Egg Guarding by Male Assassin Bugs of the Genus Zelus (Hemiptera: Reduviidae).
Psyche 84(1):103-107, 1977.
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EGG GUARDING BY MALE ASSASSIN BUGS OF THE GENUS ZELUS (HEMIPTERA: REDUVIIDAE)
BY J. SCOTT RALSTON*
Western Carolina University
Cullowhee, North Carolina 28723
Sporadic accounts of parental care of offspring in the Hemiptera- Heteroptera have appeared in the literature and are summarized by Hussey (1934) and Odhiambo (1959). In all but one or two of these cases only the female guards the brood; the only well documented exception is the reduviid Rhinocaris albopilosus in which only males guard the broods (Odhiambo, 1959). The present paper is a summary of observations on the form and function of brood guarding behavior of a reduviid species (Zelus sp.) in which males guard the brood. My observations were made during January and February 1975, in the vicinity of Cali, Colombia in a dry tropical forest zone (Espinal and Monte- negro, 1962). There the bugs are common in the outer branches of Pithecelobium duke (Leguminosae), a tree locally known as the "chiminango." During the course of this study I observed approx- imately 60 different males with broods.
Egg Structure and Placement
The cylindrical, dark brown eggs occur in tight masses, with about 5 to 15 eggs in each mass. The eggs are about 0.3 mm in diameter, about 1.2 mm long and are attached to a branch or leaf by one end. Each egg projects from the substrate at a right angle and is in direct contact with other eggs in the mass. Freshly laid eggs are brown with fine, cream-colored seals at their exposed ends. Older unhatched eggs have darker seals and are partly covered with a slimy substance. Hatched eggs lack the seal, which is broken as the nymph leaves the egg.
*Present address: Department of Biochemistry, N.C. State University, Raleigh, N.C. Manuscript received by the editor August 8, 1977.
Brood Guarding Behavior
Generally the female deposits more than one egg mass in a sniall area and this appears to be the number of egg niasses a male will guard. I observed individual males guarding as few as one to as many as seven egg masses. The niale (body length 9-10 mni) usu- ally assumes a guarding position directly over the egg niasses or will stand not more than 3-4 cni from the nearest egg mass. Two simple tests were conducted to compare the behavior of guarding and non-guarding (not positioned near any egg mass) niales. In the first test, which may have simulated the approach of a large predator, I passed my hand within about 10 cni of each niale. Thirteen guarding niales were tested; one took flight and the others simply dodged to one side to avoid my hand. With re- peated passes of niy hand no escape behavior other than dodging was triggered. Seventeen non-guarding males were similarly tested: nine flew away; another five dodged to avoid niy hand at first but with one or two repeated passes they too flew away; the remaining three only dodged to avoid niy hand. In the process of grasping 44 guarding niales for marking or removal during another part of this study, I found that none took flight; the only reaction was to dodge my hand. Application of the chi-square test to these data shows that guarding males are significantly (p < 0.01) less likely to flee from a potential predator than are non-guarding males. The second test involved a model parasite made of a bit of black tape attached to the end of stiff nylon line on the end of a long hollow glass tube. I carefully presented the model parasite so that the male would only perceive the model. Of the ten guarding males tested, one avoided the model completely, one exhibited no reac- tion, and seven others readily attacked the model by grasping it with the forelegs. The attacks appeared especially aggressive when the model came close to the egg masses. The tenth male, which was about 7 cm from his egg masses, avoided the model when it was brought near him. However, when the model approached the egg masses the male was guarding, he rapidly moved to attack it. The non-guarding males were less ready to attack the model wasp. Of eight non-guarding males tested, only one readily attacked the model. Four males avoided the model parasite but when touched by it they did attack. The other three non-guarding males avoided the model completely. The chi-square test results show that there
19771 Ralston - Egg Guarding by Zelus 105 was a significant (p< 0.01) difference between the guarding and non-guarding males in readiness to attack the model parasite. After hatching, the nymphs appeared to stay very close to the eggs and guardian male for several days (a maximum of seven days in one case). In one instance in the field I watched a male capture a small insect about 15-30 cm from the egg masses where the nymphs were gathered. He promptly returned to the nymphs with the insect and the nymphs fed on it. I attempted unsuccessfully to repeat this observation in the lab with a few males, several nymphs, and tiny insects. Dr. William Eberhard (personal communication) reports seeing recently (February 1977) a guarding male holding a prey with a cluster of nymphs gathered around apparently sucking the prey.
Rates of Parasitism
On January 28, I finished marking 22 males guarding egg masses (control group) as well as the sites of the egg masses. The males were marked with white airplane dope on the anterior dorsal sur- face of the thorax. This did not appear to interfere with their normal functioning. I also removed 22 males from the masses they were guarding and marked the sites of the egg masses (experi- mental group). Each tree in which I made the study contained both control and experimental groups distributed roughly at ran- dom. However, if different groups of egg masses were very close together, I labeled them all control or all experimental so that the presence of a nearby male would not affect parasitism of unguarded eggs. The purpose of this experiment was to see if the presence of the guarding male affected the rate of parasitism of the eggs. On February 20, I collected 57 egg masses from the marked sites with guarding males and 63 egg masses which had been left un- guarded. Of the 57 guarded egg masses, 12 (21%) had been para- sitized (distinguished by eggs with unbroken seals and tiny exit holes near their attached ends); whereas 35 (55%) of the 63 unguarded egg masses had been parasitized. A chi-square test shows that the difference between the rate of parasitism of unguarded and guarded egg masses is statistically significant (p < 0.01). Five tiny wasp parasites of the genus Telenomus (Scelionidae) hatched from one egg mass in the experimental group soon after it was collected.
Odhiambo (1959) points out that broodguarding Hemiptera ex- hibit a strong tendency to remain with egg masses when they are disturbed in ways that in other situations would cause them to take flight. I found this to be true for Zelus males. A few direct observations and some circumstantial evidence in- dicate that broodguarding in Hemiptera helps to protect eggs from egg parasites (Odhiambo, 1959). I have tested this hypothesis for Zelus in two ways and the results of both tests strongly support the hypothesis. There is no evidence, such as that found by Eberhard (1975) in his study of egg guarding by pentatomid bugs, that the guarding male makes the eggs more vulnerable to certain parasites. My observation and an observation by Eberhard show that Zelus parental care continues during early nymphal life in the form of feeding and perhaps protection.
Further study of Zelus broodguarding should be directed towards answering questions such as the following: What is the genetic rela- tionship between the males and the eggs which they guard? I would predict that the guarding male is the genetic father of at least some of the eggs he guards; otherwise there would be little or no selective advantage in the guarding behavior. How common is the parental feeding which was observed? Exactly how do the males react to real parasites and predators? How does the number and distribu- tion of egg masses affect egg rearing efficiency? The author wishes to extend his thanks to the following: Dr. William Eberhard, University del Valle, Cali, Colon~bia, for his guidance and suggestions during the course of this study; Dr. Fred- erick Coyle, Western Carolina University, for helping to arrange this study and for his interest and help in writing this paper; J. L. Herring, U. S. Department of Agriculture, Beltsville, Maryland, U.S.A., for identifying Zelus; and P. M. Marsh, U. S. Department of Agriculture, Beltsville, Maryland, U.S.A. for identifying Tele- nomus.
EBERHARD, W. G.
1975. The ecology and behavior of a subsocial pentatomid bug and two scelio- nid wasps: strategy and counter-strategy in a host and its parasites. Sn~ithsonian Contrib. 2001. 205: 1-39.
19771 Ralston - Egg Guarding by Zelus 107 ESPINAL, L. S. AND E. MONTENEGRO.
1962. Formaciones Vegetales de Colon~bia. Institute Augustin Codazzi, Bo- gota.
HUSSEY, R. F.
Observations on Pachycoris torridus (Scop.), with remarks on parental care in other Hemiptera. Bull. Brooklyn Ent. Soc. 29: 133-45. ODHIAMBO, T. R.
1959. An account of parental care in Rhinocoris albopilosus Signoret (Hem- iptera-Heteroptera: Reduviidae), with notes on its life history. Proc. Royal Ent. Soc., London. (a) 34: 175-185. REPRINTS OF ARTICLES BY W. M. WHEELER. -The Cambridge Entomological Club has available for distribution numerous re- prints of articles by Professor W. M. Wheeler. These were stored in Professor Wheeler's office at Harvard University at the time of his death in 1937 and they have recently been turned over to the Cambridge Entomological Club by Dr. Ralph Wheeler and Miss Adeline Wheeler. Included are about 12,700 individual copies of 250 publications by Dr. Wheeler.
In accordance with a vote of the society in April of this year, a committee has been appointed to administer the distribution of the reprints. The price of the reprints has been set at the rate of 5(1; a page (including postage); for orders under $5 there will be in addition a handling charge of 50~. A list of the reprints is avail- able for $1.00 from the W. M. Wheeler Reprint Con~n~ittee, Cam- bridge Entomological Club, 16 Divinity Ave., Cambridge, Mass. 02138. Checks should be made payable to the Cambridge Ento- mological Club. - F. M. Carpenter, editor.
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