Mark McCostlin, and Karen McCostlin, Appellants, vs. Monsanto Company, Respondent.

Missouri Court of Appeals·Decided June 24, 2025·No. ED112532·Published

Opinion

In the Missouri Court of Appeals Eastern District

DIVISION FOUR

MARK McCOSTLIN, AND KAREN ) No. ED112532 McCOSTLIN, )

)

Appellants, ) Appeal from the Circuit Court of ) St. Louis County

vs. ) Cause No. 19SL-CC03421 )

MONSANTO COMPANY, ) Honorable Brian H. May )

Respondent. ) Filed: June 24, 2025

Introduction

Mark McCostlin (“Mr. McCostlin”) and his wife, Karen McCostlin (collectively, “the McCostlins”) appeal the trial court’s grant of a directed verdict in favor of Monsanto Company (“Monsanto”). The McCostlins assert five points on appeal. Points I–III contend the trial court erred in granting a directed verdict in favor of Monsanto because: (1) the expert witnesses’ testimony established a submissible case on causation, which is an issue for the jury; (2) the trial court interpreted the evidence in the light most favorable to Monsanto; and (3) the directed verdict was based on the trial court’s credibility determinations which lie within the jury’s province. In Point IV, the McCostlins argue the trial court erred in denying their motion to recall one of their expert witnesses because the directed verdict depended upon an interpretation of his testimony. Lastly, in Point V, the McCostlins argue the trial court erred in sustaining Monsanto’s objection

during redirect examination of one of their expert witnesses when he attempted to give his opinion regarding Mr. McCostlin’s exposure days to Monsanto’s product.

As to Points I–III, this Court holds the trial court did not err in granting a directed verdict in favor of Monsanto because the McCostlins’ causation expert failed to demonstrate a causal connection between Monsanto’s products containing glyphosate (“Roundup”) and Mr. McCostlin’s non-Hodgkin’s Lymphoma (“NHL”). Also, the record directly contradicts the McCostlins’ contentions the trial court interpreted the evidence in the light most favorable to Monsanto or made any credibility findings regarding one of their expert witnesses. As to Point IV, the trial court did not err in denying the McCostlins’ motion to recall their expert witness because the witness did not make misstatements that needed to be clarified. With respect to Point V, the attempted testimony of the expert witness was beyond the scope of redirect and the parties’ stipulation. Thus, Monsanto’s objection was properly sustained.

Accordingly, the trial court’s judgment is affirmed.

Factual and Procedural Background On April 5, 2023, the McCostlins filed an amended petition against Monsanto asserting claims for strict liability (design defect and failure to warn), negligence, misrepresentation, violating consumer statutes, and loss of consortium. In pertinent part, the amended petition alleged Mr. McCostlin’s exposure to Roundup caused him to develop NHL.

Prior to trial, the McCostlins disclosed Dr. James Clark (“Dr. Clark”), a toxicologist, and Dr. Barry Boyd (“Dr. Boyd”), a clinical oncologist, as expert witnesses. Dr. Clark reviewed Mr. McCostlin’s deposition and also interviewed him. Based on the information he gathered, Dr. Clark opined Mr. McCostlin’s “exposure to glyphosate via inhalation was equivalent to between 13 and 33 days of spraying” measured in eight-hour days. Dr. Boyd, in forming his

opinion, relied on Dr. Clark’s calculations. In his deposition, Dr. Boyd, opined it was “more than likely” that Mr. McCostlin’s large B cell lymphoma, a subtype of NHL, “was caused by exposure to Roundup[.]”

Monsanto moved to exclude Dr. Clark and Dr. Boyd from testifying on the basis that their opinions were unscientific and unreliable. The trial court granted the motion in part, and denied it part. In pertinent part, the trial court concluded Dr. Clark was permitted to “testify that Mr. McCostlin’s exposure to Roundup was above the threshold values in the dose-metric studies but not that such dosage causes cancer.” The trial court also precluded Dr. Clark “from offering opinions regarding general or specific causation, which Plaintiffs have conceded….” As for Dr. Boyd, he was allowed to testify that Mr. McCostlin’s “exposure to Roundup was above the threshold values in the dose-metric studies placing him at an increased risk of developing NHL.” But, he was barred from testifying about various studies “for the unreliable proposition that exposure to Roundup on more than two days per year increases the risk of developing NHL.” Monsanto also moved for summary judgment on the ground the McCostlins lacked admissible evidence of general or specific causation. The trial court found summary judgment was not warranted on this basis and that “[the McCostlins’] experts may testify subject to the limitations” stated above.

The matter proceeded to a jury trial in September of 2023. At trial, Mr. McCostlin testified he used Roundup outside his homes in Indiana, Ohio, Illinois, California, Colorado, and Florida from 1985 to 2014. Mr. McCostlin stated he used Roundup year-round when he lived in California and Florida, and once a week throughout June, July, and August in the other states. Further, he stated he typically sprayed Roundup for 15 to 30 minutes at a time and explained his routine “could be more walking around than actual spraying[.]” Mr. McCostlin also testified he did not use

Roundup when he lived in Texas and Kansas. In early 2017, Mr. McCostlin went to see a doctor for a sore throat that would not improve. He was subsequently diagnosed with NHL.

Following Mr. McCostlin’s testimony, Monsanto filed another motion to exclude Dr. Clark and Dr. Boyd. Therein, Monsanto argued Mr. McCostlin’s testimony materially differed from his deposition testimony, and both expert witnesses relied on Mr. McCostlin’s deposition testimony detailing his Roundup usage, including where, when, and how long he used Roundup, in forming their respective opinions. The trial court held voir dire examinations of both experts outside the presence of the jury. After the voir dire hearing, the trial court denied Monsanto’s motion to exclude Dr. Clark and took the matter of excluding Dr. Boyd under submission.

Both experts then testified before the jury. Dr. Clark explained his process of calculating Mr. McCostlin’s exposure to Roundup and concluded Mr. McCostlin had 9.59 exposure days to Roundup. Dr. Boyd was asked his opinion on whether Roundup directly contributed to cause Mr. McCostlin’s NHL based on Mr. McCostlin’s 9.59 exposure days to Roundup. Dr. Boyd stated he believed “it contributed to his risk” of developing NHL.

At the close of the McCostlins’ evidence, Monsanto moved for a directed verdict on the ground the McCostlins failed to make a submissible case on causation. Monsanto argued the McCostlins’ “only specific causation expert, Dr. Boyd, testified to the jury only that Mr. McCostlin’s Roundup use may have ‘contributed to the risk’ of [Mr.] McCostlin developing NHL.” (emphasis in original). The McCostlins, subsequently, filed a motion to recall Dr. Boyd so he could clarify his opinion.

The trial court held hearings on both motions. Following the hearings, the McCostlins’

motion to recall Dr. Boyd was denied. The trial court reasoned Dr. Boyd did not misspeak as he consistently testified about Mr. McCostlin’s risk of developing NHL. The trial court then

considered the parties’ arguments and written motions regarding Monsanto’s motion for a directed verdict. The trial court found Dr. Boyd “no longer testified concerning causation but rather risk” after Dr. Clark recalculated Mr. McCostlin’s exposure days to be 9.59. The trial court also noted the McCostlins conceded risk did not mean causation. The trial court granted Monsanto’s motion for a directed verdict based on the insufficiency of expert evidence required to establish specific causation. The trial court denied Monsanto’s motion to exclude Dr. Boyd’s testimony as moot.

This appeal follows.1

Discussion

Points I, II, and III

Points I–III assert the trial court erred in granting Monsanto’s motion for directed verdict.

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Mark McCostlin, and Karen McCostlin, Appellants, vs. Monsanto Company, Respondent. (Mark McCostlin, and Karen McCostlin, Appellants, vs. Monsanto Company, Respondent.) — published by Counsel Stack Legal Research, free access to 12M+ legal documents.

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