Biology of Lebia subgrandis Madge, A Natural Enemy of the Colorado Potato Beetle

I investigated the biology of the Mexican carabid, Lebia subgrandis Madge, a potential biological control agent of the Colorado Potato Beetle (CPB), Leptinotarsa decemlineata (Say). The consumption of CPB eggs and 1st through 3rd instar larvae increased with temperature for both male and female L. subgrandis. Under laboratory conditions, confined pairs consumed up to 108 CPB eggs / day at zs0 c (mean = 44.5 CPB eggs / day). Early-summer females produced more offspring than late-summer females. Apparently, mating was infrequent; I found no difference in oviposition rates when the females were confined with males for 0, 1, 2, or 3 days, or for the entire experiment. First instar L. subgrandis larvae lived an average of 8.3 days. They are ectoparasites of CPB prepupae and pupae and actively seek their hosts in the soil. Adult L. subgrandis seek their prey both day and night. The host range is narrow: previously starved or not, this species refused eggs and larvae of Coleomegilla maculata DeGeer, and eggs and nymphs of either Oplomus sp. or a reduviid predator. Adults lived four to five months. Reproductive capacity was temperature dependent. L. subgrandis might be considered as a candidate to control the CPB in the northeast U.S. .

To my wife,

FLOREN:
Our future is full of good things and we shall be sharing it always; and To my daughter,

ANAFWREN:
It is through the shining of your eyes that I have found the peace in my mind and my happiness.  America and Europe (Gimingham 1950, Gauthier et al. 1981, Casagrande 1987, Groden 1989, Hare 1990) and it has been the object of many control efforts.
Chemical controls. The first attempts to control the CPB used products like tobacco water, coal tar mixed with water, lime sulphur, ashes, and white hellebore.
These failed. The first reported success came with the discovery of the insecticidal properties of Paris green (copper acetoarsenite ). After 1865, Paris green was used widely (Casagrande 1987). Following Paris green, growers have used a wide variety of chemical insecticides (Gimingham 1950, Gauthier et al. 1981, Casagrande 1987.
The CPB has developed resistance to most of these chemicals (Cutkomp et al. 1958;Gauthier et al. 1981;Harris and Svec 1981;Forgash 1981Forgash , 1985Argentine et al. 1989;Hare 1990). Efforts to deter the CPB from feeding on potato foliage by using fungicides reduced CPB populations (Hare et al. 1983, Hare 1984.  reported that tannins extracted from white oak bark and leaves also inhibit the CPB from feeding on potato foliage. Cultural controls. Before Paris green, growers attempted to control the CPB by hand-picking, using pinchers, and through predation by turkeys (Casagrande 1987). Since 1980, attention has again focused on alternative management strategies. Casagrande (1987) and Hare (1990) reviewed cultural practices used to manage CPB populations. In the Soviet Union, Sorokin (1981) and Koval (1986) cited several studies concerning potato cultural that enhance populations of predatory carabids.
1 in Table 1, with selected references. Interest in biological control is reflected in the number of studies begun in the 1980's. Thus far, no single natural enemy has proven by itself to provide significant control of the CPB.
In the Soviet Union, the carabid beetles Carabus hampei, Poecilus cupreus, and Pterostichus melanarius have been studied as natural enemies of the CPB (Koval 1986). Sorokin (1981) recorded 14 species of carabids that feed on all stages of the CPB. He found that the most efficient species were Pterostichus cupreus, Ophonus rufipes, and Broschus cephalotes. However, there is not enough evidence that most of the agents mentioned might be used on a large scale to regulate CPB populations.
Most of the natural enemies investigated are limited in one way or another in their abilities to control the CPB.
Because Central Mexico is considered to be the evolutionary home of the genus Leptinotarsa (Tower 1906), several researchers have made field trips to this region to search for natural enemies suitable as biocontrols of the CPB. There are no reports of carabids from these trips (Logan et al. 1987). Galindo (1990) lists natural enemies of L. decemlineata collected within the municipality of Cuemavaca (northern Morelos). She does not report lebiine carabids in her paper, but repeats most of the predators and parasites previously reported by Cappaert (1989).
Beginning in 1987, extensive field studies in Mexico suggested the importance of ground beetles as possible biocontrol agents of the CPB. Cappaert (1989) surveyed predators and parasites of the CPB in the State of Morelos during 1987 and1988. He recorded seven species of carabids that feed on CPB eggs and larvae, but did not evaluate their potential for biological control. Little information is available about the biology of these species. In collections made from early May  Saminakova et al. 1981;Clark et al. 1982;Ignoffo et al. 1983;Campbell et al. 1985;Anderson et al. 1988Anderson et al. , 1989Loria et al. 1983;Watt and LeBrun 1984;Hajek et al. 1987. Cantwell and Cantelo 1981Cantwell et al. 1983;Ignoffo et al. 1982;Ferro and Gelertner 1989;Zehnder and Gelertner 1989. Hostounsk:y 1984. Drummond 1986, 1988Drummond et al. 1984a, 1988.  Jansson et al. 1987;Lashomb et al. 1987a,b;Ruberson et al. 1988. Tamaki et al. 1983aDrummond et al. 1984b, 1987. Tamaki and Butt 1978. Groden 1989;Hemenway and Whitcomb 1967;Chamboussou 1939;Trouvelot 1931. through mid-July in 1988, and late August 1989, I

Laboratory experiments
L. subgrandis beetles were sexed using the preapical notch on the mesotibia of males (Madge 1967 x 1 is the temperature, Xi is the stage of CPB offered as food, and x 3 is the sex grouping (i.e., males, females, pairs).

10
Host specificity of adult L. subgrandis. All trials were performed in 90 mm plastic petri dishes lined with moistened paper towel. I placed the dishes in a growth chamber set at 25°C at the greenhouse at URI and at room temperature (22-26°C) at the UAEM.
Coleomegilla maculata De Geer ( Coleoptera: Coccinellidae) is an important predator of the CPB in potato fields ). It was important to note any possible attack by the Mexican L. subgrandis on this coccinellid. For this purpose, I set up two petri dishes, each containing four starved L. subgrandis. One dish received 110 and the other 124 C. maculata eggs placed on segments of potato leaves. In a second trial, I offered newly emerged larvae of C. maculata to L.
subgrandis, placing 50 larvae per dish in two sets of dishes containing four starved carabids each. Consumption of C. maculata eggs was recorded after 72 hours, when most of the eggs started to hatch. I recorded the consumption of C. maculata larvae after 24 hours. I did not remove residues or add new food to the dishes during the trials, but added water to keep the dish moist.
In a second test, I provided six L. subgrandis with an excess of CPB eggs and larvae for three days. Then I took all intact CPB eggs or larvae and residues out of the dishes, and left the insects to starve for two to three hours. I then placed 102 C.
maculata eggs on segments of potato leaves in one dish with six carabids and recorded the insects' behavior for the next five hours. Later, I checked the dish regularly and kept moisture constant. After 24 hours, most of the C. maculata eggs hatched and I placed 50 of the new larvae on a clean dish; I offered the larvae to L.
subgrandis and observed the insects' behavior for 5 hours. Daily consumption was recorded for four days. I did not remove residues or add new food to the dishes or offer any other type of prey to L. subgrandis during this time.
In a third experiment, I offered 26 eggs or 20 newly hatched nymphs of I also offered 10 second instar nymphs of a reduviid CPB predator (Hemiptera: Reduviidae) to one set of two starved female L. subgrandis. The trial lasted less than 48 hours due to mortality of the nymphs. New nymphs were not added during this time. I removed dead nymphs, recorded consumption, and checked the dead nymphs for signs of carabid attack. Water was added to keep the dish moist.
Finally, L. grandis Hentz did well on a diet of aphids (Homoptera: Aphidae) (Groden 1989). Because this species is related to L. subgrandis, I also offered aphids to five pairs of the mexican carabid. The aphids were taken from S. rostratum plants kept in the greenhouse at URI. I recorded the number of aphids consumed by the carabids, removed the residues, and added more aphids during two days. It was not possible to conduct the test under these conditions for a long time due limited numbers of aphids; however, I was able to observe whether L.
subgrandis beetles would survive eating aphids which I offered for more than 2 months, when available. Seasonality of L. subgrandis fecundity, I. In this experiment, I tested the influence of season and mating status on fecundity. Individual fecundity was measured as the number of L. subgrandis first instar larvae produced per female. I used five mating categories, each using five females in individual dishes. The categories were: females confined with males for 1, 2, or 3 days, or for the entire experiment. A fifth group of females was never collflned with males. I offered CPB eggs, 1st, 2nd, or 3rd instar larvae in excess, depending on what was available at that time. The females stayed on the soil for 6 days, after which I moved them to new dishes containing clean soil. The soil was changed nine times for the "early summer" females, and five times for the "late summer" females. After every change, I kept the "old" soil at room temperatures (22-26°C) for 7 days of incubation. After 7 days, I emptied each petri dish and spread the contents on a paper towel. All active L.
subgrandis first instar larvae were collected and counted. The soil was moistened after checking and returned to the shelf for further incubation. I checked the dishes daily for 8 days. An Analysis of Variance (PROC GLM, SAS Institute 1990) was used to estimate the influence of season and mating status on the production of first instar larvae by female L. subgrandis.
Seasonality of L. subgrandis fecundity, II. This experiment was designed to test recently emerged female L. subgrandis for fecundity. I confined two female L.
subgrandis, caught in the emergence cages, with males (1 female and 2 males per dish). I kept three more isolated from the time they were captured in the emergence cages. The insects were placed in plastic petri dishes containing a 2 mm layer of Mexican soil. I offered CPB eggs, 1st, 2nd, or 3rd instar larvae in excess, depending on what was available at that time. The insects stayed on the soil for 6 days and then I moved them to new dishes containing clean soil. The soil was changed five times for females confined with males and four times for the ones that were never mated. I processed the soil following the same steps described above.
Survival of L. subgrandis first instar larvae. I used the larvae obtained in the fecundity experiments to estimate 1st instar longevity. Larvae were kept in plastic petri dishes lined with moistened paper towel. Twenty dishes were set up with a density of larvae that varied from day to day, from a minimum of 18 to a maximum of 246. I kept the dishes at room temperature ( Effect of plant hei1:ht. plant density. and foliar density on the incidence of L.
subgrandis. I found strong, positive regressions between the totals of adult L.
subgrandis over the totals of each CPB stage in all categories of Solanum sp. plants.
The coefficients of determination (R 2 ) for the linear regression analysis ranged from 0.703 to 0.982 (p = 0.001 to 0.018) ( Table 2). Ninety-six L. subgrandis were found on plants that were grouped and had more prey available. I observed four carabids in plants that were scattered beyond the plots. The incidence of adult CPB's varied similarly: In plants that stood isolated, I counted 28 adult CPB's throughout the study, in contrast to 554 adult CPB's when the plants were grouped (Table 3).
Ni1:htly activity of L. subgrandis. During the July 29 observation, L. subgrandis was less active at dusk, but its numbers increased before midnight. The numbers of carabids decreased sharply after midnight, apparently due to light rainfall (Fig. 2).
Once the rain ceased, I observed as many as eight individuals within my plot from 4 to 6 a.m. During the early morning ( 6 a.m.) the number of carabids decreased again (perhaps due to the dew on the plants). By mid-morning (9-10 a.m.) the number of L. subgrandis increased again. On the second night of observations,   Consumption rate usinK RI CPB. I found significant differences in the consumption rates of CPB's by L. subgrandis due to temperature, sex, and stage of CPB offered as food. The interactions were significant also (Table 4 ). In a simple effects test (Keppel 1982) for the interaction, sex/ CPB stage, I found that consumption rates were not different between stages at 13°C, but were significant at 16°C and above for all sex groupings eating and CPB stage ( one exception (Fig. 4a) where I observed no relation at all.
The mean consumption rate of CPB stages by L. subgrandis increased at warm temperatures (16 to 28°C), but it was low when temperature dropped to 13°C (Table 7). It is doubtful that L. subgrandis will consume any stage of CPB at temperatures below 13°C. It is also possible that differences in metabolism and reproductive physiology between female and male L. subgrandis may influence the consumption rate.
Host specificity of adult L. subgrandis. When offered C. maculata eggs, starved L. subgrandis approached the food immediately, just as they do when offered CPB eggs or larvae. Some tried to bite the eggs, then retreated. Others bit the surface of the leaf just around the base of the eggs. This kind of behavior was common initially, though later the beetles settled down and seldom approached the eggs. I recorded no consumption of eggs. Although some of them showed signs of having been "tasted," the chorion was intact. Also, no C. maculata larvae were consumed, and most died naturally with no signs of carabid attack.
When I used non-starved L. subgrandis, as soon as I placed C. maculata eggs into the petri dish, the beetles approached the eggs, as if they were not satiated by the previous abundant feeding. Then, they suddenly stopped as if they were "tasting" the area around the eggs. They ''bit" (or at least that was the motion they were performing) the leaf surface, touched the eggs, and left without eating. L.
subgrandis did not eat C. maculata larvae. The carabid did not even approach them.
By the end of the trial, almost all of the larvae were dead, yet none had been eaten.
When I offered pentatomid eggs or nymphs, the beetles did not even approach them; neither eggs nor nymphs were eaten. If offered reduviid nymphs as food, the    Seasonality of L. subgrandis fecundity, I. I found a strong significant difference in the oviposition rate between "early summer" and "late summer" L.
subgrandis (F[l,40) = 10.46, p < .05); the rate difference due to mating status was moderate (F[4,40] = 3.23, p < .05). It seems that mating need not be frequent to assure fertilization. "Early summer" females laid more eggs than those collected by the end of summer, possibly because a considerable number of "late summer" females were newly emerged. The interaction was not significant (F[ 4,40) = 0.63, p < .05). The TUKEY test for mating status ( dt = 4.03) showed that the number of first instar larvae produced by females confined 1 or 2 days with males was significantly different from the other conditions. However, the TUKEY test detected no difference between the conditions of 2 days or 1 day confinement with males.
Seasonality of L. subgrandis fecundity, II. Females obtained from the emergence cages (never confined with males) produced a mean of 44.8 first instar larvae; females also from emergence cages (never confined with males, produced 12.7 first instar larvae. It is possible that most of the females that emerged in the cages were overwintering individuals that mated before the onset of the dry season. Survival of L. subgrandis first instar larvae. I found that the first instar larvae of this carabid lived an average of 8.3 + 2.5 days (range 6 to 13 days) at room temperature (22 to 26°C).
Development of ovaries in L. subgrandis. I dissected 57 females in July, 1990.
Of 38 females from early July, 22 had eggs completely developed, and these eggs filled most of the insects' abdomens. The mean number of eggs was 34.3 (range 20 to 49, s.d. = 8.5). Two of the females had two and three eggs respectively left in the ovarioles, which looked shrunken. Seven of the females were immature (no eggs get developed), and six were mature (eggs developing). One female had eggs completely developed, but these were difficult to count because most of them were destroyed (with no apparent cause) and it was not possible to single them out.
By mid-July, of 17 females dissected, I recorded six females with eggs completely developed, with a mean of 54.0 eggs each (range 41 to 74, s.d. = 13.6).
Eleven females were immature. Of two females dissected in late July, in one female the eggs had started to develop, while one other was still immature.
Of six females dissected in mid-August, I observed that they had ovarioles which I considered "regressed." These ovarioles looked extremely loosened and were pale-brown or grayish in coloration. I assumed that these were females that had laid eggs recently.
The dissection of females was continued in Rhode Island with females collected in Mexico during the summer of 1990. In mid-October, I found that 12 females collected by the end of the summer had the ovarioles "regressed", while in early November one female had the ovarioles still active, showing eggs at different stages of maturation: Its end chambers were filled with 23 developed eggs.
However, no eggs were found on the lateral oviducts or on the common oviduct.
That is, there was no evidence that eggs were going to be laid soon. In mid-November, one "early summer" female had ovarioles that were very thin (thread-like) and difficult to distinguish from each other. However, the end parts of these egg chambers showed an amber coloration and tiny dark-brown marks at their very tip. It is at this time perhaps that "early summer" females become reproductively inactive, with no production of oocytes, which is why the ovarioles looked very thin. One female collected in the late summer had the ovarioles still active, with eggs at different stages of maturation, and the end of the chambers filled with 23 developed eggs. No eggs were observed in the lateral oviducts or on the common oviduct. In late November, the ovarioles of the females collected in the early summer were empty, shrunken, and with pale-yellowish coloration. In mid-December, five females from the late summer had the ovarioles "regressed." Overwinterin2 of adult L. subgrandis. All female and male L. subgrandis died by Februrary 15, 1991. I observed that some individuals may resist temperatures as low as 10°c, where they could still move but had almost no feeding. Miscellaneous observations. L. subgrandis may be confined in small groups under laboratory conditions. In the field, I have observed small gatherings of two or three insects sharing a hiding place. Sometimes, however, the crowding of the insects in containers may stimulate a defensive behavior. In this behavior, one of the beetles expels a pungent ammonia-like liquid (which immediately turns to gas), which can kill the other beetles in the container in several seconds. I do not know exactly what the conditions are for L. subgrandis to trigger this behavior, but one beetle can provoke a reaction by all of the other insects in the container, each expelling their own gases. In response, the insects appear to be agitated: they lift the elytra and vibrate the hind wings (although they do not attempt to fly), and run around the container with the elytra lifted. In one of my observations, one of the females expelled the gas, lifted the elytra, and appeared to be very agitated. When it faced the other three females in the container, although I expected a quick responce, there was none. If the container was not ventilated or the insects removed promptly, they would die in 30 to 60 seconds.
In spite of this defensive mechanism, sometimes I could see small gatherings of two or three insects sharing a hiding place. Also, under laboratory conditions, adults of L. subgrandis were observed to share prey (especially large thrid instar CPB larvae) without displaying any threatening behavior. Possibly this species does not exhibit strong territoriality, which is advantageous if several individuals have to search the same host plant for prey.
L. subgrandis depends largely on the immature stages of the CPB to complete its own development. Accordingly, synchrony with its prey appears to be an important characteristic. L. subgrandis can, to a certain extent, synchronize its emergence with the numbers of CPB present in the same area. L. grandis populations are well syncronyzed with the CPB in both Rhode Island and Michigan (Groden 1989). In Morelos, adults of L. subgrandis start to appear later in the season (early July) when adult L. decemlineata (which appear by early to mid-June) are abundant enough to produce sufficient eggs or larvae to sustain the carabid.
The dependency of L. subgrandis on CPB stages for food was so striking that when populations of the CPB became very low, the carabids were totally absent. I think that when this occurs, adult carabids will crawl or fly towards surrounding weeds to look for alternative prey during the day and night, until the numbers of CPB increase again. Generally, whenever CPff s became sparse in the field plots, I found most of the adult carabids in the weeds surrounding the plots, and none within the plots. There must also be an alternative prey available at nights, but I have not been able to determine any likely hosts.
The numbers of adult L. subgrandis fluctuated over time, but remained correlated with the density of CPff s regardless of plant size, foliar density, or plant density. As seen from the data, L. subgrandis prefers grouped plants which offer more places for hiding and attract more CPff s. On the other hand, isolated plants are more exposed to a wide array of other predators (including those that prey on L. subgrandis plus others that prey on CPB); this may be why CPB is also less frequent on isolated plants. Because L. subgrandis is a secretive insect that seldom flies, unless disturbed, it will seek places that offer more refuge from its enemies, shelter from midday sunlight and rain, higher probabilities of finding a mate, and adequate prey for adults, as well as CPB pre-pupae to support the first instar carabid larvae.
Thus, to seek clusters of the host plant is advantageous. Adults of the carabid will not move out of an area as long as the plants have an abundant canopy. Koval (1986)  Although L. subgrandis is primarily active during the daytime, significant numbers could be found active at night when weather was favorable (no rainfall).
Cappaert (1989) mentions that the day and night activity of several Lebinii species from Morelos is distinctive. On the other hand, the· R.I. native predator L. grandis is mostly nocturnal, and only a few individuals are active during the day (Groden 1989); burlap trap catches and night counts of L. grandis were also affected by weather conditions (high humidity and rainfall). As in the case of L. grandis, L subgrandis never was caught in pitfall traps, and when disturbed it dropped to the ground (it seldom flew away to the surrounding weeds). I never captured L.
subgrandis in the "aerial" pitfall traps. Because L. subgrandis is active both day and night, it is well suited to take advantage of more opportunities to prey and for longer periods of time. L. subgrandis avoids extreme midday heat by seeking refuge under stones, in small crevices close to the roots of the plants, in stems or under the leaves of surrounding weeds or in practically any site that offers some shade. It also can keep active during the hottest periods of the day if the CPB's host plant, Solanum sp., has a thick canopy, which in turn mitigates temperature extremes.
From the food preference tests, I could see that L. subgrandis did not eat fourth instar CPB larvae under laboratory of field conditions. This is differenct from L. grandis, which feeds on all CPB immature stages (Groden 1989). CPB eggs were the food consumed most frequently by L. subgrandis: The preference for eggs has clear advantages in any CPB biological control program. That is, a pair of adult L. subgrandis can consume 42.5 CPB's per day as eggs, but only 0.8 per day as third instars (room temperatures).
Groden (1989) mentions that under laboratory conditions L. grandis showed significant difference in consumption of eggs over larvae (but no specific preference for any particular prey stage). However, the difference she mentions is among proportions of food items, which can mask a real preference. In later experiments I expect to establish an actual preference or choice for food based on the biomass of the prey (caloric content or nutritional value), densities of prey and predator, as well as the searching capacity of the predator.
In the field, whenever a L. subgrandis adult finds an acceptable prey of any stage, it will remain stationary, eating until the whole prey is consumed. Cappaert (1989) observed that "Lebia beetles" (Lebia sp. and Callida sp. together) readily accepted CPB eggs during all his trials. The mean consumption in his study was 20.0 eggs (range 9 to 37, s.d. = 1.9, 10 carabids). Apparently, the maximum impact of Lebia species on the CPB, including L. subgrandis evaluated here, is on the egg stage (Groden 1989, Cappaert 1989.
Temperature had a prominent effect upon the consumption of RI CPB by L.
subgrandis: Consumption of CPB immature stages increased with temperature over the range 13 to 28°C. While the consumption of L. subgrandis followed a linear trend between 13 and 28°C, that of L. grand.is was quadratic between 15 and 30°C (Groden 1989 A better understanding of this predator's biology will allow laboratory production of this species using inexpensive methodologies, for purposes of animal release. To date, I have made no attempt to release or evaluate this species in field conditions in Rhode Island.
Although L. subgrandis refused to prey on species other than L. decemlineata under field conditions, it consumed aphids when offered, and lived on aphids for 67 days in the laboratory. L. subgrandis did not accept any other kind of prey from the five species I offered.
The carabids displayed an interesting behavior when trying to recognize the type of food I put in the dishes. I do not know whether chemicals are used for recognition of the prey or the host plant by the predator. I did, however, observe L.
subgrandis apparently biting or "tasting" the surface of potato or Solanum sp. leaves before approaching the prey I offered. Also, the insects approached the prey, then stopped, recognized, circled, and left without consuming anything.
Although I will have to try more species of beneficial insects as prey, our preliminary results on host specificity were promising. The R.I. native L. grandis (Groden 1989) appears also to be very specific to CPB. Although L. grandis too is able to feed on aphids in the absence of CPB prey, it will not feed on aphids when CPB's are present.
Females from the early summer were able to produce more offspring than females from the late summer; the mating status was not significant in the production of first instar larvae. I think that the differences I found due to mating status are related to conditions other than length of time the female L. subgrandis were confined with males. I had females that were never confined with males (i.e., they presumably had mated the previous season) that produced more offspring than the females that were confined with males for 3 days. Also, females from the emergence cages isolated since the first day of capture laid viable eggs. Possibly, females from the late summer can mate but, due to dry conditions in the environment, they stop laying eggs and enter a reproductive diapause. They may then enter the soil until conditions are favorable the next year; these females may resume laying eggs the next rainy season. Under laboratory conditions, however, females from the late summer kept in a growth chamber at 25°C, a photoperiod of L:D 16:8, and enough food, continued laying eggs until December 4th, 1990.
Groden (1989) mentions that L. grandis summer adults will oviposit until the following summer. Possibly, she refers to late summer adults. Also, when Hemenway and Whitcomb (1967) could not get L. grandis to oviposit after several attempts in the lab, perhaps they had collected prediapausing adults. Prediapausing adults of L. subgrandis in this study and L. grandis (Groden 1989), can keep active (and still feed on CPB), but will reduce oviposition to a minimum (L. subgrandis) or will stop oviposition (L. grandis [Groden 1989]). Both species will then overwinter as adults and will resume oviposition the following summer.
The changes in the ovaries of female L. subgrandis I observed are related to the oviposition I discussed above. It was during July (females from early summer) that I noted the major production of eggs, although by late July the production of eggs diminished. Most of the females collected in late summer (August and September) had their ovaries "regressed," though some were actively laying eggs until late in the Fall. This is compatible with an hypothesis that females from the late summer might still be mating and, under field conditions, they will retreat to their hiding places already inseminated. Similarly, by late fall, females from the early summer had stopped all reproductive activity.
Although the average survival time for L. subgrandis first instar larvae (8.3 ..±. 2.5 days) was longer than that of L. grandis (4.01 ..±. 0.14 days) (Groden 1989), the rate of mortality was high by the fourth and fifth days under room conditions. To enhance rearing for this predator, keeping the larvae alive for longer periods of time will be important. L. subgrandis first larvae are strict ectoparasites of CPB pre-pupae and pupae and until an artificial diet is developed, we will be depending on the production of CPB pre-pupae and pupae to rear L. subgrandis. Further studies will be needed to assess the influence of time on the ability of L. subgrandis first instar larvae to parasitize their host.
L. subgrandis is a subtropical species which will be very difficult to adapt to cold climates. In the early spring of the Northeast, L. grandis may forage and be reproductively active at temperatures below 20°c (Groden 1989). L. subgrandis, however, had a low CPB prey consumption at 16°C, and it could barely eat and move at 13°C and below. It is doubtful that this species will be capable of any reproduction at low temperatures.
Results concerning the survival of adult L. subgrandis captured in the field are not conclusive at all. Some of the adults used in the experiment were of unknown age. This could be true also for the adults captured in the emergence cages, because as I said, they could have been buried in the soil during the previous dry season.
However, the fact that some adults may live up to 3 or 4 under lab conditions, improves chances for reproduction of this carabid in confinement. Later studies with adults emerged in the lab will allow a better measure of adult L. subgrandis lifespan.
I investigated the biology of the Mexican carabid Lebia subgrandis Madge, a potential biological control agent of the CPB. I set up field and lab investigations in Mexico in the summer of , and lab investigations in Rhode Island in 1988 subgrandis had two periods of emergence: early July and mid August. The incidence curves between adult CPB's and adult L. subgrandis were very similar: Predators were numerically correlated with adult CPB's. When the populations of CPB became very low, the carabids were totally absent within the plots, but not in the surrounding weeds, where I found them easily.
There were also strong positive regressions between the numbers of adult L.
subgrandis and the totals of each CPB stage for all categories of Solanum sp. plants.
The number of predators fluctuated over time but remained correlated with adult CPB's regardless of plant size, foliar density, or plant density. L. subgrandis prefers grouped plants, which offer diversity in microhabitats and attract more CPB's.
L. subgrandis is active day and night if the weather is favorable (no rainfall). It avoids the hottest hours of the day, unless the host plants have a thick canopy that mitigates the high temperatures.
L. subgrandis did not eat fourth instar CPB larvae. CPB eggs were more frequently consumed than CPB larvae (1st through 3rd). Pairs of L. subgrandis consumed more than the combined consumption of individual females and males.
The preference for eggs is advantageous. However, actual preference should be proved based on caloric content or nutritional value of the prey.
The consumption of CPB immature stages by L. subgrandis is temperature dependent: the higher the temperature, the more CPB's consumed for all temperatures between 13 and 28°C. The consumption rate followed a linear trend between 13 and 28°C for all CPB stages and L. subgrandis sex groupings, with the exception of the males, where consumption of CPB eggs showed no relation to temperature. This carabid may adapt to summer Rhode Island temperatures, but it is not clear that it will survive the winters of the Northeast.
Starved and non-starved L. subgrandis ate neither eggs nor larvae of C.
maculata, an important predator of CPB in the potato fields in Rhode Island. There was also no consumption of eggs or nymphs of Oplomus sp., nor nymphs of a reduviid predator of the CPB. The carabids display an interesting behavior when offered prey other than CPB but generally reject such prey. However, this species accepted a diet of aphids. Further studies will include more species of beneficial insects to prevent any harm from future field releases of the predator.
Summer females produce more offspring than fall females; the mating status was not important. It is possible that mating occurs once, and females are able to store sperm in a spermatheca. Females collected in late summer are able to mate and produce mature eggs (as I observed them in dissected females) but they lay less or none during dry periods in the environment. At the onset of dry periods, females from late summer may enter a reproductive diapause, enter the soil, and resume laying eggs in the next year's rainy season.
A long lifespan will be important to rear the predator under lab conditions. Survival of adult male or female L. subgrandis captured in the field is variable.
Females live an average of 78.0 days and males live 79.4 days under lab conditions.
L. subgrandis is a good candidate to be considered as biological control agent of the CPB in the Northeast U.S. But it is necessary to better understand its biology,