Role of Chemical Attractants and Repellents in Adult Weevil Behavior

Herbert N. Nigg

University of Florida
Citrus Research and Education Center
700 Experiment Station Road
Lake Alfred, FL 33850

Sam E. Simpson

Florida Department of Agriculture and Consumer Services
3027 Lake Alfred Road
Winter Haven, FL 33881

The title of this article implies that we will detail the repellents and attractants for Diaprepes root weevil. In fact, for repellents nothing is known about materials repellent to Diaprepes, and information on attractants is preliminary and, in some cases, confusing.

Before we discuss Diaprepes root weevil in the insect family Curculionidae, we will discuss the Diabrotica beetles which belong to the family Chrysomelidae or leaf beetle family.

The Diabrotica is a very destructive neotropical genus which includes spotted cucumber beetle Diabrotica u. howardi, banded cucumber beetle D. balteata, cucurbit beetle D. speciosa, and so forth. The Diabrotica beetles apparently have co-evolved with the Cucurbitaceae plant family. This is the gourd, squash, cucumber and melon family and contains about 900 species. The Diabrotica insect genus consists of about 338 species. About 300 of these, the fucata group, are confined to South and Central America. About 35 species (the virgifera group) are found in North America. The fucata group feeds on many plant species as adults and larvae, has many generations per year and overwinters as adults in the southern United States. The virgifera group has one generation per year and overwinters as a diapausing egg. The adults feed on pollen. Some of the virgifera group, such as the northern corn rootworm and western corn rootworm, are very damaging to corn.

The plants in the Cucurbitaceae family produce cucurbitacins. These nonvolatile, high molecular weight compounds are found in all cucurbit plant parts except the seed. Cucurbitacins are extremely bitter to humans, but for the Diabrotica beetles, cucurbitacins are feeding stimulants. Diabrotica beetles will feed on all sorts of nonfood items as long as they contain cucurbitacins. The beetles metabolize ingested cucurbitacins to 23,24-dihydrocucurbitacin D and store this compound. Mammals, on the other hand, perceive cucurbitacins and their derivatives as some of the bitterest substances known. Cucurbitacins are more toxic to mice than most of our registered agricultural pesticides. Cucurbitacins are also feeding deterrents for many other insect species. So it appears that cucurbitacins are protective against herbivores.

The Diabrotica beetles do not randomly select plants to taste. They are attracted to Cucurbitaceae blossom volatiles. These volatiles have been isolated and identified, and mixtures of blossom chemicals serve as attractants for various Diabrotica beetles. Some of these mixtures can attract specific species (Metcalf and Metcalf, 1992).

Some plant chemicals are termed kairomones and come in three varieties: attractants, arrestants and excitants. The Diabrotica beetle story is perhaps the best documented kairomone system in the insect world. The blossom volatiles of Cucurbitaceae serve as attractants and the cucurbitacins serve as arrestants and excitants. Compare their story with the attractant research on Diaprepes root weevil which follows.

Beavers et al. (1979)

Beavers, Stanley, Agee and Lovestrand investigated the spectral (light) sensitivity of the male Diaprepes root weevil eye. The male eye was found to be most sensitive to 510-550 nm with peak sensitivity at 530 nm or yellow-green. Three trap types were then modified to accept a yellow-green bulb. These traps were tested in field cages in which 100 adult weevils had been released on a 3 ft. citrus seedling. An electrocutor trap caught 38% and 28% on the two nights testing occurred. The other traps caught 8 weevils each night. These three light trap types plus a Tedders trap with a light were field tested. After 90 days in the field, traps captured 1, 1, 6, and 18 weevils. These small catches were judged not useful for detection or for population estimation (Beavers et al., 1979).

Schroeder (1981)

This experiment was conducted with small Calamondin trees in an 8 x 8 x 8 ft cage placed adjacent to a citrus grove. At 4:00 PM, 50 weevils of the same sex were placed in this cage. At 8:00 AM, these weevils were removed from this tree and the tree was placed in another 8 x 8 x 8 ft cage which was 100 yd away from the first cage. The number of weevils in a control (never occupied) and experimental (previously occupied by weevils) trees were counted every hour until 4:00 PM. Weevils also were collected in the field from 9:00-11:00 AM, 2:00-4:00 PM and 6:00-10:00 PM and the number of single males and single females and mating pairs were counted. Also, 50 males and 50 females were placed in a 3 x 3 x 3 ft cage. Citrus foliage was replaced in this cage at 6:00 AM and 6:00 PM and the number of egg masses in the foliage and eggs per mass were counted.

Males were attracted to trees previously occupied by females and females were attracted to trees previously occupied by males. Trees previously occupied by the same sex were not different than controls for males or females. Mating pairs were higher from 2:00-4:00 PM and 6:00-10:00 PM than from 9:00-11:00 AM and 80% of eggs laid were laid from 6:00 PM to 6:00 AM.

Schroeder's results suggested that the male and the female produced pheromones, that mating occurs primarily during the day and that egg-laying occurs during nighttime hours.

Beavers et al. (1982)

These investigators collected weevils in the field, held the sexes separately and fed them citrus foliage. Frass was collected and extracted with hexane and acetone. These extracts were concentrated and separated into fractions on a chromatography column. Immature citrus foliage was extracted with hexane, heptane, xylene, methylene chloride, ethanol and 1,4-dioxane. These extracts also were divided into fractions. Volatiles in air drawn over immature citrus leaves were extracted with a Porapak Q cartridge. These extracts were tested with live weevils in an olfactometer and also with isolated Diaprepes antennae to produce an electroantennogram (Table 1).

Schroeder and Jones (1983)

An inverted wire cloth funnel with a cage on top and a wire cloth square suspended below the funnel was used as a trap. Frass was collected from a colony of 600 adults (1:1 sex ratio). Cotton dental wick, glass wool and rubber septa were used to dispense the chloroform extracts of frass. Each day over 17 days the frass extracts were applied to the dispensers. Traps were placed at 2 ft, 8 ft, and 13 ft (tree top) above the ground. Ten traps were tested at each height, 3 traps per tree for a total of 30 traps.

Baited traps captured more weevils than unbaited traps: 136 (glass wool), 75 (rubber septa), 70 (cotton dental wick), and 32 (blank). The 10 traps in the tree top captured 349 weevils, 8 ft traps captured 138 weevils and 2 ft traps captured 19 weevils. The tree top trap captured 2.18 females for every male it captured, but for all traps overall, more males were captured.

Jones and Schroeder (1984)

Solvent extracts of male and female frass, male frass, female frass and dried citrus leaves were produced with various solvents. These extracts were used to bait cup traps which were placed so the skirt of the trap touched the upper foliage of the tree. Weevils were removed and counted by sex every 24 h. Four separate tests were conducted. Male plus female frass extract baited traps captured more males and more females than unbaited traps. More males than females were captured with male frass and citrus leaf extracts, but citrus leaf extract was not different than an unbaited trap. The conclusion drawn by Jones and Schroeder was that weevil excrement contained one or more substances attractive to Diaprepes root weevil.

Schroeder and Jones (1984)

Schroeder and Jones tested two traps. One was an inverted plant pot with a funnel and a cage on top of the funnel. The second trap was two wire mesh panels at a right angle to one another with the same funnel and trap as the first. These traps were placed just above the apex of the foliage in 6 ft tall citrus trees. Weevils were counted and removed daily over 14 days and both traps were rebaited daily with frass extract. The wire mesh trap captured 26 males and 17 females. The plant pot trap captured 7 males and 2 females.

Schroeder and Beavers (1985)

Extracts of male frass and female frass were made and bioassayed in the laboratory. This extract was tested in a 10 inch mesh cylinder. The extract was placed on a cotton wick on one end of the cylinder and a wick with the extraction solvent on the other end. Twenty male weevils were released into the cage and their position counted each hour from 8:00 AM to 2:00 PM. Approximately twice as many males were in the frass extract end of the container after 3 hours. For females, there were no differences.

Pavis (1989)

Pavis used an olfactometer with 5 males or 5 females per test. The materials tested were adult males and adult females, frass of both sexes mixed, a hexane extract of 3 days of frass from one weevil, and the immature leaves of 'Tahiti' lime. Compared to solvent control, live males, live females, and male and female frass, immature lime leaves, frass of females consuming lime leaves and frass of males consuming lime leaves attracted 3 times both male and female weevils compared to the unbaited control.

Harari and Landolt (1997, 1999)

Harari and Landolt studied the attraction of Diaprepes root weevil to weevil, frass, and food odors and to food plant odors. Attraction was tested in the laboratory with a Y tube olfactometer.

Females were attracted to broken green beans over intact beans, males, females and males, and females held together. Females were also attracted to female frass over females, to male and female frass about equally and to males which had been held apart from other weevils over females which had been held apart. Either males or females held together or males and females held together were not differentially attractive. These data indicate that food volatiles, female frass, and 'virgin' males were attractive to females (Harari and Landolt, 1997).

Males were attracted to broken green beans over intact beans, females, males, female frass, and male frass. Males were attracted to male frass over males, females over female frass, males and females held together over separately held males and females, and males held together over separately held males. These data indicate that food volatiles, females, males, female frass, and male frass were attractive to males. In addition, mixed sex weevils were attractive to males (Harari and Landolt, 1997).

In their second study, Harari and Landolt (1999) tested the attraction of Diaprepes root weevil females to green beans and to immature citrus leaves. In one experiment females were fed citrus leaves and were then presented with green beans. About 67% chose the control side of a Y tube olfactometer which was empty. The same result was obtained when green beans were fed and immature citrus leaves were presented. That is, they chose the empty side of the olfactometer. In a second experiment, green beans were fed first and immature citrus leaves were fed second. Females were then presented with green beans or with citrus leaves. When presented with beans, 64% went to the control (empty) side of the Y tube. When presented with citrus, 69% went to the citrus side. When citrus leaves were fed first and green beans were fed second, weevils went to the green beans (Harari and Landolt, 1999). Because these weevils were starved 24 h after feeding and before a choice, Harari and Landolt (1999) defined these data as evidence of learning. These females chose the food upon which they had last fed.

Table 2 summarizes the published attractant research for Diaprepes root weevil. There have been two other projects on Diaprepes root weevil attractants, but these are not published. Both failed because of the inability of researchers to repeat positive bioassay results. Our (HNN and SES) effort was funded by the Florida Citrus Production Research Advisory Council and was carried out at the University of Florida, Lake Alfred. The second effort by Robin Giblin-Davis was carried out at the University of Florida, Ft. Lauderdale. Both research efforts concentrated on plant attractants.

The research efforts summarized above and in Table 2 were not necessarily successful. For example, in the 530 nm light study very few weevils were captured, the study lacked controls and the weevil population in the areas of the traps was not known. Most of the research in Table 2 has used male, female or mixed frass as attractants. Most of these studies have, in fact, mixed food volatiles with frass because food volatiles are found in frass.

Some of the laboratory studies in Table 2 did not report weevils which did not respond. When these non-responders were reported they were about 30%. The total of non-responders and weevils which did not find the 'target' was equal. That is, only about 50% of the weevils actually went to the target.

There are reasons that 50% of weevils do not select a target, the most important of which is physiological condition. In other insects, for example, only virgin females respond to male pheromones. So it is not surprising that whether or not a weevil is mated or whether or not a weevil has fed would have an effect on a bioassay. This is particularly true of a field collected weevil where we know nothing of its history. One lament of the earlier researchers was that they had to use field-collected weevils because laboratory weevils were not available in abundance.

Present research is being directed to isolation and identification of plant volatiles from known hosts and non-hosts. New and improved bioassay techniques that incorporate plant volatiles and light stimuli could provide needed answers on attraction or repellency of adult Diaprepes root weevil for feeding or egg deposition.

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Selected References

Beavers, J. B., J. M. Stanley, H. R. Agee and S. A. Lovestrand. 1979. Diaprepes abbreviatus response to light traps in field and cage tests. Florida Entomol. 62:136-139.
Beavers, J. B., T. P. McGovern and V. E. Adler. 1982. Diaprepes abbreviatus: Laboratory and field behavioral and attractancy studies. Environ. Entomol. 11:436-439.
Harari, A. R. and P. J. Landolt. 1997. Orientation of sugarcane rootstalk borer weevil, Diaprepes abbreviatus, to weevil, frass, and food odors. J. Chem. Ecol. 23:857-868.
Harari, A. R. and P. J. Landolt. 1999. Feeding experience enhances attraction of female Diaprepes abbreviatus (L.) (Coleoptera: Curculionidae) to food plant odors. J. Insect Behav. 12:415-422.
Jones, I. F. and W. J. Schroeder. 1984. Capture of Diaprepes abbreviatus (Coleoptera: Curculionidae in frass extract-baited traps in citrus. J. Econ. Entomol. 77:334-336.
Metcalf, R. L. and E. R. Metcalf. 1992. Plant kairomones in Insect Ecology and Control, Chapter 4, pp. 64-108. Chapman and Hall, New York, NY.
Pavis, C. 1989. Relations plante-insectes et insectes-insectes chez le charancon des agrumes Diaprepes abbreviatus L. (Coleoptera: Curculionidae). Proc. 25th Cong. Caribbean Plant Pathology Soc., Guadaloup. July 1989. 5pp.
Schroeder, W. J. 1981. Attraction, mating, and oviposition behavior in field populations of Diaprepes abbreviatus on citrus. Environ. Entomol. 10:898-900.
Schroeder, W. J. and I. F. Jones. 1983. Capture of Diaprepes abbreviatus (Coleoptera: Curculionidae) in traps: Effects of location in a citrus tree and wick material for release of the attractant. J. Econ. Entomol. 76:1312-1314.
Schroeder, W. J. and I. F. Jones. 1984. A new trap for capturing Diaprepes abbreviatus (Coleoptera: Curculionidae). Florida Entomol. 67:312-314.
Schroeder, W. J. and J. B. Beavers. 1985. Semiochemical and Diaprepes abbreviatus (Coleoptera: Curculionidae) behavior: Implications for survey. Florida Entomol. 68:399-402.

Table 1. Olfactometer attraction of Diaprepes root weevil to frass and foliage.*

Source	n	Females	Males
Female frass	250	44.8%	52.8%
Check		29.2%	20.8%
No response		26.0%	26.4%
Male frass	250	60.0%	48.0%
Check		24.8%	24.8%
No response		15.2%	27.2%
Citrus foliage	250	59.6%	52.8%
Check		28.8%	31.6%
No response		11.6%	15.6%
*Data from Beavers, McGovern and Adler, 1982.

Table 2. Summary of Diaprepes root weevil attractant research.

Sex	Attracted to	Bioassay	Reference
Male	530 nm light	Cage and field	Beavers et al., 1979
Male	Trees previously occupied by females	Cage	Schroeder, 1981
Female	Trees previously occupied by males	Cage	Schroeder, 1981
Female	Female frass	Laboratory olfactometer	Beavers et al., 1982
Male	Female frass	Laboratory olfactometer	Beavers et al., 1982
Female	Male frass	Laboratory olfactometer	Beavers et al., 1982
Male	Male frass	Laboratory olfactometer	Beavers et al., 1982
Female and male	Male and female frass extract	Field	Schroeder and Jones, 1983
Female and male	Male and female frass extract	Field	Jones and Schroeder, 1984
Male	Male and female frass extract	Cage	Schroeder and Beavers, 1984
Male and female	Male frass extract	Laboratory olfactometer	Pavis, 1989
Male and female	Female frass extract	Laboratory olfactometer	Pavis, 1989
Male and female	Immature lime leaves	Laboratory olfactometer	Pavis, 1989
Female	Female frass	Laboratory olfactometer	Harari and Landolt, 1997
Female	'Virgin' males	Laboratory olfactometer	Harari and Landolt, 1997
Female	Broken green beans	Laboratory olfactometer	Harari and Landolt, 1997
Male	Female frass	Laboratory olfactometer	Harari and Landolt, 1997
Male	Male frass	Laboratory olfactometer	Harari and Landolt, 1997
Male	Broken green beans	Laboratory olfactometer	Harari and Landolt, 1997
Female	Broken green beans, last food eaten	Laboratory olfactometer	Harari and Landolt, 1999
Female	Citrus flush, last food eaten	Laboratory olfactometer	Harari and Landolt, 1999

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