Border Patrol apprehended 94,487 immigrants in June, according to a leak to the Washington Examiner. This constitutes a 29 percent decline compared to the 132,887 immigrants received in May. Nonetheless, apprehensions in June were higher than every month from FY 2009 to 2019 except the last two, April and May 2019 (Figure 1), and nearly three times the level in June 2018. They were also higher than March 2019, when the agency described nearly 93,000 apprehensions as a “system‐​wide emergency.”

The June decline is similar to declines almost every summer since the year 2000. The only exception was the short‐​lived “Trump effect” in 2017. Migrants moved up their crossing dates to before his 2017 inauguration and others stayed away until they realized little had changed. This caused numbers to rise throughout the year for the first time. Every other June saw declines in apprehensions.

Excluding 2017, the average decline from May to June for fiscal years 2000 to 2019 was 20 percent. The decline in 2019 was 29 percent (Figure 2). Three other years in the 20 years of available data had larger declines in June. The most recent was 2010, when apprehensions fell 30 percent. From May to July, the average decline was 25 percent. Six other years had apprehension declines from May to July above 29 percent. The most recent was 2014, when apprehensions fell 33 percent.

The seasonal declines are mostly the result of rising temperatures that make crossings much more dangerous. In 2019, however, June’s drop was larger than the average over the last two decades, which could imply that some other factors made it larger than expected. The most likely culprit is increased enforcement by Mexican authorities.

Indeed, June 2019 saw an increase in migrant apprehensions in Mexico from 23,419 in May to 29,153 in June. But that increase would explain just 15 percent of the drop in apprehensions. One might think that the increase had a deterrent effect on other new arrivals, but if Mexican apprehensions have a deterrent effect, it is a recent phenomenon. From January to May 2019, Mexican apprehensions increased by 175 percent, and U.S. apprehensions still increased by 177 percent.

Some deterrence, either from the Mexico’s interdiction or the U.S. Border Patrol’s inhumane treatment of immigrants, may have influenced the numbers coming to the border in June, but only very modestly. The most important factor by far was the weather. If this is the case, the flow will return as temperatures drop unless something changes. Congress needs to reform legal immigration to make illegal migration unnecessary.