**4. Conclusions**

DM, first synthesized in 1973, was the most potent synthetic insecticide ever at the time and was heralded for its high selectivity to insects compared with mammals, a ratio of 13,000 [7]. Upon stereoselective synthesis, the solution contains the (*RRR*) and (*RRS*) stereoisomers (Figure 2). The latter crystallizes more readily, leaving the more soluble (*RRR*) isomer in solution, which can be epimerized at the cyano-bearing carbon atom to produce more *(RRS*). This is fortuitous in that (*RRS*) is the most active insecticide. Stereoisomerism is an essential feature of biological specificity.

More commonly, insecticides are supplied as mixtures of stereoisomers, which can greatly increase the complexity of crystallographic characterizations and give rise to variable crystallization outcomes (See Table S1 for crystallization conditions). The activity of crystalline contact insecticide is dictated foremost by the rate of absorption at the interface between the crystal and the target organism. Yet little attention has been paid to insecticide crystallography. This knowledge gap is exemplified by the twelve insecticides recommended for IRS by the WHO, five of which were not previously characterized crystallographically. Herein, we have reported the characterization of seven new crystallographic forms of six contact insecticides, expanding a comparatively small structural knowledge base. As mentioned above, our laboratory has demonstrated a convincing correlation between contact insecticide activity and the respective free energies of their crystal polymorphs. This behavior remains to be validated for the new characterized materials described herein.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/insects13030292/s1, Table S1: Preparation of insecticide crystals from solution crystallization, Figure S1: Heterogenous nucleation of λ-Cyhalothrin Form II on α-Cypermethrin crystals, Figure S2: Raman spectra of thiacloprid Form I and Form II, Figure S3: Single-crystal images with Miller indices.

**Author Contributions:** B.E., A.S. and X.Z. characterized polymorphs. B.E., A.S., X.Z. and C.T.H. performed the single-crystal analysis. B.E.K., A.S. and M.D.W. directed the project. B.E., A.S., M.D.W. and B.K wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The NYU X-ray facility is supported partially by the NSF under Award Number CRIF/CHE-0840277 and by the Tropical Infectious Disease Consortium (MP4—RBPS: 108352) This work was supported partially by the MRSEC Program of the National Science Foundation under Award Number DMR-1420073.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Crystal structure data reported in this paper have been deposited at the Cambridge Crystallographic Data Centre with accession numbers found in Table 1.

**Conflicts of Interest:** The authors declare no conflict of interest.
