The DUP has claimed another Ulster Unionist scalp after a senior local councillor dramatically defected.
Former UUP Limavady councillor Aaron Callan is likely to be cheered to the rafters at this weekend's DUP conference.
And there was speculation last night that a further UUP defection is imminent.
Mr Callan's former UUP colleagues dismissed his departure, in a terse statement, saying he was "unlikely to be missed".
The defection came just a month after DUP leader Arlene Foster predicted new UUP recruits in a speech to young unionists.
At the time Mr Nesbitt rejected the speculation and made a further appeal last weekend to "occasional whispers and malcontents" within his party ranks.
Speaking at his own annual conference, Mr Nesbitt said: "To anyone looking to press the panic button; to the very few who have jumped ship; to the occasional whisperer and malcontent, I say this: are you so weak, you want to unravel four years work, just because we didn't get all we wanted first time around?
"Are you so easily led that a siren voice in your ear is enough to make you change course?"
Mr Callan yesterday set out the reasons for his departure, saying he had been "uncomfortable" with the direction and leadership of the UUP for some time.
The Causeway Coast and Glens councillor said he had been "greatly disheartened" because it appeared east Londonderry was becoming increasingly irrelevant to the UUP Assembly group.
"I am firmly of the view that the DUP is taking Northern Ireland forward in a positive way and under the leadership of Arlene Foster, is providing a confident vision for Northern Ireland underpinned by a plan that has widespread support," he went on.
Welcoming the new member, Mrs Foster also predicted he will not be the last to defect.
"He is a talented and enthusiastic public representative who cares deeply about his local area," the First Minister said.
"Aaron's move is yet another indication that the DUP is now the only unionist party that is capable of commanding support across the province.
"Many from an Ulster Unionist background are uniting around the DUP and have already joined with us since the Assembly election and I know Aaron will not be the last."
In a statement issued before the resignation had been confirmed, Mr Nesbitt's party said: "We have built our success in recent years on teamwork. On that basis Councillor Callan is unlikely to be missed."
Mr Callan is the first defector to emerge since North Down and Ards councillor Daniel Allen, who quit the UUP in a Assembly selection tussle and then announced he was joining the DUP.
But there has also been travel in the other direction with former DUP MLA Maurice Devenney signing up to Mr Nesbitt's party group on Derry City and Strabane District Council.
At the time he was the most high-profile 'defection' from the DUP to the UUP since Lisburn councillor Jenny Palmer, whose disclosures to a Stormont committee brought a DUP disciplinary threat for bringing the party into disrepute.
Mr Callan was among 33 UUP candidates lauded by Mr Nesbitt four months ago as evidence that his party was reinvigorated.
He was elected as a UUP councillor in Limavady two years ago after polling 452 first preference votes.