Peter Ellsworth, integrated pest management (IPM) specialist with the University of Arizona, and Steven Naranjo of the USDA-Agricultural Research Service, have studied whiteflies for more than 20 years.
They know all too well the damage whiteflies can do to a field of open cotton, and the physical and economic damage sticky cotton can consequently cause to a textile operation if not detected.
“Cotton producers in the West are aware of this possibility as well, and work closely with their local pest control advisers or pest managers to avoid it at all costs,” says Ellsworth.
“In today’s farming economic environment, all producers are looking for ways to reduce input costs without sacrificing those inputs that increase yield and/or protect their crop from pests like whiteflies,” he says.
One of Ellsworth’s brightest Entomology Ph.D. graduate students, Tim Vandervoet, has been immersed in research that, if successful, will go a long way to help growers protect their cotton from this damaging pest while simultaneously lowering input costs.
“Conservation biological control (bio-control), or the use of abundant populations of natural enemy predators that effect suppression of crop damaging pests and the research to verify its effectiveness, has been going on for decades,” Vandervoet explains.
“Measuring at what levels these predators (or parasitoids) become effective at suppressing whitefly populations is difficult,” he says, “in part because there are many factors that influence and cause variations in those biological interactions.”
Synthetic insecticides were commercialized around World War II, and were quickly adopted by growers since they provided inexpensive and effective pest control – often, unfortunately, at the expense of existing biological controls due to their broad spectrum activity.
Today’s chemistries offer more selective control, often permitting or enabling bio-control.
Key early research led to the development of a ‘sampling protocol’ which created a standard for whitefly population measurement. Later, the development of whitefly ‘action thresholds’ were established allowing pest managers to know what population levels put a field of cotton in danger.
Eventually, the researchers confirmed predators were a key factor in whitefly mortality in untreated fields.
On average, more than 95 percent of whiteflies in untreated fields die before adulthood due to natural factors - including predation. Researchers are now attempting to measure bio-control population levels for effective assimilation into IPM. The ability to measure those levels could even help predict rapid whitefly growth.
The good guys
Vandervoet says there are more than 20 natural enemy insects and spiders in our cotton fields that prey upon, or ‘parasitize’ whiteflies. Four common predators have been identified whose populations track closely with whitefly populations: big-eyed bugs; minute pirate bugs; collops beetles, and crab spiders.
While there may be a relatively high number of these predator insects eating whiteflies at any one time, Vandervoet knows that ‘controlling’ whiteflies is another thing altogether. He wants to quantify this with his research.
With a 38 centimeter standard sweep net, he recommends 100 sweeps per field through the top of cotton’s canopy – which is also how lygus (western tarnished plant bug) populations are sampled, so the same sweep counts can be used to evaluate predator populations as well.
Know the zones
“We see three ‘zones of control’ for each of these four predator pests, and have calculated the ratio of those individual populations to whitefly population suppression/non-suppression,” Vandervoet says.
In the optimum zone, the “Zone of Control,” bio-control limits whitefly population growth rendering insecticide applications unnecessary. A “Loss of Control” zone has too few predators, or too many whiteflies, allowing whitefly populations to expand. A grower should have already used chemical control in these conditions.
The final is the “Zone of Uncertainty”- a time of active whitefly management when bio-control measurement plays a critical role helping pest managers make difficult control decisions.
“Collops love soft-bodied insects like caterpillars, but with so much Bt cotton around, they’ll satisfy their appetites with what is plentiful – whiteflies. When whitefly populations are within the ‘Zone of Uncertainty,’ 1 or 2 collops in sweeps (depending on the measured whitefly density in the field) could indicate bio-control is still viable which could mean a control application may be deferred,” explains Vandervoet.
In the same “Zone of Uncertainty,” if 2 to 8 pirate bugs, 1 to 5 crab spiders, or 1 to 3 big-eyed bugs are found per 100 sweeps, predator insects at those densities help indicate that bio-control is functioning – there again, depending on whitefly densities.
Minute pirate bugs can be some of the first beneficials to enter cotton each year, while big-eyed bugs and crab spiders may be reliably present all season long.
Staving off resistance
Whiteflies reproduce very quickly. More than five generations can be hatched during one growing season, so a trait (like a resistance trait) that is expressed in one whitefly can quickly spread throughout future generations. Male whiteflies express half a genome while females express a full genome which can boost the expression of such a trait.
“Male whiteflies develop from unfertilized eggs (haploid), and females are developed from fertilized eggs (diploid). Males have half the number of chromosomes than a female. Consequently, resistance development is something we have to monitor very closely,” stresses Vandervoet.
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Two years ago, there was an outbreak of brown stink bugs. For control, growers were limited to broad-spectrum insecticides, but their use really knocks down natural enemy pest populations – which can consequently cause secondary outbreaks of whiteflies.
“We recommend selective insecticides if whiteflies are being actively sampled,” Vandervoet says. “They’re effective anytime, but especially early in the season, to extend the period when broad spectrum insecticides are not used. If heavy outbreaks occur, pest managers might recommend a move to a less selective class of insecticides later in the season.”
Vandervoet, Ellsworth, and Naranjo hope their work will lead to bio-control recommendations that help everyone make better decisions that lead to more accurate, more timely, and possibly fewer sprays – all outcomes that help producers reduce costly inputs while protecting their cotton from honeydew-secreting whiteflies.