California tree nut growers should know first-hand by now what the Navel orangworm is and, more importantly, the damage it can cause to maturing nuts. Still, what is this insect and why should farmers care?
The Navel orangworm, or NOW, is a primary pest in almonds and is the most damaging caterpillar in pistachios, according to the University of California. The insect is also a pest of concern in figs, pomegranates and walnuts because of the feeding damage it leaves behind and its ability to expose pistachios to fungal organisms that produce aflatoxins.
The Bug Guide, a website hosted by the University of Iowa, describes the insect as a lepidopera moth identified more than a century ago. It is said to exist in California, Mexico, and from Texas to Florida, and north to Tennessee. While it's a particularly troublesome pest in California tree nuts, its host range expands to various fruits and is said to also feed upon Florida oranges and grapefruits.
Monique Rivera, an entomologist with the University of California whose focus is southern California citrus crops says she has not seen the pest in California citrus and surmises that it is more attracted to tree nuts – pistachios, almonds, walnuts – than it is the softer citrus fruit. Much the same is reported in Arizona as Glenn Wright, an associate Extension specialist with the University of Arizona, says he's not seen the pest in Arizona citrus, though its name reportedly originates from its discovery on navel oranges there after first being discovered in Mexico about 100 years ago.
Adult NOW moths are silver and gray with black irregular lines. They are generally 9.7 to 10.9 mm in length. Larvae tend to be one-half to three-quarters of an inch long and transition from reddish orange when first hatched to a cream color after molting. As they develop their color changes depending on their food source.
Larvae tend to overwinter in mummy (unharvested) nuts left behind in the trees or remaining on the ground. These overwintering larvae develop into adults in early spring and almost immediately begin reproducing and laying eggs in remaining nut mummies or developing nuts. This points to the importance of winter sanitation and the necessity to remove and destroy overwintering nut mummies by late January, before the weather warms and the larvae can develop into adults.
Depending on the location, NOW moths can generate three to four generations per year. The warmer locations in the southern San Joaquin Valley can regularly see a fourth generation of moths, while points north into San Joaquin County and the Sacramento Valley tend to experience three generations per season, according to University of California entomologists.
NOW moths primarily cause feeding damage in maturing nuts. This feeding damage makes the nuts unmarketable. In some cases, the moths have been linked to dangerous aflatoxins, particularly in pistachios.
Over the next few months we will chronical tools used to control the NOW moth in tree nuts and look at the relative success some are having by employing these practices.