A new study produces evidence showing that light traps for moths are becoming less effective over the past 25 years. They use data on one pest species, the Corn Earworm (Helicoverpa zea), for which the moth's prevalence over time has been monitored using light traps (using fluorescent black-light bulbs) and pheromone traps operating in close proximity.

The paper is publicly available: Ian Battles et al., “Moths are less attracted to light traps than they used to be” Journal of Insect Conservation, published 19 April 2024 (https://link.springer.com/article/10.1007/s10841-024-00588-x ). New York Times subscribers may also wish to read commentary on the article in the April 19, 2024 issue (Veronique Greenwood, “Like Moths to a Flame: We May Need a New Phrase”).

(continuing discussion of the Battles et al. journal article)

In 25 years of monitoring in Delaware starting in 1998, light-trap catch averages declined from about half as many moths as pheromone traps in the first 5 years to less than 5% as many moths by the end of the period. A similar pattern was observed over a shorter 10-year period of sampling in New Jersey. On the other hand, a 15-year period of sampling in Minnesota showed no difference in the efficacy of the two trapping methods.

The causes of this decline in light trap efficacy are uncertain. Competition from light sources in the area is one possibility, but the data on competing light sources is poor and no statistically significant results are found. The researchers speculate that the more likely cause is that rapid evolution has decreased the flight-to-light response of this species. The paper notes that the Corn Earworm seems particularly susceptible to rapid evolutionary change as it has multiple generations per year and high dispersal rates: it cannot survive winter temperatures in Canada and the northern US and migrates each spring from the southern US.

Overall, this is not good news for moth observers. Perhaps other means of trapping may need to be explored, such as bait, pheromones or suction traps. On the other hand, it remains to be seen whether this finding is applicable to other species as well.