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Project description

A new look at an old pest: what makes Lygus hungry for cotton squares? (01FE025)
Program UC IPM competitive research grants program
Principal
investigator
J.A. Rosenheim, Entomology, UC Davis
Host/habitat Cotton
Pest Lygus Bug Lygus hesperus; Lygus Bugs
Discipline Entomology
Review
panel
Field Ecology
Start year (duration)  2001 (Three Years)
Objectives Quantify the diet, including plant and prey-feeding, of Lygus hesperus through direct focal observations of nymphs and adults foraging freely in cotton.

Identify environmental conditions, including plant nutritional status and the availability of arthropod prey, that may influence Lygus decisions to feed on cotton squares.

Identify "physiological state variables", including Lygus age, stage, sex, nutritional status (stored carbohydrates, lipids, and proteins), and history of plant vs. prey-feeding that may influence Lygus decisions to feed on cotton squares.

Use small-scale manipulative experiments to provide definitive tests of the influences of key environmental and physiological variables on Lygus feeding on cotton squares.

Improve existing decision rules for Lygus control by incorporating the propensity of a Lygus population to feed on squares.

Use large-scale manipulative experiments and demonstration plots to test the new decision rule in a way that growers will find meaningful.

Final report The impact of lygusbugs on cotton yield is poorly understood. Growers have long observed that there is a large amount of unexplained variability between the number of lygus observed in a cotton field and the amount of damage the crop sustains (the "lygus enigma"). We have spent the last three years investigating this phenomenon. Results from 2001 showed that lygus does not act as a facultative omnivore in California. Therefore, we shifted our focus in 2002 toward quantifying stage and sex-dependent sampling bias for lygus monitoring. We found that nymphs are progressively underestimated (with decreasing instar) and that females are more difficult to sample than males. In 2003, we expanded our sampling by adding 21 additional fields. These fields were chosen as control or "enigmatic" fields, as designated by PCAs working in the fields. Our sampling revealed that the enigma is real and that the enigmatic fields show a level of plant damage that significantly deviates from the overall relationship between lygus and damage. It appears, however, that lygus are generating the expected amount of feeding damage (to anther sacs) in all fields, and that the enigmatic fields are instead showing plant-based differences in their propensity to abscise damaged squares.

The analyses from 2002 and 2003 also suggest that fourth and fifth instar lygus nymphs impose the most feeding damage and square abscission, relative to earlier instars and adults. Controlled field experiments in 2004 showed that (when confined to a cage) adult lygus can cause even more damage than fourth and fifth instars. In 2004, we also followed up on our previous results by investigating factors that affect the propensity of a cotton plant to abscise a square that is damaged by lygus. Our 2004 experiments revealed that young plants do not adjust the probability of abscising a damaged square when other, adjacent squares are damaged or removed. In 2005, we will be examining the effects of soil nutrients (specifically phosphorus) on the abscission response in both young and older cotton plants (through a Cotton Incorporated grant to J. A. Rosenheim). We will also examine the effects of boll load, pollination asymmetries, and damage asymmetries on the overall abscission response of cotton plants, to lygus feeding (through a USDA fellowship to A.G. Zink).

Third-year
progress
Results from 2001 showed that Lygus does not act as a facultative omnivore in California. Therefore we shifted our focus in 2002 toward quantifying stage and sex-dependent sampling bias for Lygus monitoring. We found that nymphs are progressively underestimated (with decreasing instar) and that females are more difficult to sample than males. In 2003 we greatly expanded our sampling by adding 21 additional fields. These fields were chosen as control or "enigmatic" fields, as designated by PCAs working in the fields. Our sampling revealed that the enigma is real, and that the "enigmatic" fields show a level of plant damage that significantly deviates from the overall relationship between Lygus and damage. It appears, however, that Lygus are generating the expected amount of feeding damage (to anther sacs) in all fields, and that the enigmatic fields are showing plant-based differences in their propensity to abscise damaged squares. In addition, our analyses suggest that 4th-5th instar Lygus nymphs impose the most feeding damage and square abscission. Adults made a smaller contribution to observed damage, and 1st-3rd instars had no impact on square damage or abscission.

Second-year
progress
The impact of lygus bugs on cotton yield is poorly understood. Growers have long observed that there is a large amount of unexplained variability between the number of Lygus observed in a cotton field and the amount of damage the crop sustains. We have begun to investigate this phenomenon. While many laboratory studies have suggested that Lygus feed extensively as predators, our field observations have shown that (in California cotton) lygus feed as strict herbivores. Thus, omnivory does not appear to be the basis for the variable impact of Lygus on cotton. However, data from 2001 showed that Lygus behave very differently depending on their age and sex. This led us to ask the question: could differences in age and sex among populations affect sampling accuracy and/or damage to cotton squares? In our 2002 field season we documented a wide range of age structures (adults versus nymphs) and sex-ratios (males versus females) across 10 different Lygus populations. Experiments using "whole cage sampling" allowed us to examine the efficiency of sweep sampling for assessing the absolute age structures and sex-ratios of these populations. These results suggest that sweep samples do predict Lygus presence, but that they are more efficient in capturing adults versus nymphs and in capturing males versus females. Because past work suggests that nymphs can cause damage that is equal to that of adults, underestimation of nymphs due to sweep sampling bias (or misidentification) could be a serious problem. In 2002 we found a positive correlation between damage to squares (specifically anther sacs) and nymph densities. However, there was no correlation with adult densities, suggesting that nymphs may be a better predictor of square damage. Abortion of squares was positively correlated with damage to anther sacs, but surprisingly square abortion did not correlate with adult or nymph densities. We will investigate this enigma (and the associated Revised Objective 4) in the 2003 field season. In particular, we plan to document 1) the natural distribution of damage produced by Lygus of different ages and 2) the relationship between different types of square damage and eventual square loss, within the context of the damage history and reproductive capacity of the host plant.
First-year
progress
The impact of lygus bugs on cotton yield is poorly understood. Growers have long observed that there is a large amount of unexplained variability between the number of lygus observed in a cotton field and the amount of damage the crop sustains. We have begun to investigate this phenomenon. Many laboratory studies have suggested that lygus feed extensively as predators. Our field observations have shown, however, that in California cotton, lygus feed as simple herbivores. No instances of predatory behavior were observed in approximately 100 hours of field observation. Thus, omnivory does not appear to be the basis for the variable impact of lygus on cotton. Our observations do, however, point to other possible bases for the appearance of a variable impact. First, lygus are difficult to sample. We found that whereas the nymphs and adult female lygus do the most feeding on cotton squares, it is the adult males that spend time on the upper surfaces of leaves in the top of the plant canopy, where they are the primary stage collected using the industry-standard sweep-net samples. Thus, the sampling technique may provide a poor indicator of the density of damaging bugs. Second, although lygus damage is easily visible in the anther sacs of squares, we have found that damage to anther sacs does not cause square abscission.

Future work will therefore focus on the type of feeding damage that triggers square abscission, and how the plants may vary in their response to a given type of damage.

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