Jacobs Journal of Agriculture

Abundance of Liriomyza trifolii (Diptera: Agromyzidae) and its Parasitoids on Five Vegetable Crops in Southern Florida

*Dakshina R Seal
Department Of Agriculture, Tropical Research And Education Center, 18905 SW 280th St, Homestead, FL 33031, United States

*Corresponding Author:
Dakshina R Seal
Department Of Agriculture, Tropical Research And Education Center, 18905 SW 280th St, Homestead, FL 33031, United States
Email:dseal3@ufl.edu

Published on: 2018-07-26

Abstract

In southern Florida and elsewhere, the American serpentine leafminer, Liriomyza trifolii (Burgess) (Diptera: Agromyzidae), is one of the most destructive pests of snap beans Phaseolus vulgaris L. (Fabaceae), cucumbers Cucumis sativus L., squash Curcurbita pepo L. (both Cucurbitaceae), and tomatoes Solanum lycopersicum L. (Solanaceae). In tests with these crops and with cabbage, Brassica oleracea L. (Brassicaceae), we determined and compared numbers of individuals in different L. trifolii life stages, proportions of the larvae surviving to adulthood, and numbers of parasitoids. Highest numbers of L. trifolii mines, larvae, pupae, adults, and parasitoids were typically found on snap beans followed by cucumbers or squash, then often tomato, and tended to be lowest on cabbage. Beans appeared to be the most preferred crop with cabbage the least preferred. The most abundant parasitoid species was Opius dissitus Muesebeck (Hymenoptera: Braconidae), and adult parasitoid abundance seemed to generally depend on the abundance of its host, L. trifolii. For the means of dates in Tests 1 and 2, however, a higher percentage of the larvae survived to adulthood on cabbage than on the other crop species, and for means of dates in Test 1, a smaller percentage of larvae became adults on beans than on cabbage or tomato plants. A consistent occurrence with different life stages of L. trifolii on beans was that fewer insects were found on the last sample date compared with the initial three dates, and for the other crops, fewer insects appeared on the first or final on than the second or third sample dates.

Keywords

Leafminer; Snap Bean; Cucumber; Squash; Tomato; Cabbage

Introduction

Florida produces the highest acreage in the United States of snap beans, Phaseolus vulgaris L. (Fabaceae), with 44% of the total, and Miami-Dade County has the highest acreage among the 67 counties in Florida [1]. In addition to snap beans, four other commonly grown crops in southern Florida include cucumber Cucumis sativus L., squash Curcurbita pepo L. (both Cucurbitaceae), tomato Solanum lycopersicum L. (Solanaceae), and cabbage Brassica oleracea L. (Brassicaceae). The American serpentine leafminer Liriomyza trifolii (Burgess) (Diptera: Agromyzidae), is one of the most destructive insect pest species attacking snap beans and is also a major or secondary pest of the other four crops [2,3]. The larvae cause major damage by tunneling between the upper and lower leaf surfaces and feeding on the mesophyll cells [1,4]. High L. trifolii infestations reduce the rates of photosynthesis and yields therefore potentially leading to serious economic losses [5,6]. Liriomyza trifolii feeds on more than 400 cultivated and wild plant species in more than 25 families and is found in tropical, subtropical, and temperate regions of North, Central, and South America and the Caribbean, Africa, Asia, Oceana, and Europe [4,7,8].The L. trifolii adult is a small fly 2.5-3 mm long with a yellow abdomen, and it oviposits in feeding punctures on the upper surfaces of leaves [9]. The larva excavates a meandering mine, 

which enlarges as it grows, and after completing development, it exits the mine and drops to the ground to pupate [9]. Similar to many other pest insect species, the most ideal temperature range for L. trifolii growth and development is 20 - 30 °C, and temperatures above 35 °C are lethal [10,11]. Depending on temperature, L. trifolii completes development in 21 - 28 days [10,12,13]. In Florida, L. trifolii generations are continuous throughout most of the year, and because the larvae are protected within leaf tissues, successful chemical control requires careful timing [1]. Because of these attributes, the polyphagous feeding habits, and its ability to develop resistance to chemical insecticides, L. trifolii poses a major threat to vegetable production. On snap beans in Florida, silverleaf whiteflies (Bemisia argentifolii Bellows & Perring) and melon thrips (Thrips palmi Karny) are the main pests. Liriomyza trifolii is a secondary pest that occasionally becomes a major pest [1] especially on healthy plants with low B. argentifolii infestations. Greatest damage to beans occurs on the first two (primary) leaves of young seedlings before the appearance of true leaves [1]. Although L. trifolii damage is often highly visible, healthy bean plants can tolerate considerable damage without yield loss [1]. On cucumbers, the major pests include T. palmi, melonworms Diaphania hyalinata L., pickleworms D. nitidalis Stoll (Lepidoptera: Crambidae), and spider mites such as Tetranychus urticae Koch (Arachnida: Acari: Tetranychidae) [14, 15]. Similar to snap beans, L. trifolii is occasionally a major pest on cucumbers and squash in southern Florida [9]. Mossler & Nesheim [16] reported that in Florida, L. trifolii is a principal pest of squash along with B. argentifolii, D. hyalinata, D. nitidalis, squash bugs Anasa tristis (DeGeer) (Hemiptera: Coreidae), and aphids. On cucumbers and squash, L. trifolii infestation can be more severe late in the growing season, when it may defoliate these crops. Sun-scalding of fruit may also occur, particularly if the adults migrate from nearby crop residues onto the current crop [9]. As on snap beans, B. argentifolii is a major pest of tomato plants causing uneven fruit ripening and readily transmitting gemini viruses [17]. Liriomyza trifolii, too, ranks among the major insect pests of tomatoes, which also include armyworms, tomato fruitworms (Heliocoverpa zea Boddie, Noctuidae), loopers, and aphids [17,18]. Among the insect pests found on Florida cabbage, Elwakil and Mossler [19] reported the greatest problem involved diamondback moths (Plutella xylostella (L.), Plutellidae), while the cabbage looper (Trichoplusia ni (Hübner), Noctuidae), once a major pest, has become less problematic. Liriomyza trifolii and B. argentifolii are among the most troublesome on cabbage in southern Florida, where L. trifolii is a minor pest and B. argentifolii is a major pest [19,20]. Many major pests of Florida cabbage have become minor pests through control treatments for diamondback moths and are hence only occasional problems [19].

There are often interactions between host plants, herbivorous insects, and their natural enemies which are tritrophic, or involve all three kinds of organisms. Volatile compounds produced by plants can influence the success of natural enemies in finding their host insects [21, 22]. Plants in the Fabaceae, Solanaceae (eg. Capsicum annuum L.), Cucurbitaceae, Apiaceae, Rosaceae, and Vitaceae have been found to release few volatiles in low concentrations when they are healthy, but when injured, the plants often increase volatile production and release, which may attract parasitoids [22]. Several parasitic wasp species attack the larval and pupal stages of leafminers and can maintain their populations below damaging levels in the absence of broad-spectrum pesticide applications [1]. The parasitoids also show preferences for L. trifolii, which vary with different host plant species [23]. Knowledge of the infestation levels of L. trifolii and its parasitoids on different host plants can help in developing effective management plans. Hence, the present study sought to determine and compare infestation levels of L. trifolii and parasitism by its parasitoids on five vegetable crop species grown in southern Florida: snap beans, cucumbers, squash, tomatoes, and cabbage. Specific objectives included determining if there were differences among the crop species in numbers of individuals of each L. trifolii life stage or in the proportions of larvae surviving to adulthood on different dates and for the means of dates.

Materials and Methods

The study was conducted at the Tropical Research and Education Center, Homestead, FL, using two field trials (Tests 1 & 2) at different sites and time periods in 2014. In Test 1, planting was done in May with samples collected in May and June, while in Test 2, planting occurred in September with samples collected in October.

Environmental Conditions

At 60 cm above ground level, the Florida Automated Weather Network (FAWN) station at Homestead, FL, recorded the following monthly average temperatures (minimum-maximum in parentheses): 25.2°C (16 - 33°C) and 25.7°C (18 - 34°C) for May and June 2014, respectively (Test 1), and 26.1°C (21 - 33°C) and 24.4 °C (14 - 34 °C) for September and October 2014, respectively (Test 2). Relative humidity averaged 75 and 84% for May and June 2014, respectively, and 86 and 83% for September and October 2014, respectively [24].

Field Preparation, Planting, and Crop Management

The soil type was Krome gravelly loam (loamy-skeletal, carbonatic, hypothermic, lithic, udorthents) and consisted of 34 - 76% limestone pebbles (> 2 mm diam), was well drained, had a low organic matter content (< 2%), and a pH of 7.4 - 8.4 [25, 26]. The field was 84 x 30 m and included 12 beds each 84 x 0.9 m. The beds were separated by bare row spaces 0.9 m wide, which resulted in their centers separated by 1.8 m and raised 15 cm. Each treatment plot included 3 parallel bed sections each in a different row. When unused row space is included, each plot was 11.5 m long, 4.5 m wide, and had a buffer zone 1.5 m wide of non-planted bed space between plots. Within the rows, plots were separated from each other by 13 cm of non-planted space and were covered with blackand-white polythene mulch (1.5 mil thick) with the white side facing upwards (Grower’s Solution Co., Cookeville, TN). Holes (13 cm diameter) and spaced 25 cm apart within the row were cut into the plastic, where seeds or plants were planted. Cultivars of five different crop species were used including snap beans (P. vulgaris ‘Prevail’), cucumbers (C. sativus ‘Diomede’), squash (C. pepo ‘Enterprise’), tomatoes (S. lycopersicum ‘Rocky top’), and cabbage (B. oleracea ‘Escazu’). Beans, cucumbers, and squash were seeded directly into beds as follows: within each hole cut in the plastic, another hole 1.5 cm deep was dug into the soil, where 3-5 seeds were planted, and upon germination, seedlings were thinned to one plant per hole. But for tomatoes and cabbage, a single 4-wk-old seedling was planted into each hole. All crops were planted in the field on the same date in 2014 (1 May for Test 1 and 12 September for Test 2), and each test involved a randomized complete block design with four replications.

Each bed was irrigated using drip tape (T-systems, DripWorks Co., Willits, CA) with two parallel lines spaced 60 cm apart and each line parallel to and 30 cm from the bed center. Each T-tape dispensed water through 2-mm-wide openings spaced 13 cm apart with the system supplying 14,000 liters/ha/d. Twenty-one days before planting, the pre-emergent herbicide halosulfuron methyl (Sandea®, Gowan Co., Yuma, AZ) was applied at 51.9 g/ha to control weeds. Before placement of the plastic mulch, granular fertilizer (N-P-K: 6-12-12) was applied at 1345 kg/ha in a 10-cm-wide band on each side of the bed center, then mixed with the soil. Liquid fertilizer (N-P-K: 4-0- 8) was also added at 0.56 kg N/ha/d through the drip system at 3, 4, and 5 wk after planting. Additionally, the following fungicides were sprayed every two weeks: chlorothalonil (Bravo®, Syngenta Crop Protection Co., Greensboro, NC) at 1.75 liters/ha and copper hydroxide (Kocide® 3000, BASF Ag Products, Research Triangle Park, NC) at 0.8 liters/ha. To control Lepidopteran pests including D. hyalinata, D. nitidalis, and P. xylostella, insecticides derived from Bacillus thuringiensis Berliner var. kurstaki were applied. These included Dipel DF® at 1.1 kg/ha and Xentari DF® at 1.2 liters/ha (Valent Biosciences Co., Libertyville, IL), and they were sprayed biweekly beginning 21 d after planting.

Sampling and Data Collection

Sampling began 11 d (Test 1) or 26 d (Test 2) after planting (DAP), when plants had at least two primary (noncotyledonous) leaves fully unfolded. All the samples were from the middle 6 m of the middle row of each treatment plot. Five plants were randomly selected from each plot, and from eachplant, one leaf was randomly collected and placed into a plastic container (20-cm ht x 15-cm diam). Mines and larvae per plot of L. trifolii were recorded. Sample leaves were maintained in the laboratory (28 ± 1.5 o C, 75 ± 5% RH, and 14:10 h L: D photoperiod) until the larvae pupated, then, numbers of pupae per sample were recorded. Pupae from each plot were maintained in a Petri dish (5 mm x 60 mm) and observed for further development. Liriomyza trifolii adults and their parasitoids were counted every day, and the parasitoids were stored for further identification. Based on the counts, ratios of adults to larvae were determined per sample time per plot to indicate survivorship to the adulthood.

Statistical Analyses

Analysis for each sample date or for the means of dates included comparisons among crops for the numbers of mines, larvae, pupae, adults, proportion of larvae surviving to adulthood, and parasitoids. Three independent statistical analyses were performed beginning with factorials (twoway ANOVAs), which tested for interaction between crops and dates. However, repeated measures ANOVAs tested for differences among crops on different dates. A one-way ANOVA was also performed independently for each date and for the mean of dates in each test. This method produced all the mean separation data by Waller-Duncan Multiple Range tests (P < 0.05), which are not available with repeated measure ANOVAs; it was also used to analyze data for means of dates in each test [27]. To normalize error variances, all data were squareroot transformed (x + 0.25) before statistical analyses except for proportions of larvae surviving to adulthood, which were arcsine square-root transformed (arcsine√x). However, only non-transformed data are shown in the graphs.

Results

Mines

In tests 1 and 2 respectively, numbers of L. trifolii mines per 5 leaves or leaflets were significantly affected by crop (F4, 59 = 96.27, P < 0.0001; F4, 59 = 338.03, P < 0.0001), date (F3, 59 = 61.40, P < 0.0001; F3, 59 = 65.50, P < 0.0001), and the interaction of crop and date (F12, 59 = 14.9, P < 0.0001; F12, 59 = 28.90, P < 0.0001). In Tests 1 and 2, mean numbers of mines on all four sample dates and for the means of dates were nearly always significantly higher for beans than for the other crops (Figure 1). The exception was Test 2 (47 DAP), when beans had statistically the same number of mines as squash, though numerically more than the other crops. However, among the four sample dates and the means of dates in Tests 1 and 2, numbers of mines were nearly always significantly lower for cabbage than for the other crops. The exception was on the first sample date for Test 1 (11 DAP), when cabbage yielded statistically the same number of mines as squash or tomato, though significantly fewer than beans or cucumbers (Figure 1). Cucumbers, squash, and tomatoes were more intermediate and had statistically equal numbers of mines based on Test 1 (11, 25, and 32 DAP) and Test 2 (26 DAP and the mean of dates). Cucumbers and squash produced significantly more mines than tomatoes for Test 1 (18 DAP) and Test 2 (33 DAP). Also, significantly more mines were found on leaves of cucumbers than tomatoes in Test 1 (mean of dates) and Test 2 (40 DAP); in Test 2 (47 DAP), squash yielded significantly more mines than tomatoes.

Figure 1. Mean number of L. trifolii mines per five leaves on five vegetable crops during A. Test 1 (May - June 2014) and B. Test 2 (September - October 2014). Data were transformed by (√x + 0.25) before statistical analyses, but only non-transformed means and standard deviations (SDs) are shown. Means within a sample date having the same or no letters did not differ significantly based on analyses of variance followed by Waller-Duncan K-ratio t-tests (P ≥ 0.05, SAS Institute 2014).

Larvae

Numbers of L. trifolii larvae resulted in similar distributions among crops as the numbers of mines. In tests 1 and 2 respectively, L. trifolii larval numbers were significantly affected by crop (F4, 59 = 111.56, P < 0.0001; F4, 59 = 303.00, P < 0.0001), date (F3, 59 = 60.77, P < 0.0001; F3, 59 = 71.03, P < 0.0001), and the interaction of crop and date (F12, 59 = 15.64, P < 0.0001; F12, 59 = 29.94, P < 0.0001) (Figure 2). Mean numbers of larvae in Tests 1 and 2 on all sample dates and for the means of dates were generally significantly higher for beans than for the other crops (Figure 2). However, Test 2 (47 DAP) was the exception with beans having statistically the same number of larvae as squash, though numerically more than all the other crops. Similarly, mean numbers of larvae were significantly lower for cabbage than for all the other crops, except on the first sample date (Test 1, 11 DAP), when cabbage had fewer larvae numerically than all the other crops, but statistically the same number as squash or tomato. Cucumber, squash, and tomatoes were more intermediate and had statistically equal numbers of larvae for Test 1 (11, 25, and 32 DAP and the mean of dates) and Test 2 (26 DAP and mean of dates). Cucumber and squash had significantly more larvae than tomatoes for Test 1 (18 DAP) and Test 2 (33 and 40 DAP). Also, squash yielded significantly more larvae than tomatoes in Test 2 (47 DAP). Mean numbers of larvae on squash and cucumber did not differ significantly within any sample date or for the seasonal averages.

Figure 2. Mean numbers of L. trifolii larvae. Data transformation and mean separation were the same as described for Figure 1.

Pupae

Mean numbers of pupae from leaf samples were generally similar to previous results for mines and larvae (Figure 3). In tests 1 and 2 respectively, mean numbers of pupae were significantly affected by crop (F4, 59 = 103.96, P < 0.0001; F4, 59 = 252.76, P < 0.0001), date (F3, 59 = 54.33, P < 0.0001; F4, 59 = 68.95, P < 0.0001), and the interaction of crop and date (F12, 59 = 14.57, P < 0.0001; F4, 59 = 30.71, P < 0.0001). Numbers of pupae from Tests 1 and 2 leaf samples were significantly higher on beans than on squash, cucumbers, tomatoes, or cabbage during nearly all the sample dates and for the means of dates. Test 1 (32 DAP) was the exception, however, with beans having statistically the same number of pupae as cucumbers or tomatoes, although beans still exceeded all the other crops numerically 

Figure 3. Mean number of L. trifolii pupae. Data transformation and mean separation were as described for Figure 1.

In contrast, cabbage had the lowest number of pupae statistically during nearly all sample dates and for the means of dates. The exception was Test 1 (11 DAP), when cabbage produced the same number of pupae statistically as cucumber, squash, and tomato, though numerically fewer than all the other crops. Cucumbers, squash, and tomatoes were more intermediate with statistically equal counts of pupae in Test 1 (11, 25, and 32 DAP and the mean of dates) and in Test 2 (26 DAP and the mean of dates). Cucumber and squash yielded significantly more pupae than tomatoes in Test 1 (18 DAP) and Test 2 (47 DAP). Squash also had significantly more pupae than tomatoes in Test 2 (33 DAP), and cucumber had significantly more than tomatoes in Test 2 (40 DAP). However, squash and cucumber did not differ significantly in numbers of pupae on any sample date or for the seasonal averages.

Adults

In tests 1 and 2 respectively, mean numbers of L. trifolii adults that emerged from pupae were significantly affected by crop (F4, 59 = 88.56, P < 0.0001; F4, 59 = 210.98, P < 0.0001) and by the interaction of crop and date (F12, 59 = 11.72, P < 0.0001; F4, 59 = 22.18, P < 0.0001) (Figure 4). Population distributions for L. trifolii adults among the vegetable crops were generally similar to previous comparisons for mines, larvae, and pupae.

Figure 4. Mean number of L. trifolii adults. Data transformation and mean separation were as described for Figure 1.

In Tests 1 and 2, there were always numerically more and usually significantly more adults on beans than on the other four crops on each of the four sample dates and for the mean of dates. Exceptions to the statistical differences were as follows: Test 1 (32 DAP) did not yield any statistical differences, though beans still held the numerical maximum; Test 2 (40 DAP), when beans and cucumbers were statistically equal but greater than the other crops; and Test 2 (47 DAP), when squash was statistically the same as beans. In contrast, cabbage had the lowest numbers statistically of L. trifolii adults on most sample dates and for the mean of dates. Exceptions included Test 1 (11 DAP), when cabbage yielded the same number of adults statistically as squash and tomato, though numerically fewer than all the other crops, and Test 1 (32 DAP), which had no statistical differences. Cucumber, squash, and tomatoes each had intermediate numbers of adults, which were statistically equal based on Test 1 (11, 25, and 32 DAP and the mean of dates) and Test 2 (26 and 47 DAP and the mean of dates). Cucumbers and squash also resulted in significantly more L. trifolii adults than tomatoes in Test 1 (18 DAP), and squash yielded significantly more than tomatoes in Test 2 (33 DAP). Mean numbers of adults on squash and cucumber did not differ significantly within most sample dates or for the means of dates except for Test 2 (40 DAP), when cucumbers produced significantly more adults than squash or tomatoes.

Proportions of Larvae Surviving to Adulthood

Mean proportions of L. trifolii larvae surviving to adulthood tended to follow a nearly opposite relationship among crops compared with the previous relationships within each life stage or for the parasitoids (Figure 5).

Figure 5. Mean percentage of larvae surviving to adulthood with data transformed by (arcsine√x) before statistical analyses. Mean separation was the same as described for Figure 1.

This was especially true with cabbage, which tended to yield higher survival rates than the other crops. However, exceptions were on the fourth sample dates in each test including Test 1 (32 DAP) and Test 2 (47 DAP), when no larvae or adults were found on cabbage. Within individual sample dates for each test, analyses by repeated measure ANOVAs found no significant differences among crops in proportions of larvae surviving to adulthood. For means of dates in each test, however, oneway ANOVAs yielded significantly higher proportions of larvae surviving to adulthood on cabbage than on the other crops in Tests 1 and 2 (Figure 5). Also, a significantly lower percentage of larvae survived to adulthood on beans than on tomato or cabbage in Test 1.

Parasitoids

Numerical distributions of adult parasitoids emerging from samples produced similar results to individual stages of their host, L. trifolii (Figure 6). In tests 1 and 2 respectively, mean numbers of parasitoid adults were significantly affected by crop (F4, 59 = 32.74, P < 0.0001; F4, 59 = 32.07, P < 0.0001), date (F3, 59 = 47.38, P < 0.0001; F4, 59 = 19.25, P < 0.0001), and the interaction of crop and date (F12, 59 = 11.72, P < 0.0001; F4, 59 = 22.18, P < 0.0001).

Figure 6. Mean number of L. trifolii parasitoids. Data transformation and mean separation were as described for Figure 1

In tests 1 and 2, numbers of parasitoids were usually numerically and significantly higher in beans than the other crops during individual sample dates and for means of dates. In Test 1 (25 DAP) and Test 2 (40 DAP), beans and squash were statistically the same: each yielded more parasitoids than cabbage. In Test 2 (40 DAP), cucumbers and tomatoes each had quantities of parasitoids that were statistically equal to all the other crops, while in Test 1 (25 DAP), cucumbers and tomatoes each yielded significantly more parasitoids than cabbage but fewer than beans. Test 1 (32 DAP) was the only date when beans did not result in the most parasitoids numerically: squash and tomatoes each had more. Test 1 (32 DAP) and Test 2 (47 DAP) each yielded no statistical differences in numbers of parasitoids. For the second sample taken in each test, which included Test 1 (18 DAP) and Test 2 (33 DAP), cabbage and tomato each led to significantly fewer parasitoids than the other three crops. However, cucumber and squash were each statistically intermediate with significantly more parasitoids than cabbage or tomato, but fewer than beans. In Test 2 (26 DAP), numbers of squash parasitoids were statistically intermediate between beans and cabbage. For the Test 2 mean of dates, cucumber, squash, and tomatoes were each statistically intermediate with fewer parasitoids than beans though more than cabbage. From the mean of dates in Test 1, cucumbers and squash each had statistically intermediate parasitoid counts with fewer than beans but more than cabbage. Similarly, parasitoids numbers on cucumbers and tomatoes in Test 1 (25 DAP) were statistically intermediate: fewer than beans but more than cabbage. For Test 1 (25 DAP) and Test 2 mean of dates, cabbage yielded significantly fewer parasitoids than all the other crops.

There were no apparent differences between crops in the proportions of each parasitoid species composing the total parasitoid complex. Opius dissitus Muesebeck (Hymenoptera: Braconidae) was the most abundant parasitoid species in all the crops and composed more than 60% of the total population. The other parasitoid species included Diglyphus sp. and Diaulinopsis callichroma Crawford (Hymenoptera: Eulophidae).

Discussion 

The initial crop x time factorial analyses generally indicated that the crop, time, and crop x time interactions were all significant with strong interactions, which suggested there were strong effects of each variable (crop or time) on the other. To meet the objectives of the present study in each test, mean numbers of mines, larvae, pupae, adults, and parasitoids were to each statistically compared between crops within each date or means of dates, but not between dates. However, they tended to appear higher for beans during the first three sample dates than the final date, and higher for the second and third than the first or final sample dates in the other crops. When comparing results from the two tests, similar patterns were evident: for example from beans, the first sample dates (11 d for Test 1 and 26d for Test 2) seemed to yield more L. trifolii than the corresponding last dates (32 d for Test 1 and 47 d for Test 2). However, all sample dates in the second test were 15d later than corresponding dates in the first test. It is unknown why they were so similar in comparative numbers of L. trifolii per sample date per crop. Because bean plants appeared to be a preferred host, they may have permitted a more rapid population buildup, hence a less-apparent lag, compared to the other crops on the first sample date. In all crops, the low numbers for different stages of L. trifolii consistently counted on the fourth sample date may have represented poor nutrient quality, low food supply, or perhaps competition with other insect species. Reduced numbers of L. trifolii on the last date more likely resulted from these food supply factors instead of from control by parasitoids because parasitoid population distributions over time were similar to those of individual L. trifolii life stages.

In each test, mean numbers of mines, larvae, pupae, adults, and parasitoids were typically highest on snap beans followed by cucumbers or squash, then often tomatoes, and tended to be lowest on cabbage. Hence, beans generally appeared to be the most preferred and cabbage the least preferred host. Liriomyza trifolii choses host plants based on many factors including nutritional chemistry, allelochemicals, plant morphology, and natural enemy activities [28]. Several factors affect host-plant selection by adult females including visual, chemical, acoustic, and tactile [29, 30]. Insects respond to specific ratios of plant volatiles from their host plant species, which may affect their levels of preference [31]. Thompson and Pellmyr [28] reported that a polyphagous insect will feed and oviposit more eggs on a most-preferred host plant species and fewer eggs on a lesspreferred species, such as in a choice test. An adult female will select a particular host plant if it is most favorable to the development of its offspring [32, 33, 34] or to the female itself [34, 35, 36]. Because adult females of L. trifolii choose host plants for oviposition and the offspring larvae cannot move from leaf to leaf, the fitness of larvae depends heavily on this initial host-plant selection.

According to Zhao and Kang [37, 38], leafminers showed different in electroantennogram (EAG) responses indicating they were more attracted to the odors of tomato than to cotton plants. Zhao and Kang [39] found that leafminer EAG responses were higher on bean or tomato plants than on squash or cucumber with the lowest responses on Chinese rose, tobacco, and morning glory. Similarly, beans and tomatoes were the most preferred host plant species by Liriomyza in Kenya [40]. In the present study, however, the often lower numbers of mines, larvae, pupae, adults, and parasitoids on tomato leaves than on bean, cucumber, or squash leaves may have resulted from the smaller leaf areas of tomatoes compared with the other crops. Similarly, Pang et al. [41] reported that preferences of leafminer adults were highest for plants in the Fabaceae followed by the Asteraceae, Cucurbitaceae, Solanaceae, and Apiaceae with the lowest preferences found in the Brassicaceae. Furthermore, the acceptance and rejection of host plant species often seemed to depend on levels of tannin and flavone in the leaf along with leaf trichome densities [42]. Leafminer activity was negatively correlated with levels of tannin and flavone in the leaves, and because cabbage has a high flavone content, it had the fewest leaf mines [42]. In the present study, we also found that cabbage generally produced the fewest leaf mines, larvae, pupae, and adults on each sample date per test. This lower infestation level on cabbage plants also, may have resulted from leaf epicuticular waxes, which can render it more difficult for an herbivorous insect to adhere, feed, or oviposit on the leaf surface than on plants with less leaf wax [43].

In the present study, the often reduced activity of L. trifolii on cucumber and squash compared with bean plants may have resulted from cucurbitacin, which is found in plants of the Cucurbitaceae and deters the feeding of many insect species [44]. Mekuria et al. [45] also found that L. trifolii adults often appeared to avoid oviposition on plants in the Cucurbitaceae possibly because of the high levels of cucurbitane, glucosides, and triterpenoids. Differences in leaf nutritional value in the present study may also have affected oviposition and feeding preferences. Oviposition, feeding, and fecundity of L. trifolii females have been correlated with leaf nitrogen content, and leafminers often demonstrated a preference for plants with high levels of nitrogen [46,47]. In Miami-Dade County FL, high L. trifolii activity on snap beans, cucumbers, and squash may have occurred because of continuous feeding for many generations and because adults of many pest species show preference for the same crops they initially began feeding on as larvae [37]. Contrary to numerical distributions among the five crops shown by L. trifolii mines and different L. trifolii life stages, higher proportions of the larvae survived to adulthood on cabbage than on the other crops based on the means of dates in Tests 1 and 2 (Figure 5). In Test 1, the mean of dates yielded a lower percentage of larvae surviving to adulthood on beans compared with cabbage or tomato plants. The descrepancies in proportional survival may have resulted from mortality because of competition, lower food quality, increased disease infection because of crowded conditions, or possibly higher production of plant secondary compounds in response to plant damage. These factors also may help account for consistent population reductions that occurred on the final (fourth) sample date compared with the first three dates. According to Petitt et al. [48], O. dissitus, the most common leafminer parasitoid in the present study, was more attracted to odors of leafminer-infested lima bean plants than to those of infested cotton or eggplants. This agrees somewhat with our finding significantly more L. trifolii parasitoids in infested bean leaves than in squash, cucumber, tomato, or cabbage leaves. Similarly, Zhao and Kang [38] reported that Diglyphus isaea (Walker) (Hymenoptera: Eulophidae) was more attracted to plants infested with leafminers than to non-infested plants.

Conclusion

We found that mean numbers of L. trifolii mines, larvae, pupae, adults, and parasitoids were typically highest on snap beans followed by cucumbers or squash, then often tomato, and tended to be lowest on cabbage. Beans generally appeared to be the most preferred host with cabbage the least preferred. Opius dissitus was the most common parasitoid species Parasitoid abundance seemed to follow and thus appeared to be based on host abundance of L. trifolii, but in turn, the parasitoids apparently exerted no effects on host abundance. Contrary to these crop preferences, however, a higher percentage of larvae survived to adulthood on cabbage than on the other crop species for the means of dates in Tests 1 and 2, and a smaller percentage of larvae survived to adulthood on beans than on cabbage or tomato for the means of dates in Test 1. In addition, each life stage of L. trifolii resulted in fewer individuals consistently appearing on beans on the final sample date than the initial three dates, and with the other crops, lower numbers were found on the first or final than on the second and third sample dates. Based on the numbers of L. trifolii per sample date and their proportional survival to adulthood, the main limitations to population growth were likely poor nutritional quality, low food supply, or perhaps competition from other insects. This was most apparent from observing beans compared with cabbage and on the fourth compared with the first three sample dates. Useful sequel studies might examine the chemistry of these five vegetable crops. Because cabbage plants had the fewest L. trifolii mines larvae, pupae, and adults, chemical analyses of cabbage leaves may help to determine any ovipositional or feeding deterrents present, which may suggest control options.

Acknowledgments

We thank R. Rijal-Devkota, C. M. Sabines, B. Panthi, C. Carter, and J. Teyes for assistance.

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