Japanese Beetle Emergence and Longevity

The persistent and destructive Japanese beetle (JB) got a 'jump start' on its activity with the hot temperatures of May and June 2018. The first reported observation of an adult JB was on June 12, 2018.  Emergence was gradual through late June and early July with the peak population happening in mid-to-late-July.  The lacy, brown foliage typical of JB feeding came on gradually, but heavy infestations and defoliation of favored plants (lindens, crabapples, grapevines, and roses) eventually happened.  Observers report that population numbers are declining, possibly ahead of their expected demise in early fall.  Some adult beetles continue to linger and just how soon they all will be gone is a discussion among gardeners and landscapers.  Below are observations about JB emergence and longevity.

It would be better for us if JB emergence was synchronized so all of the adults came out at once. Unfortunately, insect emergence is not perfectly synchronized, so new JB adults continue to emerge over several weeks. Population emergence can be affected by how well the grub stage developed last year, how many grubs were feeding in the same area, how harsh the winter was to this pest (how deep the frost penetrated in the soil and how long it lasted), and favorable spring soil temperatures.

Each week that beetles emerge extends the overall period that JB will be around, unfortunately. So the last beetles to emerge (early August?) will prolong the feeding activity well beyond the time that the first emerging beetles become history.  According to Walter Fleming [1972, USDA Technical Bulletin 1449], the generally accepted lifespan of a JB is 30 to 45 days [4 to 6 weeks is also quoted by Extension Entomologists], but Fleming reported that males can live 74 days and females can live 105 days under laboratory conditions.

Other factors that complicate how long JB will be active during a given year include:

  • Temperature extremes: Adult JB function well between 55-95F. Above and below this temperature range, JB doesn't do well. Will the cooler day time highs [75-80F] we are experiencing now extend their life compared to temperatures in the 90- 100F?
  • Host quality: The better the host food quality and variety, the longer the beetles can live. Interesting that our intentional diversification of the ornamental landscape may actually benefit JB.  Who would have thought?
  • Predators: How many birds (e.g., starlings, robins, crows, grackles, blue jays, etc.) are present in an area? The higher the bird population, the greater the predation by birds.

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