Yes, you do need the double slits in order to witness the interference pattern which suggest that the particle is no longer a particle but a wave.

When a particle is propelled through a single slit it displays the characteristics of a particle on the display of impacts on the backstop. But should it not also retain that characteristic when the second slit is presented? There should be two vertical representations of impact on the backstop. Scale up to machine gun bullets firing at a single slit. The results is a single vertical record of impacts indicating that some of the rounds passed through the slit at various heights. If you fire the machine gun at a double slitted barrier you would not be surprised by a double vertical impact pattern on the backstop.

When you propel particles such as electrons through a single slit you get the same pattern as the bullets. When both slits are open shouldn't these particles also duplicate what the more massive bullets did? The results instead is a horizontal band of non impact areas interspaced by high impact areas. This points to a results that can only come from something acting in a wave form and not a solid particle. For the various bands were caused by the 'interference pattern' of waves. Peaks of waves from one slit meeting peaks from the other slit causing an increase in amplitude as well as when troughs meet troughs. But, when a peak meets a trough there is cancellation of the energy causing the dark bands on the backstop. Only a wave can cause this.

Imagine the experiment set up in water. Cause a steady and constant series of waves to be generated from a single point toward the double slit barrier. As the energy of the wave passes through each slit it begins to radiate outward on the backside of the barrier in a semi-circle. As the peaks and troughs make contact from each of the slits they do one of three things. They increase in height (amplitude) where a wave peak meets a wave peak. They increase in depth (amplitude again) when a trough meets a trough. And they cancel each other out when a peak meets a trough. The back stop would be impacted in several but discernible areas of peaks, troughs and calm.

Now here's the weird part. Fire a single photon or electron propelled once every ten seconds at a double slitted barrier. Our intuition suggest that there should be two vertical impact areas on the backstop. Instead, we get horizontal representation of an interference pattern. A photon that passes through the right slit should not care that there also happens to be a left slit, and vice versa. But somehow it does. The interference pattern generated requires an overlapping and an intermingling between something sensitive to both slits, even if the electrons are fired one by one. The explanation is resolved by assigning a probability wave to each electron. The electron's probability wave "sees" both slits and is subject to the same kind of interference from intermingling. Places where the probability wave is augmented by the intermingling are locations where the electron is likely to be found' places where the probability wave is diminished by the intermingling, are locations where the electron is unlikely or never to be found. Electrons hit the backstop one by one, distributed according to this probability profile, and thereby build up an interference pattern.

By examining the electrons to see which slit it passes through results in the disappearance of the interference pattern and the two vertical impact areas are revealed. To verify the experiment ruins the experiment.

Richard Feynman proclaimed that each electron that makes it through to the backstop actually goes through both slits. He argued that in traveling from the source to a given point on the backstop each individual electron actually traverses every possible trajectory simultaneously.