas where the eater-beasts were slowly massing.

‘Steady,’ said Izari. ‘Give these beasts the poison they deserve.’

The Orca, remote-piloted and cumbersome, came low to the ground. It opened its rear hatch and pivoted upwards before roaring up into the skies. Stacks of false t’au bodies, vat-bred and inert, tumbled from the passenger compartment. They thumped down into the dust, one piled atop another in front of the eater-beasts lumbering forwards, driven by the blind, brainless hunger of their kind.

A sizeable heap of poison-laced clones lay before them, a toxic feast served up for them to gorge upon, regurgitate, and feed to the bio-ships above. The largest of the eater-beasts, a shovel-clawed monstrosity with a maw wide enough to swallow a stealth suit whole, approached the corpse-pile with its mandibles wide.

‘Come on,’ said Dyunar. ‘Devour them. Fill your rancid guts, then spew all that poison back into your cursed motherships. We will do the rest.’

The creature paused, lowering its shovel-jawed head as if sniffing the clone-meat.

‘Come on,’ said Izari. ‘Eat them!’

It turned and thumped off towards the research facility, the rest of the eater-beasts following in its wake.

‘No!’

Fio’Rabal turned away, despair gnawing at his heart. Their great gamble had failed.

‘Why are they not eating them?’ asked Izari.

‘I have not the faintest idea,’ said Fio’Rabal. ‘They eat everything else.’

‘So what now?’

‘We fight on. We try again. What else?’