In a previous post, I quoted an entomologist who thought the name “Mormon Fly” was “an insolvable mystery.”  He went on to say that “there was somewhat more plausible ground for calling the Chinch bug the ‘Mormon louse;’ for that little pest really did swarm for the first time in Illinois about the same year that the Mormons settled there.” The bug in question, Blissus lucopterus, is a small beast, usually less than 4 mm aft-to-stern, but it has destroyed great swaths of wheat and other grasses throughout the US for the past two centuries and change by feeding on roots and blocking the flow of nutrients. The earliest reference I’ve found comes from 1856:
[The Chinch bug] made its appearance there simultaneously with the establishment of the Mormons at Nauvoo (1840-1844 [sic]) and many ignorant people firmly believed they were introduced there by these strange religionists, and ‘Mormon lice’ became the name by which they were currently designated, through that district. When we have such instance of credulity and ignorance of our own day and generation, let us not smile at our patriotic grandsires for deeming that the Hessian soldiers were breeding and shaking off pestilent vermin and scattering them over the country wherever they marched.” 
The much-loathed Hessians appeared in many of the citations.
In parts of North Carolina it receives the name, Hessian Fly or, Hessian Bug, on the supposition that they were left in the country by German soldiers, as they were first observed after a detachment had passed through. In Illinois, they were similarly called Mormon lice, upon the supposition that the Mormons were somehow responsible for them. 
Different circumstance, different bugbear—othering is nothing if not plastic. I find interesting here the construction of an elitist identity: the writers smugly assert their intellectual superiority to “strange religionists,” previous generations, and plebeians. Such constructions differ from many of the others we’ve been discussing in that the construction distinguishes the “rational” from the “credulous,” rather than the moral from the immoral. For Mormons, mere “strangeness” indicated, at least rhetorically, a much greater degree of social acceptance. The growing authority of Science in the late nineteenth century changed/coincided with (re)constructions of Mormon identity. For an increasing part of the population, Mormonism became “just another irrational religion.”  Clean cities and racial homogeneity invited admiration in the new paradigm. Mormon industry had always inspired favor (or envy or fear), so approval did not so much increase as the loathing of a “moral abomination” decreased. Mormons shifted from evil non-Whites who happened to be good—and therefore dangerous—colonizers to good, White—and therefore useful—colonizers who happened to hold weird ideas. But all that was still mostly in the future. By the 1890s the sobriquet “Mormon lice” had mostly petered out even though Mormons and B. leucopterus both still held national attention.  The timing requires explanation: the second-relic-of-barbarism 1880s were hardly the decade for negative Mormon references to die off. Possible factors include the fact that the name never gained more than local traction; the chinch bug’s wide range and previously established name obscured the new name; and Mormons didn’t implicate it in a faith-promoting story.  Like most common names, “Mormon lice” lacked taxonomic precision and some folks might have applied the name to bed bugs (Cimex lectularius). I also found one reference to “Mormon bed-bugs”:
Fleas are, in western phrase, ‘tolerable bad,’ but bed bugs are intolerable; both in numbers and voracity those of Utah beat the world, particularly in the country towns, and among the poorer classes of foreign-born Mormons. In certain settlements their ravage is incredible, and Mormon bed-bugs seem as much worse than others as their human companions. Like the latter, too, they seem to regard the Gentile as fair prey. 
The author scores big points in nativist pinball, hitting almost all of the standard issues: overwhelming numbers, individual ferocity, poverty, foreign-ness, and lack of decency. In particular, the dig undermined one of the Mormons’ most (/the only) successful claims, pioneering excellence. If bed bugs vexed, Mormons hadn’t been as successful as claimed in civilizing the frontier. 
At any rate, the big issue with Mormon lice is, of course, the nits. Haun’s Mill Massacre participant William Reynolds famously explained his execution of a child with, “Nits will make lice, and if he had lived he would have become a Mormon.”  As Paul Reeve demonstrated at MHA this year, the “nits make lice” idea surfaced during conflicts with Native American Peoples from the eighteenth through the nineteenth century.  However, I have not yet found any documentary evidence connecting Reynolds’s nits with “Mormon lice.”
Even without the possible equation with Indians, “Mormon lice” keyed on a fundamental assumption for most White, nineteenth-century Americans: lice signified poverty, disgrace, uncleanness, and moral degradation. “Lice-ridden” almost always made up part of the assigned identity for Blacks, immigrants, and the poor. Thus, even though I find no direct evidence changing “Mormon lice” from a troponym to a charactonym, I think it highly likely that it connoted disreputability and performed maintenance othering. 
The standard caveats: When I write “The earliest instance…,” etc, the “that I’ve found to date” is implied. When I write something like “Most of the Mormon records used…”I am reporting a non-quantitative, subjective impression (unless otherwise stated).
Related Posts: Twin Barbarians 1: Mormon Crickets
What Put the “Mormon” in “Mormon Fly” Might Not Go Well with Breakfast
All God’s Creatures—Including Mormos, the Other Mormons
A Preliminary History of the Phrase “Happy Valley”
 Benjamin D Walsh, “The Bughunter in Egypt: A Journal of an Entomological Tour into South Illinois by the Senior Editor,” American Entomologist 1 no 1 (1868 Sep):6-7.
 Asa Fitch, First and Second Report on the Noxious, Beneficial and Other Insects of the State of New York (Albany: C van Benthuysen, 1856) p282.
 CW Woodworth, “Report of the Entomologist,” Second Annual Report of the Arkansas Agricultural Experiment Station at Arkansas Industrial University, Fayetteville, Ark. AE Menke, director (Little Rock: Mitchell and Bettis, 1890) p161.
 In some senses, however, it only changed the form, as polemicists used Science to justify and confirm their preferred ideas.
 “This pernicious insect is a very small bug, of a black color, with white wings. In some localities they are called “Mormon lice.”” S. Edwards Todd, The American Wheat Culturist: A Practical Treatise… (New York: Taintor Brothers, 1868) p419-20. “The chinch-bug or Mormon louse of Walsh, Micropus (Rhyparochromus devastator,) (Micropus) (Blissus) leucopterus, (Fig. 26,) is one of our most destructive insects to wheat, corn, &c….” Townend Glover, “Report of the Entomologist,” in Frederick Watts, Report of the Commissioner of Agriculture for the Year 1875 (Washington: Government Printing Office, 1876) p122. LO Howard, “The Chinch Bug (Blissus leucopteris, Say),” in CV Riley, “Report of the Entomologist,” in Report of the Commissioner of Agriculture, 1887 (Government Printing Office: Washington DC, 1888) p52 (51-87). There is one exception: Bernard Jaffe, Outposts of Science: A Journey to the Workshops of Our Leading Men of Research (New York: Simon and Schuster, 1935) p288, which is only available in snippetview at gbooks includes this sentence: “And as he sat writing entomological definitions for the Century Encyclopedia and “inflicting new atrocities on the English language,” he was rudely disturbed by the Mormon louse. This gnatlike insect, also…”
 Almost all the instances of “Mormon lice” I encountered came as asides in scientific or historical reports about a local or historical curio. Mormon crickets and Mormon flies had strong geographic attachments—the crickets to the Mormon-dominated West and the flies, though appearing all over the US, to the Oh-my-billions-of-bugs Upper Mississippi hatches; the chinch had been around longer, so already had a well-established name; and, as noted in a previous post, Mormons talked about the crickets.
 The word “chinch” comes from a Spanish word for “bed bug,” so it’s not a huge leap. The cite comes from a family history, but it seems to be quoting a diary: “‘How do we get rid of Mormon lice?’ At first he did not know what they were talking about. After examining them, he found that ‘Mormon lice’ were bed bugs, which the Indians had never seen before” Kenneth B Tidwell, “Seely, Sarah,” John Henry Owen Willcox Family Organization, no date. I’m not sure I buy the idea of Indians not knowing what beg bugs were, but that’s my one hint. Richard F Burton in City of the Saints refers to the chinch bug, but uses it for the Cimex lectularius or common bedbug; he makes no reference to the idea of Mormon lice (160). Mormon bed bugs: John Hanson Beadle, Life in Utah or, The Mysteries and Crimes of Mormonism… (Philadelphia: National Publishing Company, 1870) p473.
 As usual for this sort of thing, emphatically incompatible ideas got along very well together. Here the defect is that Mormons are poor and not assimilating to American culture; elsewhere the challenge was Mormonism’s “vast” financial resources and efficiency at infiltrating American culture.
 Andrew Jenson, “Haun’s Mill Massacre,” The Historical Record 7 no 11-12 (1888 Dec): 673. One supposes that Reynolds refers to head or body lice, Pediculus humanus capitis de Geer and Pediculus humanus humanus Linnaeus, respectively.
 For synopsis, see JI’s notes. American John Heckewelder reversed the roles in his 1819 treatise on Indians in Pennsylvania. “But what shall I say of the conduct of the British agents…who, at the commencement of the American revolution, openly excited the Indians to kill and destroy all the rebels without distinction? ‘Kill all the rebels,’ they would say, ‘put them all to death, and spare none.’ A veteran chief of the Wyandot nation…observed to one of them that surely it was meant that they should kill men only, and not women and children. ‘No, no,’ was the answer, ‘kill all, destroy all; nits breed lice!’ The brave veteran was so disgusted with this reply, that he refused to go out at all.” Emphasis in original. John Heckewelder, “An Account of the History, Manners, and Customs, of the Indian Nations, Who Once Inhabited Pennsylvania and the Neighbouring States,” Transactions of the Historical & Literary Committee of the American Philosophical Society… (Philadelphia: Abraham Small, 1819) vol 1 no 1 p 337.
 I also find no evidence of nineteenth-century speakers applying “lice” as a pejorative in any particularly Mormon sense. In the twentieth century I’ve found only one instance, and that in a 1940 historical fiction: “‘One side—you goddamned Mormon lice!’ roared a tall, bony-bodied corporal.” Paul Bailey, For This My Glory: A Story of a Mormon Life (Los Angeles: Lyman House, 1940) p229.