Scientific Notes 391

PATTERNS OF ACTIVITY ON STRICTA (CACTACEAE), A NATIVE HOST- OF THE INVASIVE MOTH, (LEPIDOPTERA: PYRALIDAE)

MEREDITH ROBBINS AND TOM E. X. MILLER* Department of Biological Science, Florida State University, Tallahassee, FL 32306-1100

*Corresponding author and present address: Tom Miller, Department of Ecology and Evolutionary Biology, Rice University, Houston, TX 77251-1892

Many ant species defend against herbi- was conducted in spring/early summer, a period of vores in exchange for extrafloral (EFN), an overlap between cactus moth oviposition (S. Le- example of facultative mutualism. While protec- gaspi et al., personal communication) and O. tion by against native is well doc- stricta EFN production (M. Robbins, pers. obs.). umented (Bronstein 1998), less is known about As in many ant-tended cacti, EFN production by the interactions between plant-tending ants and O. stricta is concentrated near the areoles of de- exotic herbivores. Studies of these interactions veloping cladodes and flower buds. We therefore are needed in order to determine whether mutu- expected differences in ant activity between alism between resident ants and plants can con- young cladodes (identified by the presence of true fer ecological resistance to invasion by exotic her- ) and older, lignified cladodes. Data were bivores. Here, we report patterns of ant visitation collected during censuses at 7-10-d intervals to Haworth (Haworth), an EFN- (18 Mar-1 Jun, 2008) from 50 randomly selected producing cactus, native to Florida that is vulner- plants in an O. stricta population located within able to attack by Cactoblastis cactorum Berg the St. Mark’s National Wildlife Refuge (Wakulla (Lepidoptera: Pyralidae), an invasive exotic her- Co., FL). For each plant on each census date, we bivore. recorded worker ant abundance during a 30-s ob- Cactoblastis cactorum is native to servation period, ant location (young or old pads), and was intentionally introduced to a wide array and numbers and locations of moth eggsticks. Ant of countries and continents for the biological con- observations were made between 9 AM and 12 trol of weedy cacti in the genus Opuntia (Zimmer- PM, and our data set included a total of 500 obser- man et al. 2004). The moth unexpectedly arrived vation periods. Cactoblastis cactorum and the re- in the Florida Keys in 1989 (Habeck & Bennett lated, native cactus moth Melitara prodenialis 1990). Since then, the moth has spread along the Walker (Lepidoptera: Pyralidae), both occurred at Atlantic and Gulf coasts (Hight et al. 2002), our study site, lending the opportunity to com- where the larvae attack native Opuntia spp. pare the 2 moth species. Thus, C. cactorum, a classic success story of bio- We found that ants visit O. stricta within the logical control, is now a threat to native and culti- North American invasive range of C. cactorum. vated cacti throughout (Simonson We observed a total of 408 ant workers on O. et al. 2005). There is an urgent need for informa- stricta, with a mean ± SE of 1.05 ± 0.13 ants per tion on factors that could limit the invasive suc- plant per observation period. The ant assemblage cess of this species (Mahr 2001). associated with O. stricta at our study site con- Many cacti attract ants with EFN and ant vis- sisted of 4 species (Table 1; species identification itation can confer protection against cactus herbi- confirmed by Dr. Joshua King, Florida State Uni- vores (Oliveira et al. 1999; Miller 2007). Little is versity). The most abundant species, Brachy- known about interactions between C. cactorum myrmex patagonicus (Mayr), is non-native and EFN-feeding ants. Female moths deposit egg- (MacGowan et al. 2007), while Pheidole dentata sticks (stacks of 50-90 eggs) on the surfaces of cla- (Mayr), Forelius pruinosus (Roger), and Dory- dodes (cactus pads), where they are vulnerable to natural enemies. Research in South , where the moth was intentionally released, has impli- TABLE 1. ANT SPECIES AND WORKER RELATIVE ABUN- cated ants in the high mortality of C. cactorum DANCES OBSERVED ON OPUNTIA STRICTA AT eggs (Robertson 1984). However, no prior studies ST. MARK’S NATIONAL WILDLIFE REFUGE, have documented ant activity on susceptible host- WAKULLA CO., FL. plants in the moth’s North American range. Our objectives were to (1) determine if and Species Relative abundance which species of ants collect EFN from O. stricta, Brachymyrmex patagonicus 0.44 (2) describe within-plant spatial patterns of ant Pheidole dentate 0.39 activity, and (3) contrast patterns of ant activity Dorymyrmex bureni 0.09 with locations of moth oviposition to evaluate the Forelius pruinosus 0.08 potential for ant-moth interactions. Our study

392 Florida Entomologist 92(2) June 2009

myrmex bureni (Trager) are native to the South- occurrence between the invasive and native east U.S. All species except F. pruinosus were ob- moths reflects a difference in female oviposition served collecting nectar from cactus extra-floral preference or a difference in post-oviposition egg nectaries. removal/predation. However, a separate study We compared the within-plant distribution of showed that eggstick survival rates were equal on ants with the expected distribution based on young and old cladodes (Miller, unpublished numbers of young and old cladodes on our study data), and so we suspect that the difference in oc- plants. Pooled across species, ants occurred more currence reflects a difference in female prefer- frequently on young cladodes, where extrafloral ence. nectaries are located, than expected by chance The ant activity on O. stricta documented here (Fisher’s Exact test: P < 0.001). Thus, young cla- indicates potential for resident ants to act as dodes may be better defended than old ones. How- plant mutualists and limit the success of C. cac- ever, partitioning the data by ant species revealed torum. However, at least 3 factors might influence interesting differences (Fig. 1A). The two most the effectiveness of ants as agents of ecological re- abundant species, B. patagonicus and P. dentata, sistance. First, the combination of ant and moth occurred most frequently on young cladodes occurrence data suggests that the low overall ant (Fisher’s Exact tests: P < 0.001, P < 0.002), respec- activity on old cladodes may provide a spatial ref- tively, while F. pruinosus did not deviate from the uge for successful C. cactorum oviposition and egg random expectation (P = 0.68); we did not have development. In fact, our data suggest that ants sufficient data for similar analysis of D. bureni. may be more likely to encounter the native moth Both C. cactorum and M. prodenialis deposited than the invasive moth. Second, ant species dif- eggsticks on our study plants. As above, we com- fered in their relative abundances and frequen- pared the locations of eggsticks to the null expec- cies of occurrence on young versus old cladodes. tation. This analysis included a total of 15 C. cac- Consequently, C. cactorum and M. prodenialis torum and 27 M. prodenialis eggsticks. Eggsticks may interact with different subsets of the cactus- from C. cactorum eggsticks occurred on old vs. tending ant assemblage. Our results indicate a young cladodes more frequently than expected need for experimental studies that quantify vari- (Fig. 1B), though the deviation was only margin- ation among ant species in their protective abili- ally significant (Fisher’s Exact test: P = 0.08). In ties and the net effects of ant visitation on C. cac- contrast, M. prodenialis eggsticks occurred most torum population growth. Finally, C. cactorum is often on young cladodes, where overall ant activ- known to have 3 oviposition periods per : Apr, ity was greatest (P < 0.001; Fig. 1B). These data Aug, and Oct (S. Legaspi et al., personal commu- do not indicate whether the difference in eggstick nication). Our observations suggest that EFN

Fig. 1. Relative frequencies of (A) ant workers and (B) moth eggsticks on young (black) and old (white) Opuntia stricta cladodes, compared to the expected frequencies based on cladode relative abundance. Stars indicate devia- tions from expected frequencies based on Fisher’s Exact tests (*P < 0.1, **P < 0.001).

Scientific Notes 393 production by O. stricta is seasonal, with a peak panding geographic range of Cactoblastis cactorum during the Apr oviposition period. Thus, if ant ac- (Lepidoptera: Pyralidae) in North America. Florida tivity is motivated by EFN, then 2 of the 3 cactus Entomol. 85: 527-529. moth generations per year may have limited in- MACGOWAN, J. A., HILL, J. G., AND DEYRUP, M. A. 2007. teractions with ants. Given the threats posed by Brachymyrmex patagonicus (Hymenoptera: Formi- cidae), an emerging pest species in the southeastern C. cactorum, further investigations into these United States. Florida Entomol. 90: 457-464. sources of variation in ant-plant-moth interac- MAHR, D. L. 2001. Cactoblastis cactorum (Lepidoptera: tions are warranted. Pyralidae) in North America: a workshop of assess- ment and planning. Florida Entomol. 84: 465-473. SUMMARY MILLER, T. E. X. 2007. Does having multiple partners weaken the benefits of facultative mutualism? A test We present evidence for interactions between with cacti and cactus-tending ants. Oikos 116: 500- cacti and cactus-tending ants within the invasive 512. range of C. cactorum. There is potential for ant- OLIVEIRA, P. S., RICO-GRAY, V., DIEZ-CASTELAZO, C., plant mutualism to limit the success of this invasive AND CASTILLO-GUEVARA, C. 1999. Interactions be- . However, significant within-plant varia- tween ants, extrafloral nectaries, and insect herbi- tion in both ant and moth activity may influence the vores in Neotropical sand dunes: herbivore deter- rence by visiting ants increases set in Opuntia frequencies and outcomes of ant-moth interactions. stricta (Cactaceae). Functional Ecol. 13: 623-631. This study lays the groundwork for subsequent re- ROBERTSON, H. G. 1984. Egg predation by ants as a par- search on ant-cactus interactions that could contrib- tial explanation of the difference in performance of ute to control strategies against C. cactorum. Cactoblastis cactorum on cactus weeds in South Af- rica and , pp. 83-88 In Proc. VI Symp. Biol. REFERENCES CITED Contr. Weeds, Vancouver. SIMONSON, S. E., STOHLGREN, T. J., TYLER, L., GREGG, BRONSTEIN, J. L. 1998. The contribution of ant-plant W. P., MUIR, R., AND GARRETT, L. J. 2005. Prelimi- protection studies to our understanding of mutual- nary assessment of the potential impacts and risks ism. Biotropica 30: 150-161. of the invasive cactus moth, Cactoblastis cactorum HABECK, D. H., AND BENNETT, F. D. 1990. Cactoblastis Berg, in the U.S. and . Intl. Atomic Energy cactorum Berg (Lepidoptera: Pyralidae), A Phycitine Agency, Vienna, Austria. New to Florida. Entomology Circular 333, Florida ZIMMERMAN, H. G., BLOEM, S., AND KLEIN, H. 2004. Bi- Dept. Agric. Consumer Services, Div. Plant Indus- ology, history, threat, surveillance and control of the tries. cactus moth, Cactoblastis cactorum, pp xxx In Joint HIGHT, S. D., CARPENTER, J. E., BLOEM, K. A., BLOEM, FAO/IAEA Programme of Nuclear Techniques in S., PEMBERTON, R. W., AND STILING, P. 2002. Ex- Food and Agriculture. IAEA, Vienna, Austria.