Fall Pest Pressure on Desert Vegetable Crops (November 27, 2013)

As is typical this time of the year, the weather has finally broken and it feels like “winter” has arrived in the desert. With cooler temperatures, you can expect a steady decline in insect abundance until temperatures begin to warm up again. Last fall, I felt that the insect pressure on produce and melons crops in the Yuma was as heavy as I’d seen in many years. This year it didn’t seem quite as heavy, but I’ve heard many PCAs complain of heavy insect pressure, particularly during November which was unusually warm. A quick look at both recent and historic data on pest abundance recorded from our research plots here in the Yuma area suggests that insect pressure this fall was a little lighter than last year, but still heavier as compared to previous years. First, whitefly adult numbers on fall melons and produce were extremely high in August, but monsoon rainfall in early September appeared to suppress their numbers for a couple of weeks thereafter. Much like last fall however, untreated melons plots on the Ag Center wilted and died rapidly as a direct result of heavy whitefly infestations. Sticky trap captures of whiteflies near cantaloupe fields from Wellton to Texas Hill, were lower than last fall, but were still higher than the previous four years. However, numbers of adults caught on traps in the Roll/Tacna area were higher in late-August this fall than we’ve ever recorded. Similarly, CYSDV incidence at harvest in cantaloupe fields in these areas was very high this fall. In all cases, high traps catches were found in melon fields in near proximity to cotton fields. Based on counts from untreated lettuce at the Yuma Ag Center, beet armyworm and cabbage looper population abundance this fall was slightly lower than last fall. However, worm pressure the past two years was higher than we’ve seen the previous 5 years. Populations began infesting plots in early September, and remained steady throughout October and into early November. Egg deposition and larval development has only recently declined significantly. Corn earworm numbers were lower than what we observed last year, but could still easily be found in untreated plots. Finally, Bagrada bug infestations were abundant in untreated broccoli plots at YAC for the 4th consecutive year. As expected, low-moderate population appeared in early September, but increased steadily through September and peaking in early October. Overall, the peak numbers observed were not as high as either 2010 or 2012, but much higher than in 2011. The infestation levels in our untreated broccoli plots this year remained at damaging levels throughout October and into mid-November. This is the first year we have seen Bagrada bugs this abundant in November. In contrast, similar to last fall, western flower thrips population numbers have been relatively low, but unlike last fall, we’ve been picking up winged green peach aphids and colonies on sticky traps and in untreated lettuce plots since early November. How these aphid and thrips numbers will translate into potential population pressure in January and February is unknown, but you should anticipate their abundance as usual. I’m often asked why we see such differences in insect pest numbers each year. It’s obviously a very complex question, and I don’t have a good answer. The bottom line; insect abundance and outbreaks are dictated by many abiotic, biotic and crop management factors within our cropping system. Moreover, it’s nearly impossible to consider all the factors necessary to draw a reliable conclusion. Nonetheless, graphics showing these recent trends in Whitefly, CYSDV, Lep Larvae and Bagrada abundance can be found at Pest Abundance on Desert Produce and Melon Cops in 2013.