Spongy Moth

The spongy moth invasion of North America is a century-long case study in Amara's Law misjudgment. Introduced to Massachusetts in 1869 for silk production, it escaped and spread with three distinct phases: rapid expansion 1900-1915, slow spread 1916-1965 (nearly 50 years), then rapid spread again 1966-1990. Observers in each phase systematically extrapolated current rates and were systematically wrong. During rapid phases, predictions of unstoppable spread to the Pacific coast seemed inevitable. During the slow phase, containment appeared successful. The reality: spread rate depended on suitable host tree density, climate patterns, and natural enemy populations, creating phase transitions no one predicted. The moth demonstrates why linear thinking fails for biological invasions: the system operates in sigmoid curves with lag phases, exponential bursts, and carrying capacity constraints. What appeared to be 'control measures working' during 1916-1965 was actually the invasion encountering less suitable habitat and building up parasitoid populations that suppressed spread. When those constraints lifted in 1966, rapid spread resumed. The moth now infests 90 million hectares in North America, defoliating hardwood forests in periodic outbreaks. The invasion history reveals a deeper pattern: biological systems don't grow smoothly—they exhibit punctuated dynamics with unpredictable transitions. Observers optimized for linear change cannot predict phase shifts. The lag phase (1869-1900) allowed population establishment. The first exponential phase (1900-1915) triggered alarmism. The slow phase (1916-1965) created complacency. The second exponential phase (1966-1990) revealed limits to control.